摘要:

FARADAY DISCUSSIONS OF THE CHEMICAL SOCIETY NO.79 1985 Polymer Liquid Crystals THE FARADAY DIVISION THE ROYAL SOCIETY O F CHEMISTRY LONDONFARADAY DISCUSSIONS OF THE CHEMICAL SOCIETY NO.79 1985 Polymer Liquid Crystals THE FARADAY DIVISION THE ROYAL SOCIETY O F CHEMISTRY LONDONA GENERAL DISCUSSION O N PoIymer Liquid Crystals lst, 2nd and 3rd April, 1985 A GENERAL DISCUSSION on Polymer Liquid Crystals was held at the University of Cambridge on lst, 2nd and 3rd April 1985. The President of the Faraday Division, Professor P. Gray, FRS, was in the chair: about 200 Fellows of the Faraday Division and visitors from overseas attended the meeting. Among the overseas visitors were: Dr W. Adams, U.S.A. Dr M. Ballauff, West Germany Dr A. Baumgartner, West Germany Professor G.C. Berry, U.S.A. Professor J. Blackwell, U.S.A. Professor A. Blumstein, U.S.A. Professor R. B. Blumstein, U.S.A. Miss C. Boffel, West Germany Mr C. Brembati, Italy Dr L. L. Chapoy, Denmark Professor E. Chiellini, Italy Dr C. Corno, Italy Dr G. Costa, Italy Mr R. Eidenschink, West Germany Miss M. Engel, West Germany Miss P. Fabre, France Dr G. Farrow, U.S.A. Dr D. Freitag, West Germany Dr K. H. Gardner, U.S.A. Dr T. Geelhaar, West Germany Dr J. Genz, West Germany Dr D. G. Gray, Canada Professor A. C. Griffin, U.S.A. Dr R. S . Irwin, U.S.A. Dr W. J. Jackson Jr, U.S.A. Mr D. Judas, France Professor M. Kliman, France Dr H-J. Kock, West Germany Mr W. Kreuder, West Germany Professor W. R. Krigbaum, U.S.A. Dr H. Kromer, West Germany Miss S . L. Kwolek, U.S.A. Dr M.Laun, West Germany Dr F. Laupretre, France Professor R. W. Lenz, U.S.A. Dr R. R. Luise, USA Dr E. Marsano, Italy Professor A. F. Martins, Portugal Miss G. Mazelet, France Dr J-P. Meraldi, Switzerland Professor R. B. Meyer, U.S.A. Professor L. Monnerie, France Mr K. Mueller, West Germany Professor M. Muthukumar, U.S.A. Dr P. Navard, France Dr C. Noel, France Professor T. Odijk, The Netherlands Dr U. Pedretti, Italy Professor V. Percec, U.S.A. Dr S . J. Picken, The Netherlands Dr H. Pielartzik, West Germany Mr B. Reck, West Germany Professor H. Ringsdorf, West Germany Dr G. Roessling, West Germany Professor P. S. Russo, U.S.A. Professor E. T. Samulski, U.S.A. Dr D. J. Sikkema, The Netherlands Dr L. S . Singer, U.S.A. Mr P. Sixou, France Mr A. Stroobants, Belgium Professor E. L.Thomas, U.S.A. Professor B. Valenti, Italy Dr S . van der Zwaag, The Netherlands Professor B. Wessten, Sweden Mrs B. Wessten, Sweden Dr K. W. Wissbrun, U.S.A. Miss B. A. Wood, U.S.A. Dr D. Y. Yoon, U.S.A. Dr A. E. Zachariades, U.S.A. Mr R. Zentel, West GermanyOrganising Committee Professor B. R. Jennings (Chairman) Dr B. Griffin Professor A. J. Leadbetter Professor A. Ledwith Dr M. R. Mackley Professor G . Williams Dr A. H. Windle Dr D. A. Young ISBN: 0-85186- 598-4 ISSN: 0301-7249 Printed in Great Britain by J. W. Arrowsmith Ltd, BristolCONTENTS Page 7 21 33 41 55 73 85 125 133 141 149 161 175 191 20 1 215 225 229 Introductory Lecture: Macromolecular Structure and Liquid Crystallinity by E. T. Samulski Balancing Mesogenic and Non-mesogenic Groups in the Design of Ther- motropic Polyesters by R.W. Lenz Structure- Property Correlations in some Nematic Main-chain Polyesters by A. Blumstein, M. M. Gauthier, 0. Thomas and R. B. Blumstein Conjigurational Characteristics and Nematic Order of Semiflexible Ther- motropic Polymers by D. Y. Yoon, S. Bruckner, W. Volksen, J. C. Scott and A. C. Griffin Molecular Correlation in Thermotropic Copolyesters by A. H . Windle, C. Viney, R. Golombok, A. M. Donald and G. R. Mitchell X-Ray Analysis of the Structure of Liquid-crystalline Copolyesters by J . Blackwell, A. Biswas, G. A. Gutierrez and R. A. Chivers GENERAL DISCUSSION Measurements of the Anisotropic Viscous and Elastic Properties of Lyotropic Polymer Nematics by R. B. Meyer, F. Lonberg, V. Taratuta, S.Fraden, S-D. Lee and A. J. Hurd Phase Studies of Binary Mesogenic Systems by W. R. Krigbaum Rheological and Rheo-optical Studies of a Constitutive Equation for Nematogenic Solutions of Rod-like Polymers by G. C. Berry Optical Textures Observed during the Shearing of Thermotropic Liquid-crystal Po 1y m e rs by N. J. Alderman and M. R. Mackley A Model for Domain Flow of Liquid-crystal Polymers by K. F. Wissbrun GENERAL DISCUSSION Dielectric, Nuclear Magnetic Resonance and Electron Spin Resonance Studies of Relaxation Processes in a Liquid-crystalline Polyester by F. Laupretre, C. Noel, W. N. Jenkins and G. Williams Electro-optic Efects in Side-chain Polymer Liquid Crystals by H . J . Coles Defects and their Relationship to Molecular Conjgurations in Nematic Polymers by M . KlCman GENERAL DISCUSSION Mesophase Texture and Defects in Thermotropic Liquid-crystalline Polymers by E. L. Thomas and B. A. Wood241 Chiral Liquid-crystalline Polyesters. Structural Effects on Mesomorphic Behaviour by E. Chiellini and G. Galli Chemical Characteristics of Cellulosic Liquid Crystals by D. G. Gray The Mesophase in Carbonaceous Pitches by L. S . Singer 257 265 273 GENERAL DISCUSSION 289 LIST OF POSTERS 291 INDEX OF NAMES

ISSN:0301-7249    

DOI:10.1039/DC9857900001

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1985, 79, 7-20 Macromolecular Structure and Liquid Crystallinity BY EDWARD T. SAMULSKI Department of Chemistry and Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06268, U.S.A. Received 13th April, 1985 The relationship between low-molar-mass or monomeric liquid crystals and polymeric liquid crystals is explored by examining the historical development of experimental and theoretical aspects of the latter materials. Polymeric liquid crystals are initially contrasted with conventional, non-mesogenic, flexible polymer chains (random coils), and then semi- flexible and rigid-rod polymers are examined. Both thermotropic and lyotropic phases are described, with emphasis placed on the former; in particular, the relationship between dimers and polymers belonging to the semi-flexible category: +(mesogenic core) -(flexible spacer)-+,.The review concludes by drawing attention to the curious behaviour of polypeptide liquid crystals (cholesteric compensation). The underlying theme of the lecture is the need to recognize the synergism and affinity that characterizes research on monomeric and poly- meric liquid crystals. Polymer chemists and physicists have for many years acknowledged in a limited way the relevance of the research effort in liquid crystals to aspects of their own research. They occasionally borrowed nomenclature (‘nematic’ and ‘smectic’) to describe the kinds of supramolecular organization that they presumed must exist in condensed phases of polymers. Some of the early textbooks on high polymers even included a section on the classification of liquid crystals.Today with the increasing interest in ordered, fluid phases composed of macromolecules (polymeric liquid crystals) the research activity in these two formerly distinct areas is becoming synergetic. Subtle phenomena, difficult to investigate in Conventional liquid crystals, are often exaggerated in polymeric liquid crystals. Moreover, the process of extend- ing and refining the modelling of liquid crystals to include polymers is giving us a clearer view of the dominant molecular forces that stabilize the liquid-crystal state. In this lecture I preface a discussion of a few aspects of polymeric liquid crystals that I find particularly intriguing with a brief historical development of the field from my own and necessarily limited perspective.HISTORY Vorlander’s prodigious contribution to liquid-crystal research during the first quarter of this century displayed admirable chemical intuition. Working with little more than the notion that lath-like molecules (I) are predisposed to form liquid- l a t h - l i k e me M LC8 MOLECULAR STRUCTURE AND LIQUID CRYSTALLINITY crystalline melts, he postulated that ‘infinitely long mesogens’ would be compatible with the supramolecular organizational constraints in liquid crystals.’ In addition to this superficial reference to the plausibility of preparing a polymeric liquid crystal (PLC), Vorlander subsequently synthesized a dimer liquid crystal (DLC), a mesogen consisting of two lath-like segments (mesogenic cores) joined together with a flexible alkyl chain (spacer).2 The DLC (11) is currently a subject of active research core - spacer in model compound studies that have a direct bearing on linear, semi-flexible,PLCs (111) composed of polymer chains having the regular alternating sequence: +( mesogenic core) - (spacer)+,. After Vorlander’s early contributions, research mainchain PL C on PLCs in the second quarter of this century was, for all practical purposes, dormant. In fact, until the late 1960s, when the industrial potential of PLCs was recognized, only a very few isolated, albeit significant, discoveries relevant to PLCs were reported.A brief chronology of these contributions follows. Birefringent solutions of rod-like virus particles (lyotropic PLCs) were reported in the late 1930s and early 1940~.~ Following this observation Onsager demonstrated that a gas composed of ideal, rigid rods will exhibit a spontaneous transformation from an isotropic to a quasiparallel (nematic) distribution of rods when the density of the gas exceeds a critical value which in turn is an explicit function of the aspect ratio of the This first reasonable model of the nematic-isotropic phase transition was essentially ignored by the modest mainstream of research on conven- tional, low-molar-mass, thermotropic liquid crystals. [Herein I designate such conventional mesogens MLCs (‘monomer’ liquid crystals) irrespective of whether or not such mesogens may be polymerized.] The (Y -helical, rod-like, synthetic polypeptide, poly( y )-benzyl-L-glutamate (PBLG), was shown to form a lyotropic PLC in a variety of common organic solvents in the early 1950s.The initial interest in this seemingly esoteric biopolymer was industrially motivated: Courtaulds Ltd was seeking new fibre-forming polymers and considered synthetic polypeptides because naturally occurring polypeptides ( e.g. wool) exhibit excellent properties.’ Although the anticipated useful properties of PBLG did not materialize, it was comprehensively studied at the Courtaulds Ltd Research Laboratory in Maidenhead and played a significant role in the elucidation of the structural architecture of the ( ~ - h e l i x . ~ . ~ In fact, it was during the preparation of oriented specimeps for i.r.-dichroism studies of the a-helix that the unusual properties of concentrated solutions of PBLG were first noticed.8 Robinson and coworkers then characterized the PBLG liquid crystal, demonstrating that the chiralE.T. SAMULSKI 9 rods (right-handed a-helices) assumed a twisted nematic (cholesteric) supramolecular arrangement in the lyotropic m e s ~ p h a s e . ~ - ~ ~ At roughly the same time and also in England, Flory, using the lattice theory of polymer solutions, replaced flexible chains with rigid rods and demonstrated the formation of an ordered phase above a critical volume fraction of rods that depends on the rod aspect ratio.14 With the exception of isolated comparisons of theory and experiment on the contemporaneous lyotropic polypeptide liquid cry~tals,’~ as in the case of the Onsager model, this advance in the delineation of the role of anisotropic repulsion (steric interactions) in stabilizing mesophases did not gain widespread acceptance.It was largely overshadowed by the mean-field model of Maier and Saupe of the isotropic-nematic phase transition, which implicated anisotropic attraction (dispersion forces) as the source of mesophase stability.16 The latter model influenced the design and synthesis of new classes of thermotropic MLCs into the 1960s and 1970s. The relative importance of repulsive and attractive interactions has been recently reassessed by theorists ;17-19 this reassessment has been reinforced by the discovery of completely aliphatic MLCs (mesogens with negligible polarizability anisotropy).20 In the last decade both the Onsager and Flory models have been re-examined and modified to include the influences of rod flexibility, anisotropic attractive forces, rod length distributions and mixing of rods with random-coil polymers: Flory has recently reviewed these extensions of the lattice theory?l and a modified Onsager model is considered in the review by Grosberg and Khoklov;22 more contemporary approaches to modelling PLCs are available in ref.(23). In the mid 1960s MLCs with a single functional group (e.g. a vinyl group) were polymerized to form a sidechain PLC (IV). Melts of such sidechain polymers W sidechain PLC retained liquid-crystal textures (nematic, smectic and cholesteric) over a well defined temperature range usually in excess of that exhibited by the MLC; i.e. the PLC mesophase relative to the MLC could be ‘stabilized’ uia p~lymerization.~~ I am not aware of a scientifically compelling rationale for synthesizing sidechain PLCs in the 1960s.If activities in Tobolsky’s laboratory at Princeton University were rep- resentative of the times, sidechain PLCs were synthesized only because they could be prepared by non-synthetically inclined students of physical chemistry, e.g. cholesterol acrylate, a commercially available MLC, could be heated into its cholesteric phase, allowing the thermally induced polymerization to proceed ; a solid, dimensionally stable, cholesteric polymer film which reflected irridescent colours was ~btained.~’ Since these early crude experiments, technologically impor- tant end-uses for sidechain PLCs have been identified: (1) cholesteric PLCs function as tunable-wavelength reflectors and notch filters ;26 (2) sidechain copolymers solubil- ize dichroic dye additives to electro-optic displays;27 (3) they serve as a non- centrosymmetric host matrix for hyperpolarizable guest molecules in non-linear optical devices ;28 (4) lyotropic smectic MLCs can be polymerized to stabilize vesicles10 MOLECULAR STRUCTURE AND LIQUID CRYSTALLINITY in drug-delivery schemes.29 Extensions of the latter to smectic monolayers could stabilize thin organic dielectric layers for microelectronic device fabri~ation.~' Lyotropic sidechain PLCs having amphiphillic sidechains have been prepared and their aqueous phases characterized.26 There have also been efforts to incorporate oblate, disc-like mesogenic cores into theromotropic sidechain P L C S .~ ~ During roughly the same period that sidechain PLCs were first synthesized, linear block copolymers prepared from monomers with distinctly different chemical properties were shown to aggregate into liquid-crystalline textures in the melt or in the presence of a solvent for one of the blocks.32 Block copolymer solids with well developed, microphase-separated morphologies exhibit unique mechanical proper- ties, and preferentially swollen copolymers yield a variety of morph~logies.~~ The development of linear, mainchain PLCs was more intentional and directed. The potential end uses for linear PLCs (high thermal stability and high tensile strength) focused research on these r n a t e r i a l ~ . ~ ~ Expanding on the background provided by research on liquid-crystalline phases formed by biological macromo1ecules,35 in particular, the anomalously low viscosity of the m e ~ o p h a s e ~ ~ relative to the isotropic solution, it would have seemed logical, in retrospect, to design explicitly linear PLCs.Curiously, some polymers subsequently shown to be thermotropic linear PLCs of the semi-flexible polyester type (high aromatic content) were studied in industrial research laboratories in the early 1960s ; ultra-high-strength materials were being sought. However, the liquid-crystalline nature of the fluid phases of these polymers appears to have been o v e r l o ~ k e d . ~ ~ It was not until Stephanie Kwolek rationalized the unusually high tensile strengths exhibited by fibres spun from concentrated solutions of the poly(ary1 amides) that the critical importance of mesophase formation was Dobb and McIntyre have recently reviewed the development of the industrially important PLCS.~' The Kevlar (Du Pont) and Arenka (Akzo) poly(ary1 amide) lyotropic PLCs have received the most notoriety, although commercialization of thermotropic PLCs has just been announced.41 Academic research on mainchain PLCs started in the mid 1970s and has focused on the semi-flexible, alternating copolymer PLCs (111) initially described by Roviello and S i r i g ~ .~ ~ Exaggerated even-odd behaviour (relative to MLCs) of both thermo- dynamic and microscopic properties of these PLCs are associated with the parity of the alkyl spacer segment of the copolymer (re.whether or not there is an even or an odd number of chemical bonds in the spacer). The influence of the parity of the spacer and, more generally, the implications of connecting mesogenic cores via flexible spacers are addressed below and in several of the papers in this Discussion. Table 1 enumerates typical polymers that span a wide range of intrinsic molecular flexibility. The coarse division of the entries in table 1 serves as a mechanism for partitioning the remaining material covered in this lecture. FLEXIBLE POLYMERS In fluid phases (melts and concentrated solutions) of flexible polymers such as those in the first category of table 1, the individual chains are devoid of long-range intramolecular order. An absence of internal order extending beyond a few monomer units is their hallmark, even though such chains are comprised of a continuous succession of covalently joined monomers, each link conforming to exacting bond- length and valence-angle constraints.This is because at typical temperatures charac- terizing polymer fluids (200-500 K) there are very rapid transitions (ca. 10'' s-l) among the rather localized dihedral angle preferences at single covalent bonds. The consequence of this isomerization is that the persistence of orientational correlationE. T. SAMULSKI 11 Table 1. Coarse classification of linear polymer flexibility ; examples ~~ I flexible polymers poly (siloxanes) poly( phosphazines) poly( ethylene) I1 semi-flexible polymers cellulose derivatives: poly(p-phenylene terephthalamide) poly(p-hydroxybenzoic acid) +o@$ A,B-copolymers (regular) f (rigid core) - (flexible spacer)+ ,, I11 rigid rod-like polymers a -helical polypeptides 0 II fNHCH-C+, I R poly(p-phenylenebenzobisthiazole) polyphenyl along such chains is rapidly attenuated.The limited range of intramolecular order is graphically illustrated in fig. 1 for fragments of three flexible polymer chains differing in their chemical constitution. In the figure the bond correlation function, ( P2( bi bj)), a kind of intramolecular order ~arameter;~ is plotted against the separ- ation of a pair of bonds. (P2(6i bj)) = (i(3 cos 0, - 1)) reflects the angular correla- tions between a given bond unit vector bi and a second bond unit vector bj located j - i bonds further along the chain.? The brackets ( ) signify an average over isomerization and may be readily computed using equilibrium statistical mechanics with the rotational isomeric state (r.i.s.) approximation.44 For adjacent bonds 8, ( j = i + l), the supplement of the valence angle, dictates the magnitude of P2( bi bj), and as this angle virtually always exceeds 54.7", the initial value of the bond t The second Legendre polynomial P2( 6, - b,) = i(3 cos 8 , - 1 ) is a cohvenient parameter for expressing the extent of bond orientational correlations. For the extreme situation wherein b, 11 b,, e.g.alternate bond vectors ( j = i + 2k, k = 1 , 2 , 3 . . . ) in an alkyl chain in the all-trans conformation, (P2( b; - b,)) = 1.0. If 8, > 54.7 O (the 'magic angle'), P,(b, - b,) < 0; the extrema -1 is obtained when b , l b , .12 MOLECULAR STRUCTURE AND LIQUID CRYSTALLINITY -0.51- 00 I .O L 1 2 3 4 5 6 7 8 9 1 0 0 -a D -0 * 5 11111111111 ( i -1) Fig. 1. The bond correlation function averaged over rotational isomerization plotted against the distance separating a particular bond pair (j-i): ( a ) an alkyl chain, ( b ) an ethylene oxide chain and (c) a dimethylsiloxane chain. The averages over the respective r.i.s. isomerizations were carried out using nominally accepted statistical weights ;* all non-bonded interactions (excluded volume) were included. 1 2 3 4 5 6 7 8 9 1 0 correlation function (neglecting the self-correlation i = j) is negative. Apart from the peculiar zero at 8, = 54.7 O, ( P2( b bj)) = 0 only when isomerization averages angular correlations to zero.For flexible chains (P2(bi bj)) ---* 0 when the pair of bonds under consideration are sufficiently far removed from one another, i.e. when j - i exceeds the angular correlation 'length'. Returning to fig. 1, we see that for an alkyl chain subjected to generally accepted bond length, valence angle and dihedral angle constraint^,^^ angular correlations become negligible when j - i > 10. The correlations attenuate faster in the ethylene oxide chain because the dihedral angle preference (the largest statistical weight) alternates between gauche at the C-C bonds and trans at the 0-C bonds.& The dimethyl siloxane chain has an alternating valence angle: 8, = (LSi-0-Si) = 37 O and eb = (LO-Si-0) = 70 O. This alternation in 8, together with r.i.s. transitions, causes a loss of angular correlation after only two monomer units.Findings such as those shown in fig. 1 are generally indicative of flexible polymer chains. What does this absence of long-range intrachain order imply about interchain orientational correlations? The latter is the signet of liquid crystals.E. T. SAMULSKI 13 Fig. 2. Schematic drawing of a random-flight conformation of a flexible polymer chain. The insets depict ( a ) positive orientational correlation between two neighbouring chain fragments (parallel chain axes) and ( b ) negative orthogonal correlation. 0 5 10 h / A Fig. 3. The chain-axis (chord) Correlation function is shown as a function of chain separation rrj; [after ref. (SO) and (51)]. The nature of interchain organization in condensed, fluid phases (and also, mechanically equilibrated rubbers or glasses) composed of linear, flexible polymers is less readily accessible. There is, of course, no evidence for macroscopic order.On a smaller scale, i.e. on the order of the scale of the overall chain dimensions, neutron-scattering data indicate that the polymer chains assume a random-flight trajectory (fig. 2).48 On the submacromolecular scale, information about orienta- tional correlations between chain fragments is difficult to obtain. There seems to be a strong temptation to impose short-range order on the larger-scale randomness, i.e. to suggest that locally, chains pack with their chain axes aligned approximately parallel [fig. 2( a)]. Such local nematic-like ordering seems ‘intuitively’ proper for prolate chain fragments.Experimental data extrapolated from low-molar-mass fluids is not unequivocal on short-range order; studies of n-alkanes sug est only very weak orientational order characterized by a correlation length <10 !?’ Computer experiments (molecular dynamics and Monte Carlo calculations) relevant to this point corroborate this view but suggest that orientational correlations between nearest-neighbour chain fragments, insofar as they are detectable, have proximate segments orthogonal on average [fig. 2( b ) ] , i e . the chain-axis angular correlation function averaged over all pairs I and J of chain fragments at a given Eeparation rlJ, p2( Cl C’), is negative when the fragments contact each other (fig. 3) ; P2( C, CJ)14 MOLECULAR STRUCTURE AND LIQUID CRYSTALLINITY tends to positive values (+0.05) for rZJ = 5 - 6 A, before becoming negligible at separations on the order of 10 81.50,51 In sum, the random-chain configuration, together with minimal short-range interchain correlations, ensure that fluid phases of flexible polymers are isotropic on all scales.Does this imply that further consider- ation of flexible polymers in the context of mesophase formation should be discon- tinued? Apparently not. Several polymers in this category merit further study. (1) Poly(ethy1ene) melts at high pressures are purported to form mesophases ; extended-chain crystals are derived from the quenched melts.52 (2) Poly( diethylsiloxane) exhibits anomalous behaviour above its melting point; the term viscous-crystalline was applied to the ordered fluid pha~e.’~ Curiously, there may be an MLC analogue of the poly( siloxane) ‘mesophase’: the dimer of di-isobutyl silanediol exhibits a me~ophase.’~ (3) Poly(phosphazines), a new class of flexible elastomers with good thermal stability, clearly manifest mesophases above Tg.” Clearly, further studies of flexible polymers directed at the supramolecular organizational attributes of these polymers are warranted.SEMI-FLEXIBLE POLYMERS Much of the current research activity focuses on thermotropic, semi-flexible polymers. In industrial research laboratories, primary structures of the types shown in the first and second entries under semi-flexible polymers in table 1 are tuned (monomer stereochemistry or ratios of co-monomers are varied) to yield melt- processible materials which retain desirable properties for specific end-uses (thermal stability, high modulus etc.).In academic laboratories, semi-flexible PLCs with a regular, alternating copolymer primary structure, +(core) - (spacer)+,, are being studied. There are a variety of straightforward synthetic routes to such PLCs utilizing thoroughly characterized components (known mesogenic cores of MLCs). Addi- tionally, the ready availability of spacer homologues with variable length provides an entrt! to even-odd phenomena in this class of PLCs. Not surprisingly (especially if Vorlander’s work on DLCs is considered), all of the even-odd phenomena (oscillations in TNI, AS,, etc.) are exaggerated in PLCs relative to those observed in MLCs.Hence studies of these PLCs afford a unique opportunity to refine our understanding of one of the more subtle effects that has intrigued researchers working on liquid crystals since their discovery a century ago. Griffin rekindled interest in the polymer precursor DLCs and suggested that they may play a pivotal role in contrasting PLC and MLC b e h a ~ i o u r . ~ ~ This simple oligomer has the essential distinguishing feature of the polymer: connectivity between two mesogenic cores via covalent bonds, which in turn enables the direct coupling of orientational correlations between two mesogenic cores. Recently, considerable emphasis has been placed on the role of DLCs as models for PLCS.’~-~O Blumstein and Thomas dramatically illustrated that, relative to MLCs, the even-odd entropy change accompanying the nematic-isotropic transition as a func- tion of the number of bonds in the spacer is highly amplified in PLCS.~’ Generally speaking, ASNI for spacers having an even number of bonds (even spacer-chain parity) is ca. 1-2 J mru-’ K-’ {mru = mol of repeat unit, i.e.f-(core)-(spacer)+} roughly independent of the spacer chain length and the degree of polymerization; i.e. ASNI is about the same for MLCs and for DLCs and PLCs having even spacer parity. For odd parity, ASNI = 6-10 J mru-’ K-’ in DLCs and is roughly a factor of two larger in PLCs. For odd-parity spacers the difference between the magnitudes of the DLC and the PLC ASNI values can be rationalized. Roughly speaking, each mesogenic core in the PLC is coupled to two nearest-neighbour cores via spacerE. T.SAMULSKI 15 chains, whereas in the DLC, orientational coupling exists with only a single, covalently bonded core. The large even-odd oscillation of ASNI may also be rationalized in a qualitative manner. In nematic phases, the long-range orientational order encourages mesogenic molecules to align on average parallel to the local director, n. This has significant conformational consequences for the spacer chains when two mesogenic cores are covalently coupled: successive pairs of cores in dimers or polymers will strive, within the conformational constraints of the spacer, to keep their respective core axes parallel to n in the nematic phase. For odd-parity spacers, extended, conformationally ordered spacers (in the extreme, the all-trans conformation of aliphatic spacers) are ideally accommodated in the nematic, and, as a consequence, at the nematic- isotropic transition there will be a substantial contribution from spacer conforma- tional disordering to ASNI.This is not the case for even-parity spacers; the valence angles (approximately tetrahedral) conspire against extended spacer conformations, as such conformations disrupt coparallel juxtapositioning of successive cores ( i.e. in the extreme all-trans conformation, successive cores make an angle of 109.5 O with each other). The uniaxial nematic constraint favours a conformationally disordered spacer, and there is a correspondingly smaller contribution to ASNl for even-parity spacers. The relative magnitudes of different contributions to ASNI have been considered in a more quantitative manner.62-65 We find that for ester-linked spacers, when the calculated spacer conformational contribution is subtracted from experimental values of AS" in the PLC, this difference (ca.1 J mru-' K-' for even-parity spacers and ca. 3 J mru-' K-' for odd) lies in the range of ASNI values determined for MLCs without large alkyl chains.65 Deuterium nuclear magnetic resonance (d.m.r.) gives a more detailed picture of the spacer conformation and delineates the differences between a DLC and a PLC.66 In nematic phases the d.m.r. spectrum of a multiply labelled molecule is a superposition of quadrupolar splittings A v,, the magnitudes of which reflect the efficacy of the molecular motion (rigid-body reorientation, libration and internal isomerization) for averaging the electric field gradient at the ith deuteron; i.e.d.m.r. is a direct measure of the averaging of the orientation of the C-D bond vectors relative to the nematic director. For flexible alkyl chains the number of Av, resolved in the quadrupolar splitting pattern reflects the degree of differential averaging of the electric field gradient at the various methylene segments and is determined by the facility of isomerization at the respective positions along the chain.67 Fig. 4 shows the temperature dependence of the d.m.r. spectra of a neat DLC and a polymer in a PLC-MLC eutectic mixture, each having deuterium-labelled spacer chains: -O(CD2),o-O--. The behaviour of the former is reminiscent of that exhibited by n-alkanes (dissolved in nematic solvents) and alkyl chains appen- ded to conventional MLCs.On lowering the temperature the nematic order increases, yielding larger Avi. This increase in the Avi always occurs in MLCs with the relative Avi diverging at low temperature. When the DLC is contrasted with the polymer eutectic, the widths of both the PLC and DLC quadrupolar splitting patterns increase on lowering the temperature (increasing nematic order), but at comparable reduced temperatures the PLC Aui converge to and coalesce at some limiting value Avo = 80 kHz. Two idealized, limiting cases wherein an odd-parity spacer assumes the all- trans conformation are conceivable: ( 1 ) successive cores (and alternate bonds of the spacer) are aligned exactly parallel to the nematic director resulting in the C-D bond vectors making an angle of 109.5 O relative to n and (2) the major axis of the all-trans conformation is aligned parallel to n yielding C-D bond-vector orienta-16 MOLECULAR STRUCTURE AND LIQUID CRYSTALLINITY 50 kHz c---------$ Fig.4. Temperature dependence of the quadrupolar splittings associated with deuterium- labelled spacer chains in (a) a DLC, and ( b ) a labelled PLC in a eutectic mixture containing a MLC [see ref. (66)]. tions that are normal to n. The calculated quadrupolar splittings Av = l$q &-,,I corresponding to these two limiting cases are Av( 1) = 83.9 kHz and Av(2) = 126 kHz, respectively ( q = 168 kHz). These two extremes would, in conjunction with experi- mental measurements, enable determinations of the order parameters for the molecular director Sm ( i) = A vo/ A v( i).For the two idealized cases above we find Sm( 1) = 0.95 and Sm(2) = 0.63. X-ray measurements of the azimuthal scattering intensity from aligned, quenched PLCs with odd spacer parity yield S,,, values in the nematic phase ranging from 0.6 to 0.72.68 Proton n.m.r. measurements yield mesogenic-core order parameters that span a slightly larger range, 0.7-0.8.64*69 Clearly neither of the above limiting cases applies in a fluid phase, and even at the lowest temperatures in the PLC eutectic (fig. 4) the magnitude of Avo implies that there is still considerable mobility of the C-D bond vectors. The observed coales- cence (Avi ---* Avo) as the temperature is lowered suggests that, concomitant with the increased nematic order, the averaging of the C-D bond-vector orientation (methylene mobility) becomes independent of the methylene segment’s proximity to the mesogenic cores.Similar findings were reported for a different PLC earlier.7o The marked differences in the d.m.r. data for the DLC and the PLC (fig. 4) reflect the distinctly different abilities of the spacer chain to accommodate the increasing orientational constraints imposed on the mesogenic cores in the respective oligomers, and raise additional questions about the relationship of the dimer mesophase to that of the polymer. Are the d.m.r. measurements in the PLC simply showing the annealing out of the chain-end ‘defects’ as the temperature is lowered? (Note that ‘hairpin’ bends do not increase the number of defects in a DLC and both bent and extended DLC conformers would accommodate increasing nematic order.) Moreover, in semi-flexible, linear PLCs, certain deformations requiringE.T. SAMULSKI 17 chain ends (splay) may actually drive conformational transitions in the spacer to produce hairpin defects. As the d.m.r. observations clearly show averaged conforma- tional differences between the dimer and the polymer, it would be interesting to ascertain the critical degree of polymerization (the mesogenic-core angular correla- tion length) above which the spacer dynamics of the oligomer crosses over to behaviour characteristic of the polymer. The calculations used to simulate spectra such as shown in fig. 4 also afford an opportunity to extract predicted values of the core-order parameter (the ‘nematic order’) in a homologous series. At the N-I transition, the predicted values of the core order oscillate with spacer parity in the range from ca.0.35 for even parity to ca. 0.65 for odd parity.65 Such an oscillation is observed experimentally. 68i71 Also, in a PLC we might expect that linking successive cores together via the spacer could very well amplify biaxial librations of the mesogenic core. Hence it would be of interest to determine explicitly the biaxiality of the ordering of a PLC core and ascertain if, relative to MLCs and D L C S , ~ ~ there is a pronounced even-odd oscilla- tion in ( S , - S,,,,), and thereby refine the measurements of the ‘nematic order’ derived from n.m.r. studies. RIGID-ROD POLYMERS All of the rigid-rod polymers (the last entries in table 1) are thermally intractable, i.e.they chemically degrade at temperatures below their melting points. As a consequence, these polymers must be solubilized in order to exhibit mesophase formation. The severe solvent requirements (usually strong acids) have not, however, deterred research on these systems, as the potential reward (high thermal stability and ultra-high-strength fibres) is considerable. I will confine my remarks about this class of PLCs to the system with which I am most familiar, the lyotropic polypeptide mesophase. Building on the comprehen- sive characterization of this PLC by Robinson et a L y 3 in the 1960s and 1970s we subjected the PBLG mesophase to the kinds of investigations that were carried out with thermotropic MLCs.In every respect the lyotropic PBLG liquid crystals exhibited all of the qualities of a thermotropic MLC. The only difference between the polymer mesophase and MLCs was the expanded timescale for phenomena in the former. However, this is merely a consequence of the attenuated transport properties of polymer fluids relative to ordinary The PBLG liquid crystal could act as a nematic solvent for n.m.r.:3-75 its twist viscosity coefficient was determined,76 and, with measurements of its diamagnetic ani~otropy,~~ DuPrC deter- mined the critical magnetic field for untwisting the cholesteric structure and measured the first elastic constant in a PLC.78 Much earlier, Tobolsky and I showed the effect of liquid-crystal texture on the morphology of solid polymer films cast from cholesteric and nematic PBLG lyotropic phases.79 The cholesteric texture in PBLG solutions continues to fascinate me; Robinson demonstrated that the cholesteric pitch could be varied continuously and indepen- dently by controlling (1) rod length (molecular weight), (2) rod concentration, (3) temperature and (4) the solvent comp~sition.’~ Perhaps the most intriguing observa- tion in PBLG mesophases is that the cholesteric pitch can be compensated at a given temperature by merely adjusting the composition of the achiral solvent mixture.This observation that the solvent medium in which the PBLG rods were embedded could dictate the sense of the cholesteric twist prompted my brother and I to consider McLachlan’s formulation of the van der Waals-Liftshitz forces between dielectric particles embedded in a dielectric medium as a candidate for describing this unusual18 MOLECULAR STRUCTURE AND LIQUID CRYSTALLINITY choI e s t e r i c is0 t ropic Fig.5. Schematic representation of the transformation of tightly packaged, liquid-crystalline (cholesteric) DNA to an unravelled, isotropic suspension of random coils. compensation phenomenon.80 Osipovsl has recently reconsidered this approach, and, in an elegant extension of the model, he has accounted for multiple inversions of cholesteric sense observed experimentally in PBLG liquid crystals.82 It would be interesting to determine if this approach might be extended to include ther- motropic MLCs, as it may be readily parametrized in terms of specific molecular attributes (the excess polarizability of the particle relative to the medium).Finally, I want to close by speculating about the implications of polymer mesomorphism in biological systems. Ever since seeing B ~ u l i g a n d ’ s ~ ~ conjecture about and experimental evidence for packaging DNA in chromosomes with a concentrated, cholesteric structure, I have been intrigued by the possibility that a mesophase transition (cholesteric to isotropic) coupled with a macromolecule transi- tion (helix to random-coil) might be involved in the highly synchronous expression of genetic information during the life-cycle of a cell. Such a coupled transition is schematically shown in fig. 5. This would be a highly cooperative transition, as demonstrated by recent theoretical modeling of ‘induced rigidity’: stiffening of a polymer chain during the pretransition stage of the isotropic-nematic liquid-crystal tran~ition.’~ I thank the U.S.National Institutes of Health (NIH grant AM17497) for support for my research program. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 D. Vorlander, Z. Phys. Chem., 1923, 105, 211. D. Vorlander, Z. Phys. Chem., TeiZA, 1927, 126, 449. F. C. Bawden and N. W. 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Yu, Macromolecules, submitted for publication.20 MOLECULAR STRUCTURE AND LIQUID CRYSTALLINITY 66 A. C. Griffin and E. T. Samulski, J. Am. Chem. SOC., 1985, 107, 2975. 67 E. T. Samulski, Polymer, 1985, 26, 177. R. Capasso, P. Iannelli, A. Roviello and A. Sirigu, to be published. 69 A. F. Martins, J. F. Ferreira, F. Volino, A. Blumstein and R. B. Blumstein, Macromolecules, 1983, 16,279; see also A. Blumstein, M. M. Gauthier, 0. Thomas and R. B. Blumstein, Faruday Discuss. Chem. SOC., 1985,79, 33. 70 E. T. Samulski, M. M. Gautheir, R. B. Blumstein and A. Blumstein, Macromolecules, 1984,17,479. A. Blumstein, M. M. Gauthier, 0. Thomas and R. €3. Blumstein, Furuduy Discuss. Chem. SOC., 1985, 79, 33. 71 72 J. W. Emsley, G. R. Luckhurst and G. N. Shilstone, MoZ. Phys., 1984, 53, 1023. 73 S. Sobajima, J. Phys. SOC. Jpn, 1967, 23, 1070. 74 N. Panar and W. D. Phillips, J. Am. Chem. SOC., 1968, 90, 3880. 75 E. T. Samulski and A. V. Tobolski, Macromolecules, 1968, 1, 555. 76 C. Guha-Shridar, W. A. Hines and E. T. Samulski, J. Phys. (Paris), Colloq., 1975, 36, 270. 77 C. Guha-Shridar, W. A. Hines and E. T. Samulski, J. Chem. Phys., 1974,61, 947. 78 D. B. DuPrC, in Polymer Liquid Crystals, ed. A. Ciferri, W. R. Krigbaum and R. B. Meyer 79 E. T. Samulski and A. V. Tobolsky, MoZ. Crysr. Liq. Crysr, 1969, 7 , 433. 8o T. V. Samulski and E. T. Samulski, J. Chem. Phys., 1977, 66, 824. 82 A. Toriumi, K. Yahagi, I. Uematsu, and Y. Uematsu, MoZ. Crysr. Liq. Cryst., 1983, 94, 267. 83 Y. Bouligand, J. Phys., 1969,30, C4-90; see also M. KlCman, Furuduy Discuss. Chem. SOC., 1985, 79, 215 84 J. S. Walker and C. A. Vause, Mol. Crysf. Liq. Cryst., 1984, 110,349; P. G. de Gennes, Mol. Crysr. Liq. Crysf. Lett., 1984, 102,95; P. J. Flory and R. R. Matheson Jr, J. Phys. Chem., 1984,88,6606. (Academic Press, New York, 1982), chap. 7. M. A. Osipov, Chem. Phys., 1985, 96, 259. 81

ISSN:0301-7249    

DOI:10.1039/DC9857900007

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1985, 79, 21-32 Balancing Mesogenic and Non-mesogenic Groups in the Design of Thermotropic Polyesters BY ROBERT W. LENZ Chemical Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003, U.S.A. Received 29 th November, 1984 The molecular variables which control the structure-property relationships in thermotropic liquid-crystalline polyesters are under investigation in this laboratory. A wide variety of polymers based on rigid, linear aromatic ester mesogenic units, with and without flexible or rigid non-mesogenic spacers, have been prepared and characterized for their ability to form a liquid-crystalline melt, the type of phase formed, their transition temperatures and the morphology of the mesophase. Flexible spacers reduce both the melting and clearing tem- peratures, and the type and length of the spacer can determine whether a nematic, cholesteric or smectic phase is formed.Variations in the structure of the rigid mesogenic group, both in the specific type and arrangement of the aromatic ester groups and the presence of pendant substituent, also cause profound changes in the properties of the mesophase melt formed. Copolymers containing rigid non-mesogenic units in random sequence distributions with closely related mesogenic units have been characterized for the effects of composition on thermotropic properties, including the parameter of ‘degree of liquid crystallinity’. Of interest in all series of polymers studied is the critical limit of non-mesogenic unit content beyond which either liquid crystallinity no longer occurs or monotropic behaviour is observed.The investigations in our laboratory on liquid-crystal polyesters have been primarily concerned with delineating the relationships between structure and liquid- crystal properties of main-chain polyesters in which the rigid, linear mesogenic groups are connected together by either rigid or flexible non-mesogenic spacers. We have prepared a large number of such polymers and have investigated the relationships between the structures of both the mesogenic units and the spacers to their thermotropic properties. This report is concerned with our studies on polymers with both types of spacers. The term ‘mesogenic group’, for the purposes of this review, refers to the part of the polymer chain that is composed of the rigid, linear segments and the atoms or functional groups which link them together in a linear array.It is this part of the polymer chain that ultimately determines whether or not the polymer will be liquid crystalline, within what range the transition temperatures will occur for a thermotropic polymer and what type of mesophase can be formed. The mesogenic group must consist of at least two aromatic (or cycloaliphatic) rings connected in the para positions by a short rigid link of two or four atoms, in our case almost always an ester group, which maintains the linear alignments of the aromatic rings. In this manner a rigid element is formed which has an overall length that is substantially greater than the diameter of the aromatic group ( i e .the axial ratio). Three principal methods have been used in our investigations for modifying and controlling the properties of main-chain liquid-crystalline polymers ; these are ( 1) the use of non-rigid groups, termed flexible spacers, in combination with the rigid mesogenic groups in the main chain to reduce the axial ratio of the latter, (2) the 2122 DESIGN OF THERMOTROPIC POLYESTERS unsymmetrical placement of substituents on the mesogenic groups to disrupt the regularity of the repeating units within the polymer and (3) the copolymerization of ( a ) monomers containing different types of mesogenic units (such as a naphthalene-based unit with a p-phenylene-based unit), (b) monomers with two different types of flexible spacers or ( c ) monomers with two different types of rigid molecular structures in which one is linear and mesogenic and the other is not because of its non-linear structure.Only the structural modifications in ( I ) and (3 c), which reduce the axial ratio, will decrease the isotropization (or clearing) temperature, z, while the other modifications reduce the melting point, T,, of the polymers. The effects of these types of variations in molecular structure on the liquid-crystalline properties of their polymers will be discussed in the following sections. EFFECTS OF FLEXIBLE NON-MESOGENIC UNITS ON THERMOTROPIC * PROPERTIES Several important observations and conclusions have resulted from our studies to date on the effects of both the type and length of the flexible spacer unit, R, on the thermotropic properties of liquid-crystal polyesters.Of particular interest are the variations in properties observed for two series of polyesters which were based on a single type of mesogenic unit, an aromatic ester triad of the following structure: The results of these studies are compiled in table 1. For the polymers in table 1 containing polymethylene units, R = +CHzfn, those with spacers having up to 8 methylene groups formed either a smectic or a nematic phase on melting, although some doubt still exists about the polymer with n = 5 , while those with 9, 10 and 12 methylene groups formed a smectic phase.' The isotropization temperatures of these polymers showed a regular trend, with an odd-even effect, for the decrease in with the length of the flexible units up to n = 9 in R, but then T, increased for the 9, 10 and 12 polymers, as seen in the data in table 1.Both the T, and transition temperatures for the polymers with an even value of n were generally higher than those with n odd, as has been observed in several other series of liquid-crystal but since the effect of increasing the spacer length was greater on T, than on T, the odd members had a wider temperature range for liquid crystallinity, A T. From these results we concluded that in this series of polymers the longer flexible spacers rendered higher degrees of freedom to the mesogenic units, which permitted their alignment to form smectic layers. However, another possible explanation for the abrupt change from nematic to smectic order along the series may be that a conformational change of the polyalkylene spacer, from the fully extended trans conformation to the one with a central gauche unit, could occur in the odd-numbered or the longer spacers.' Such a conformational change would represent a higher energy state but would stabilize the mesophase order.The liquid-crystal behaviour of the polymers of the second series in table 1, that with the polyoxyethylene flexible units, also showed a very strong dependence on the length of the polyethyleneoxy spacer, n.6 In this series, however, the polymers23 R. W. LENZ Table 1. Effects of structure and length of flexible spacer unit on the liquid-crystal properties of main-chain polyesters polymer repeat unit Tm/ "C T,/"C" AT/"Cb n = 2 3 4 5 6 7 8 9 10 12 340 240 285 175 227 176 197 174 220 212 (N)365I (S)315I (N)3451 (S)267I (N)2901 (S)253I (S)2331 (S)267I (S)245I (S)2201 n = 1 2 3 4 8.7 13.2 342 (N)3651 185 (S)222N2881 180 (S)203 N257I 121 (S)211N245I 102 (N)2421 91 C 25 75 60 92 63 77 55 59 47 33 N, nematic; S, smectic; I, isotropic. AT = T, - Tm.No liquid crystal. with shorter spacer units formed smectic as well as nematic phases, while the polymer with the longest spacer in the series, in which n has an average value of ca. 9, formed only a nematic phase. When the length of the polyoxyethylene spacer was further increased to n = 13.2 (which was obtained from the next glycol monomer available at the time) the polymer did not form a liquid-crystal phase. These observations emphasize the fact that both the thermal stability and the nature of the mesophase strongly depend on a combination of both the structure and the length of the flexible spacer unit.Perhaps the ability of the relatively short diethyleneoxy spacer ( n = 2) to cause the formation of a smectic phase indicates that the presence of an oxygen atom in the spacer may have exerted a specific polar effect, which strengthened the lateral intermolecular attraction between adjacent polymer chains, thereby helping the formation of smectic layers of the mesogens. Unfortunately the changes in the ability to form either the smectic phases or even to undergo an enantiotropic transition with increasing spacer length occurred at intermediate values where polymers were not available, so it is difficult to draw specific conclusions on these effects.Polymers 11, n = 2-4, of table 1 all showed interesting behaviour when mounted on a glass slide and observed on the hot stage of a polarizing microscope. In each case, on cooling from the nematic to the smectic mesophase these samples appeared24 DESIGN OF THERMOTROPIC POLYESTERS to assume spontaneously a homeotropic orientation to the slide or cover plate, so that they were aligned parallel to the light beam, and as a result the field of view became almost completely dark. The polymer chains could be forced out of parallel alignment with the light beam by a shearing action generated by moving the cover plate parallel to the slide, and on doing so the field brightened. On removal of the shear force, however, the sample relaxed back to the perpendicular alignment and the dark field was restored. It is not known as yet if this effect resulted from some specific interaction of the polymer with a residue on the glass surface.PEN DANT-GROU P FLEX I BLE UNITS Long-chain polymethylene groups can be placed in the polymer either as main-chain units (spacers) or as lateral pendant substituents on the mesogenic group; surprisingly we have observed that similar results are obtained in terms of the effect of both unit length and odd-even structures on the thermal properties and type of liquid-crystal phase formed. In recent investigations in our laboratory two different series of polymers of this type were prepared and characterized for these effects, as follows: series I: series 11: The first is based on the rigid-rod polymer poly( hydroquinone terephthalate), which has a melting point well above 600°C.Substantial decreases in T, values were only achieved when fairly long pendant alkyl substituents were used, so polymers containing n-alkyl groups ranging from hexyl to dodecyl ( n = 5-1 1 in series I) were prepared for this purpose, with the results shown in table L7 On melting, the polymers formed a type of liquid-crystal phase which we have not fully characterized, but which may be smectic. This phase is converted into a nematic phase at a higher temperature, q, as indicated in table 2. Because the polymer samples in table 2 varied quite widely in molecular weight, as indicated by their solution viscosities, it was not possible to make exact com- parisons of the effects of substituent length on their liquid-crystalline properties ; however, as the data in table 2 show, there was only a surprisingly small variation in both T, and with alkyl-group length.It was unexpected that some of the polymers in series I, particularly those with the decyl substituent, could form a smectic phase on melting because lateral substituents usually prevent such a phase from forming. Additional characterization studies are in progress to verify this possibility.R. W. LENZ 25 Table 2. Physical properties of the poly( 2-n-alkyl- 1,4-phenylene terephthalates), series I ~~ ~ ~ n-alkyl vinh a substituent /cm'g-' T,/"C T,/"C AT/"C hexyl hexyl hexyl hexyl heptyl octyl nonyl decyl decyl decyl decyl decyl undecyl dodecyl 0.52 0.59 1.88 0.48 0.47 0.32 0.30 0.35 1.30 1.38 2.10 0.37 0.25 1.32 277 299 295 300 257 257 23 7 217 254 302 297 228 217 - 323 330 340 345 302 3 07 29 1 237 322 323 3 19 292 277 - 46 31 45 45 45 50 54 20 20 26 64 60 - - a Solution viscosity in p-chlorophenol at a concentration of Endothermic transition observed by d.s.c.to 0.2 g cm-? and at 45 "C. form a nematic phase. For the series I1 polymers containing a flexible decamethylene spacer and an aromatic triad ester mesogenic group there was a very great effect of the alkyl group and its size on both T, and T for the methyl, ethyl and propyl groups, but little change in these properties for larger groups. On replacing a hydrogen atom in the central hydroquinone unit with a methyl group in the series I1 polymer, very large decreases occurred in both T, (from 231 to 154 "C) and T (from 267 to 190 "C).Similar decreases were found with the ethyl group ( T, = 7 1 "C and = 127 "C), but still larger groups caused only minor changes in these properties. It therefore appears that the additional increase in the length of the alkyl group beyond four carbon atoms did not produce any additional steric effect that could interfere with the molecular packing in the solid state, and indeed a more or less constant melting point was observed for the polymers with the longer alkyl groups, possibly attributable to crystallization of the side-chain alkyl groups themselves rather than the polymer main chains. The clearing temperatures of the polymers in this series decreased steadily with increasing length of the substituent, although the contribution of each additional methylene unit to the depression of this transition temperature became much smaller for the butyl, pentyl and hexyl substituents.These reduced effects may be the result of the gradually decreasing contribution of each additional methylene unit to the molecular diameter, as defined by Gray,' of the mesogenic units. Finally, when the alkyl group was lengthened to more than six carbon atoms no thermotropic behaviour was observed, possibly because either (1) the clearing temperatures of these polymers may have been depressed so much that they were lower than the melting point (monotropic behaviour) or (2) the polymers were incapable of forming a liquid-crystalline mesophase. All of the polymers in this series were nematic, but unlike those in table 1 with backbone spacers, the polymers with an even number of carbon atoms had a wider temperature range for thermotropic be havi ou r .26 DESIGN OF THERMOTROPIC POLYESTERS Of considerable importance in considerations of structure-property relationships in polymers of the types in tables 1 and 2 is the question of the critical balance between the size of the mesogenic group and the size of the non-mesogenic flexible spacer at which the formation of a thermotropic phase no longer is possible.It appears that the critical point for such polymers occurs at ca. 50 wt% of each, and if the flexible spacers (either in the main chain or as pendant groups or both) constitute a much higher fraction of the repeating unit then the polymer becomes incapable of forming a liquid-crystal phase.PENDANT POLAR GROUPS Substituents other than alkyl groups were also studied for the series I1 polymers.* Highly polar substituents (e.g. -CN or -NOz) were very effective in depressing both the melting and clearing temperatures of these polymers. This depression may again be considered to be partly the result of the steric effects, which limited the molecular packing efficiency in both the crystal and the liquid-crystal states, but an opposing effect of the polarity of the group itself is apparently important also, and the latter is believed to be the cause of the higher clearing temperatures of the bromo-, cyano- and nitro-substituted polymers. These substituents are all larger in size than the methyl group, but the clearing temperatures of their polymers were found to be higher than those of the methyl-substituted polymer.The methoxy- substituted homopolymer in this series was found to be monotropic. EFFECTS OF MESOGENIC UNITS ON THERMOTROPIC PROPERTIES The variations which occur in the structure-property relationships of ther- motropic, main-chain polyesters as a function of the structure of the mesogenic units have been studied extensively for the effect of three different types of modifica- tions of their repeating unit structure, including (1) changes in the structural units, (2) changes in the length of mesogen and its axial ratio and (3) the effect of lateral substituents which are arranged in either a random head-to-tail or a regular head-to- head orientation along the chain.It has been found in this and other laboratories that even a slight change in the molecular structure of the mesogenic groups can result in a significant change in the thermal properties of the mesophase. STRUCTURAL-UNIT EFFECTS The types of profound changes which can result from relatively small changes in the mesogenic group structure are demonstrated in a comparison of the T, values of the first three polymers in table 3.9-" These polymers contain three different but closely related aromatic ester triad mesogens connected by a common flexible spacer, the decamethylene group. The major difference between the three polymers is in the specific structure of the central aromatic ester of the mesogenic units. Polymer A has a central hydroquinone residue while polymers B and C have central tereph- thaloyl residues.As seen in table 3, Ti of polymers A and B are significantly higher than that of polymer C, indicating a greater thermal stability of the liquid-crystal phase of the former. This observation cannot be explained on the basis of an expected coplanar and colinear geometry of the mesogenic units of polymer A, in contrast to the non-linear conformation resulting from the presence of a central terephthaloyl groupTable 3. Thermal behaviour of liquid-crystal polyesters with different mesogenic units designation polymer repeat unit transition temperature/"C mesophase Tln Ti AT tY Pe A F -0 0 C-0 0 0-C 0 O--(CH2h7 <)' 0 '0- 52 nematic 253 305 56 nematic 265 321 ? 4 220 267 47 smectic (head-to-head dyad orientation) 170 190 20 nematic U U (random dyad orientation) 144 133 - nematic (on cooling) r m z N28 DESIGN OF THERMOTROPIC POLYESTERS in the other two, as has previously been suggested for low-molecular-weight liquid- crystal compounds.'* A coplanar molecular geometry should favour more effective molecular packing and alignment between the polymer molecules in the liquid-crystal phase, which in turn should stabilize the me~ophase.'~ A slightly higher thermal stability of the mesophase of polymer C would be expected over that of polymer B because the mesogenic unit in polymer C is further extended through the terminal carbonyl groups, which are in resonance interaction with the neighbouring phenyl rings.However, even though this effect was not observed, the presence of the two terminal carbonyl groups enables the formation of a smectic phase in polymer C, while the other two polymers, A and B, form nematic phases, and it is difficult to rationalize this dramatic difference in liquid- crystal behaviour.Increasing the length of the linear rigid mesogenic unit enhances the thermal stability of the mesophase and leads to an increase in Ti, as would be expected from axial-ratio considerations; this is shown by a comparison of the properties of polymers A and D in table 3.14 The replacement of the middle p-phenylene unit in polymer A by a biphenylene unit is accompanied by an increase in Ti of ca. 70 "C and a much greater thermal stability the mesophase of 116 "C. Similarly, on shorten- ing the mesogenic unit the thermal stability of the liquid-crystal phase decreases, as seen in a comparison of the data for polymers A and E in table 3;' i.e.for triad units compared with dyad units, respectively. Furthermore, if a longer spacer is inserted into a dyad polyester of the structure of polymer E in table 3, in which the dyad units are also arranged in a random head-to-tail orientation, the stability of the nematic phase is decreased to the point that T, is below T,, and the liquid-crystalline phase can only be observed on cooling of the isotropic melt (i.e. the polymer is monotropic). ' While it is expected that variations in axial ratio of the mesogenic group will have the effects on T observed here, there is a great need to relate these effects on a quantitative basis, and certainly no rationale exists as yet to account for the different types of liquid-crystal organizations formed by polymers A and B versus C.In addition to these structural factors, the presence of lateral substituents in the mesogenic units also plays a very important role in controlling the thermal behaviour of the mesophase of the main-chain polymers, as was described earlier for the alkylhydroquinone terephthalate polymers. In our investigation of these triad ester polymers we have also systematically varied the lateral substituent on the middle p-phenylene ring of the mesogenic unit of polymer A of table 3, and the properties of these polymers are given in table 4.9"' The data in table 4 can be summarized as follows: (1) monosubstitution decreases T and the thermal stability of the mesophase, (2) the degree of reduction in Ti by a substituent is directly related to its size, (3) the presence of two substituents of the same type, as in polymer A5 in table 4, lowers Ti approximately twice as much as one, indicating the possible existence of additivity in the substituent effect (although, as T, is increased, so the effect on AT is even greater) and (4) polymers based on a monosubstituted hydro- quinone unit (polymers A1 to A4 in table 4) exhibit higher values of AS, than those with an unsubstituted unit (polymer A) or a symmetrically disubstituted unit (poly- mer A5). All of these observations can be rationalized on the basis of two effects: either (1) steric hindrance by the substituents causes an increased separation of the mesogenic units in adjacent polymer chains or (2) an interlocking by the substituent on adjacent chains decreases molecular mobility in the liquid-crystal phase." The former decreases Ti while the latter increases ASi, as can be seen in table 4.PolarR. W. LENZ 29 Table 4. Effect of lateral substituents on the liquid-crystal properties of main-chain polyesters AHi A Si designation polymer repeat unit T,/"C T;/"C AT/"C /kcal mol-' (e.u.) Y X Y A H H 253 305 52 0.97 1.9 A1 H c1 157 279 122 2.5 4.6 A2 H CH3 162 274 112 7.6 2.9 5.2 A3 H Br 146 270 124 2.8 A4 H C6H5 151 168 117 1.6 3.6 A5 c1 c1 200 255 55 0.94 1.8 effects do not seem to exert an important role on the thermal stability of the mesophases of these polymers, in contrast to those discussed previously. NON-MESOGENIC RIGID-SPACER EFFECTS The types of rigid non-mesogenic spacers which we have used to modify and control the thermal properties of thermotropic copolyesters are those based on bisphenols containing different central substituents between the two phenolic rings in which X is either C(CH3),, CH2, 0, S or SOz.Resorcinol was also included in this series. For each of these non-mesogenic unit monomers a series of copolyesters based on o-chloro-p-phenylene terephthalate, CHQ, and the respective bisphenol terephthalate units were prepared over a wide range of co-monomer compositions. For each series the maximum or threshold amount of each bisphenol comonomer which could be incorporated into the random copolymers without complete destruc- tion of the liquid-crystal nature of the resulting copolymers was determined, and the results are shown in table 5.11y'6 All of the liquid-crystal copolymers formed nematic phases. The results collected in table 5 clearly demonstrate that the greater the bulkiness of the central substituent, X, in the rigid bisphenol spacer unit, the lower the threshold co-monomer amount which could be accommodated in the copolymer without completely losing the liquid-crystal characteristics.The differences in non- linearity of the non-mesogenic units caused by the presence of the middle substituents of the bisphenols were all within about a 5" angle of each other, indicating that the degree of intramolecular bending caused by X was approximately the same. Certainly it is to be expected that the larger X groups, e.g.the C(CH3), and SO2 groups, will also cause an increased separation of the adjacent polymer chains to30 DESIGN OF THERMOTROPIC POLYESTERS Table 5. Maximum amount of each bis- phenol monomer which could be copoly- merized without complete destruction of the liquid-crystallinity of the CHQ copolyester X maximum amount (mol%) C(CH312 40 so2 50 CH2 60 S 60 0 70 resorcinol 60 destabilige the mesophase further, and both the stereogeometry or space-filling characteristics of the polymer units and the bulkiness of X must contribute to destabilizing the liquid-crystal phase of these copolyesters. Conversely, the elec- tronic or polar effects of the X groups on the liquid-crystal properties were not as evident as the steric effects and can be considered to be relatively minor in com- parison.The copolymers of CHQ and resorcinol were unique in that the resorcinol unit did not have a central substituent, X, as in the other bisphenols. This unit was able to maintain the rigidity of the polymer chain, but it induced a bending angle of 120" along the polymer backbone, destroying the linearity and reducing the parellel association of polymer chains in the nematic state, thereby decreasing the stability of the mesophase. Jackson and coworkers earlier arrived at the same conclusion from their study of the liquid-crystal properties of polyesters based on p-oxyben- zoate-modified poly( ethylene tere~hthalate).'~ Unfortunately, in our studies infor- mation on the clearing transitions of the liquid-crystal phases could not be obtained because all of the polymers underwent thermal degradation before reaching the T, transition. Of particular interest in this series of copolymers was the formation of a two-phase melt above T,; i.e.the presence of non-mesogenic rigid group: can sufficiently disrupt the formation of the nematic phase so that both a liquid-crystal phase and an isotropic phase are present in equilibrium, even though Ti is still too high to be observed. Of great importance to the complete characterization of these copolymers, as discussed in more detail below, will be the development of a method to characterize quantitatively the 'degree of liquid crystallinity' in such systems and to relate this parameter to their physical, rheological and mechanical properties.COPOLYMERS WITH FLEXIBLE SPACERS In addition to these types of rigid-rod polymem in which non-linear rigid units were inserted to reduce the aspect ratio of the mesogenic unit sequences, we have also prepared a series of random copolymers containing both linear mesogenic groups and non-linear non-mesogenic groups in units with a common decamethyleneR. W. LENZ 31 spacer, of the following structure: x = 1.0, 0.8, 0.75, 0.6, 0.5. As eFpected, these copolyesters also showed regular decreases in all of their transition temperatures with decreasing P-unit content, including T,, T, and their deisotropization, Td, and recrystallization, T', temperatures, as seen from the data in table 6. The copolymer containing 60 mol% of the mesogenic P unit was observed to form a barely visible nematic phase on melting on the hot stage of a polarizing microscope, but no clear Ti endotherm was found in its d.s.c.thermogram. Even for the copolymer containing 75 mol% P units the nematic phase which formed on melting contained many dark regions, again indicative of the presence of a two-phase melt which contains both liquid-crystalline and isotropic regions in equilibrium. The relative amounts of bright and dark regions and the birefringence intensities observed for samples on the hot stage of a polarizing microscope may be taken as a qualitative indication of the degree of liquid crystallinity (i.e. of the fractional amount of liquid-crystalline phase) of the thermotropic melt, as we have suggested in an earlier report.I6 The thermotropic melts of the copolymers containing 75 mol% P units or higher and the P homopolymer all showed strong opalescence on stirring their thermotropic melts, while that with 60mol% was only weakly opalescent and the 50mol% copolymer showed no opalescence on stirring.I6 Hence this property may also indicate, in a semi-quantitative manner, that either the amount or the stability of the liquid-crystalline phase is directly related to the ratio of mesogenic units to non-mesogenic units in the same manner as are the crystalline properties in a copolymer.As suggested earlier, previous experience in this laboratory with a variety of aromatic polyesters suggests that a polymer must contain no less than 50-60 wt% mesogenic units to be able to form a stable thermotropic m e ~ o p h a s e .' ~ ~ ' ~ A more quantitative indication of the degree of liquid crystallinity can be obtained from the areas of the endothermic peaks and the T d exothermic peaks (see table 6) in the d.s.c. thermograms of these copolymers, i.e. relative and very approximate estimates of either the amount or the stability of the nematic phase can be determined from these peak areas, and visual inspection of their thermograms revealed that even at 80mol% P this copolymer had a much smaller Ti endotherm compared with that of the homopolymer. The rheological properties of these copolymers are also very sensitive to the relative amounts of liquid-crystalline and isotropic phases present in their thermotropic melts, and these relationships are now under investiga- tion in our laboratory.Also of considerable interest for these copolymers is that they could be made to undergo an irreversible rearrangement from a random to a block sequence32 DESIGN OF THERMOTROPIC POLYESTERS transition temperatures/"C ~ composition heating cycle cooling cycle (unit mole fractions) qinh a P M /cm3 g-' Trn T, Tc Td 1 .o 0 0.79 265 321 229 309 0.80 0.20 0.67 238 274 208 270 0.75 0.25 0.90 233 265 217 267 0.60 0.40 1 .o 230 242 215 - 0.50 0.50 0.67 212 - 170 - 0 1.0 0.2 1 108 - - - a Inherent viscosity of solutions in p-chlorophenol at a concentration of 0.2 g cm-3 at 45 f 0.3 "C. distribution on extended thermal treatment at temperatures either just below T, or above T, and below Ti.18 We have referred to this type of reorganization in earlier studies as a 'crystallization-induced reaction' of copolymers. I thank the Office of Naval Research, the National Science Foundation, the Petroleum Research Fund and the Materials Research Laboratory of the University of Massachusetts, funded by the National Science Foundation, for support of these research programmes on liquid-crystal polymers. ' C. K. Ober, J-I. Jin and R. W. Lenz, Makromol. Chem., Rapid Cornmun., 1983,4, 49. R. W. Lenz and J-I. Jin, in Liquid Crystals and Ordered Fluids, ed. A. Griffin and J. Johnsons (Plenum Press, New York, in press). A. C. Griffin and S. J. Havens, J. Polym. Sci., Polym. Phys. Ed., 1981, 19, 951. L. Strzelecki anc&D. van Luyen, Eur. Polym. J., 1980, 16, 299. G. W. Gray and A. Mosley, J. Chem. SOC., Perkin Trans. 2, 1976, 97. G. Galli, E. Chiellini, C. K. Ober and R. W. Lenz, Makromol. Chem., 1982, 183, 2693. Q. F. Zhou, R. W. Lenz and J-I. Jin, to be published. C. K. Ober, J-I. Jin and R. W. Lenz, Polym. J. (Jpn), 1982, 14, 9. ' J. Majnusz, J. M. Catala and R. W. Lenz, Eur. Polym. J., 1983, 19, 1043. l o J-I. Jin, S. Antoun, C. Ober and R. W. Lenz, Brit. Polym. J., 1980, 12, 132. l 1 S. Antoun, R. W. Lenz and J-I. Jin, J. Polym. Sci., Polym. Chem. Ed., 1981, 19, 1901. M. J. S. Dewar and R. M. Riddle, J. Am. Chem. SOC., 1975, 97, 6658. l 3 The Molecular Physics of Liquid Crystals ed. G. R. Luckhurst and G. W. Gray (Academic Press, New York, 1979), p. 15 and references cited therein. l4 B-W. Jo, J-I. Jin and R. W. Lenz, Makromol. Chem., Rapid Commun., 1982, 3, 23. C. Ober, R. W. Lenz, G. Galli and E. Chiellini, Macromolecules, 1983, 16, 1034. l 6 R. W. Lenz and J-I. Jin, Macromolecules, 1981, 14, 1405. W. J. Jackson Jr, Brit. Polym. J., 1980, 12, 154. G. Chen and R. W. Lenz, J. Polym. Sci., Polym. Chem. Ed., 1984, 22, 3189. l9 Q. Zhou and R. W. Lenz, J. Polym. Sci., Polym. Chem. Ed., 1983, 21, 3313. 15 17 18

ISSN:0301-7249    

DOI:10.1039/DC9857900021

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

Faraday Discuss. Chem. Soc., 1985, 79, 33-39 Structure-Property Correlations in some Nematic Main-chain Polyesters BY ALEXANDRE BLUMSTEIN," MICHELLE M. GAUTHIER, OOMAN THOMAS AND RITA B. BLUMSTEIN Department of Chemistry, Polymer Science Program, University of Lowell, Lowell, Massachusetts 01854, U.S.A. Received 3 rd December, 1984 The homologous series of thermotropic nematic main-chain polyesters 0 has been investigated. In the case of even values of n, a smectic phase appears at n = 18. An odd-even oscillatih of nematic order parameters associated with the mesogen is observed near the clearing tempkrature, T,. At the same time, in the vicinity of T,, the relative order of mesogen and spacer is approximately the same for odd and even values of n. In unoriented specimens of polymer with spacer length n = 10, a layered structure develops progressively with increasing chain length, the sharpness and intensity of the SAX diffraction peak increasing with molecular weight.However, the degree of order remains that of a cybotactic nematic, based on measurements of the complex viscosity and miscibility behaviour. No SAX diff rac- tion peak is observed with spacer length n = 7. Connectivity between building blocks is the distinctive feature of polymeric liquid crystals (p,l.c.), as opposed to their low-molecular-mass (1.m.l.c.) counterparts. In p.l.c., mesogens can be located either in the side chains or in the main chain: a priori considerations would le+d one to predict a preponderance of smectic phases in the former and of nematic phases in the latter case.However, even a cursory glance at the review literat~rel-~ clearly shows that details of molecular structure profoundly influence phase behaviour in both cases. In the present paper we wish to illustrate this point by focusing on the homologous series of main-chain nematic polyesters of general formula: 0 1 where n = 3- 18. length n on mesophase order. Results are presented on the influence of the molecular mass and flexible spacer EXPERIMENTAL The synthesis and characterization of samples with n d 14 have been r e p ~ r t e d . ~ - ~ Inter- facial polycondensation was used to prepare molecular weights > 5000-6000 and solution 3334 STRUCTURE-PROPERTY CORRELATIONS polymerization was used for smaller values of Mn. Synthesis of diacid chlorides with n > 14 will be described elsewhere.The mesogen nematic order parameter S was measured by proton magnetic resonance (p.m.r.) as described in ref. ( 6 ) and (7) (2S,/kHz = 24.08s; see fig. 1 later). Rheological measurements were made on a rheometric viscoelastic tester. Data were obtained in the parallel-plate torsion geometry and oscillatory shear mode over the frequency range 2-100 rad s-’. A constant strain of 2% was maintained in order to keep the sample in the linear viscoelastic range. X-ray data on quenched samples were obtained by a technique described in ref. (5) and (8). RESULTS AND DISCUSSION CONNECTIVITY OF MESOGENIC GROUPS AND ODD-EVEN OSCILLATIONS AT THE CLEARING POINT L.m.l.c., in which the mesogen parts are not interconnected and the flexible portims of the molecule are free at one end, usually display a certain number of common characteristics.(i) The odd-even effect at the liquid-crystal/isotropic (LC/I) transition [ Tc( n), ASc( n) etc. as a function of the number of ‘flexible bonds’ in terminal chains] dampens out rapidly and is usually very weak for n > 4, (ii) the nematic phases often coexist with smectic phases at n > 7 - 8 and are eventually replaced by the latter at high values of n and (iii) no change of mesophase is observed in the odd-even alternation: both odd and even compounds display similar mesophases over the same range of n. This situation changes when two mesogens, or potential mesogens, are connected together via a sequence of flexible bonds. The odd-even effect is reinforced and the transition parameters [such as Tc( n), ASc( n) and Sc( n)] oscillate in a sustained fashion.Often, alternation between two different levels of order takes place. An example of such behaviour for a Schiff base phenyl alkyl ester of cinnamic acid was given by Coates and Gray’ for n = 0-3: However, systematic investigation of this phenomenon occurred only recently with the work on ‘Siamese twin’ model and the odd-even effect in p01ymers.~”~ The explanation, similar to the one forwarded by Coates and Gray,g is based on the alignment of two consecutive mesogen units via an extended flexible spacer. The two mesogen axes fall alternately in and out of alignment as n changes from even to odd. Several theoretical treatments of this phenomenon have recently appeared.l 4 MESOGEN AND SPACER ALIGNMENT IN THE ODD-EVEN SERIES OF 4,4‘-DIOXY-2,2‘- DIMETHYLAZOXYBENZENE ALKANEDIOYLS For the homologous series based on 2,2’-dimethylazoxybenzene, odd-even oscillations of AHIN and A S I N were investigated in the interval of n = 3-15 for high-molecular-weight samples with small biphase intervals.478 The experimental points fit the following equations: ( 1 4 - I N -4.7+0.16n kJ mru-* AS? = 7.55 + 0.87n J K-’ mru-’A. BLUMSTEIN, M. M. GAUTHIER, 0. THOMAS AND R. B. BLUMSTEIN 35 ( 2 4 (2b) AHodd- IN -0.94+0.19n kJmru-' AS:;" = 1.41 + 0.57n J K-' mru-' where mru is a mole of the repeating unit. Synthesis of the homologous series has recently beeen extended to n = 20. The range of the nematic phase of both odd- and even-numbered samples narrows progressively after passing through a maximum at n =: 6-8.For the odd-numbered series it seems to collapse for n = 17, while for the even-numbered samples the mesomorphic range is maintained for n = 18. A smectic mesophase appears at n = 18 and will be discussed e1~ewhere.l~ If one considers the intercepts and slopes in eqn (1) and (2) as the respective contributions of mesogen order and spacer conformational changes at the N/I transition, it is clear that the mesogen order is much larger for n = even than n = odd. An increase in the trans-conformer population at the I / N transition is apparent in both the odd and the even series, from the slopes of AH,,( n ) and ASIN( n). However, such measurements provide insufficient insight into the drastic differences in overall mesophase order that may be associated with even small differences in alkyl-chain flexibility. The odd-even oscillation of the mesogen orientational order was also investigated by p.m.r.and is shown in table 1. As high-molecular-weight samples do not align in the spectrometer field, polymers with = 8- 10 were prepared. Phase transitions are listed in table 1. Table 1. Mesogen order parameter S as a function of spacer length phase transitions biphasic supercooling n heating cooling range/"C at T,,/"C S" 5 6 7 8 9 10 1 1 12 ~~ ~~~ g28N 1541 g23K164N224I g12N136I 22K110N1661 g17K114N1321 g9KlO 1 N 1381 g20K96N 1221 g19K105N 1331 I1 50N20g I208N 122K13g 1135N8g I1 58N15g I1 27N73K9g I1 32N6g I1 18N15g I1 20N54K8g 10.5 23 6" 23 12 18 1 1 18 4 16 1 12 5 6 4 15 0.35 0.49 0.72 0.35 0.7 1 0.44 0.75 b " At Tred = 0.98.DP-= 15 ; sample alignment in spectrometer field was not investigated. " Narrow fraction (M,/ M, = 1.06). With the exception of n = 7, samples were unfractionated and thus display a fairly broad nematic-isotropic (N + I) biphase, as previously reported for samples of low degree of polymerization.6 The biphasic range reported here was measured as the width of the d.s.c. I/N peak on cooling. This appears to be a more accurate measure of the N + I biphase range than microscopic observation. The supercool- ing at the I/N transition is reported as the supercooling of the d.s.c. peak maximum at scanning rates of 20 K min-'. Note the odd-even oscillation of the biphasic range and supercooling at TIN (with the exception of n = lo), for which we have no explanation at present.For n = 10 and 7 we have previously shown a 'plateau effect' with levelling off of order at 8- 10 repeating units per chain. With the exception of n = 6 (which was36 STRUCTURE-PROPERTY CORRELATIONS not investigated), the samples in table 1 are at, or slightly below, that value. The mesogen order parameters S shown in the last column of table 1 were measured at an arbitrary reduced temperature Tred = T / TIN = 0.98, where TIN is the d.s.c. peak minimum on cooling. This corresponds approximately to the end of the biphasic region, although some trailing of a minor isotropic component might still be present. Table 1 unequivocally shows the odd-even oscillation of mesogen order para- meters occurring in unison with the ASIN oscillation. Note that the order parameter increases rapidly with decreasing temperature in the N + I biphase.Thus, an arbitrary definition of T, probably results in the scatter of S values observed for n =odd. Siamese-twin model compounds formed by a sequence of mesogen-spacer- mesogen also display odd-even order oscillations. As pointed out previously,'2 even-numbered spacers in such systems might induce efficient alignment of guest molecules in mixed systems. On the basis of an X-ray diffraction investigation of quenched aligned samples we have proposed for the n = odd series a 'normal' nematic level of order' and for the n = even series a 'micellar cybotactic' nematic ~rganization,~ in which the polymer chains are extended and confined to layers skewed with respect to the nematic director by 45-41' (angle between director and normal to the plane of the layer). In l.m.l.c., such systems are precursors of smectic C mesophases.'' It is possible that in this homologous series lateral substitution at the 2 and 2' positions, which results in a distorted mesogenic part," leads to a 'frustration' of smectic packing.In the odd-numbered series, formation of cybotactic nematic domains can be induced by external elongational flow, provided n is high enough to allow micro- phase separation. Thus for n = 11 a fibre with a quenched cybotactic nematic structure was obtained _by extrusion from the nematic melt at Tred =: 0.95 (sample molecular weight was M, = 34 000). No odd-numbered polyester with n < I1 gave a quenched cybotactic SAX pattern on-extrusion and no odd-numbered polyester, including n = 11, gave a cybotactic SAX pattern when oriented by a magnetic field.As pointed out above, preliminary investigation of the higher homologues sug- gests that a smectic mesophase develops at n = 18. In contrast, for n = odd, the large orientational fluctuations of the mesogen axis seem to produce an early collapse of the nematic mesophase at n = 17. We expect that the smectic mesophase of the even-numbered homologues will collapse eventually because of dilution of the mesogen by the spacer. The X-ray scattering data obtained from samples oriented by a magnetic field of 10 T and quenched at Tred = 0.95 indicate that the methylene sequence of even- numbered spacers is in an extended state.l9 However, the uncertainty in the d values (of &0.5-0.8 A) is compatible with the existence of a non-negligible gauche com- ponent in the sequence. In the case of n = 10 (DDA-9), d.m.r. investigation of a labelled spacer2' suggests that the order is uniform along the eight internal methyl- enes in the spacer, which is extended, though not all trans, throughout the nematic range. The spacer reorientational mobility decreases by ca. 40% as the temperature decreases throughout the nematic phase, indicating considerable stiffening at lower temperatures (consistent with the above X-ray data). Conversely, it follows that the spacer is quite flexible near the N/I transition. The Siamese-twin model com- pound C2H2,-9]-DDA-9, consisting of the sequence mesogen-spacer-mesogen, shows qualitatively the same behaviour although the temperature dependence of spacer mobility is much less pronounced.21 In the case of p.m.r.studies, simulation carried out by Martins et aZ? allowed decomposition of the spectrum of DDA-9 ( n = 10) into the respective contributions of spacer protons (26, fig. 1) and mesogen ortho protons (2SN, fig. 1). Thus theA. BLUMSTEIN, M. M. GAUTHIER, 0. THOMAS AND R. B. BLUMSTEIN 37 20 kHz Fig. 1. P.m.r. lineshapes of a representative even-numbered (n = 10; solid line) and odd- numbered ( n = 9; dashed line) polymer. Both spectra recorded at Tred = 0.92, with linewidth at 2/5 of peak maximum shown as 2S2,, and dipolar splitting due to mesogen ortho protons shown as 2SN.The spectrum is decomposed as outlined in ref. (21). temperature dependence of the ratio 82/5/8N can be used to follow disordering of the spacer relative to the mesogen.2' For DDA-9 this ratio varies from ca. 1.9 to ca. 1.6 as the temperature increases to the N/ I transition, suggesting that the spacer disorders faster than the mesogen. P.m.r. spectra of the rest of the homologous series were considered by analogy with that of DDA-9. In the vicinity of the N/I transition the ratio 82/5/8N was found to be ca. 1.6 for all polymers. This suggests that in the vicinity of T, mesogen and spacer display the same relative degree of order in both the odd- and even-numbered series. The odd-numbered chains may fluctuate more strongly from an extended conformation than the even-numbered ones. Thus, theoretical predictions of spontaneous chain stiffening in the vicinity of the I/ N transition 22 may have to incorporate consideration of details of chemical structure.INFLUENCE OF MOLECULAR WEIGHT It has been pointed out previously that the range of the nematic-isotropic (N + I) biphase narrows considerably with increasing molecular mass M. Other thermody- namic parameters, such as TIN, AHIN, ASIN and the order parameter S,, are increasing functions of M,6 varying rapidly for low M and levelling off at ca. 10 repeating units. Selective partitioning of the longest and best aligned chains into the anisotropic phase upon cooling through the I/N transition was inferred from the undulations observed in the S( T) plot of unfractionated samples.23 We have recently confirmed this partitioning by quenching DDA-9 samples in the N+ I biphase and physically separating the nematic and isotropic component^.^^ Another aspect of molecular-weight dependence of order in the DDA-9 ( n = 10) system is the progressive appearance of layered structures upon increasin the chain length, even in unaligned samples.An X-ray diffraction maximum at 16.5 1 develops38 STRUCTURE- PROPE RTY CO RRELATI 0 NS 1 0' lo-' 1 oo 1 0' lo2 frequency/s-' Fig. 2. Complex viscosity Iq*l for (0) DDA-9 and ( X) AZA-9 polyesters of Gn = 8000 as a function of frequency. DDA-9 at 180 "C ( Tred = 1.05) (- - -) and 140 "C ( Tred = 0.96) (-) ; MA-9 at 180 "c ( Tred = 1.05) (- - -) and I30 "c ( Tred = 0.94) (-). with increasing degree of polymerization, from a broad halo for m< 5 to a well defined (broad) peak for m> 10.(This corresponds to the spacing recorded for the quenched aligned samples.) The transition is progressive, the sharpness and intensity of the peak increasing with m.19 No such peak was found for MA-9 ( n = 7) on scanning the molecular weights up to 13 000. However, the mesophase of a DDA-9 with M, = 20 000 (m =r 45) is totally miscible with the mesophase of MA-9 and also with classical 1.m.n. nematics such as PAA. The nematic nature of DDA-9 is further illustrated by its bulk viscosity behaviour. Fig. 2 shows the evolution of complex viscosity Iq*l _as function of frequency for a sample of DDA-9 ( n = 10) and AZA-9 ( n = 7) of M,, = 8000, in the nematic and isotropic phase.It is easy to see that lq*l is lower in the liquid-crystalline than in the isotropic phase for both samples. This indicates that both polyesters are nematic; similar results are obtained for high-molecular-weight samples of DDA-9 and MA-9 (A?,, = 20 000). The difference in shear sensitivity between the two samples in the nematic phase may be due to different molecular mass distributions. This difference seems to subside in the isotropic phase. The relatively high value of DDA-9 complex viscosity, compared with MA-9 of similar Mn, is compatible with cybotactic nematic ordering of the n = 10 polymer. The cybotactic domains probably increase in size and perfection with molecular weight as the sample viscosity increases and order disruption created by chain ends lessens.In summary, development of the broad SAX peak at 16.5 A does not signify the onset of a smectic mesophase: DDA-9 remains a thermotropic nematic p.1.c. of higher order, at least in the range of up to ca. 45 investigated to date. It would be interesting to see whether a continuous transition from 'cybotactic nematic' to 'smectic C' level of order can be detected upon further increase in DDA-9 molecular mass.A. BLUMSTEIN, M. M. GAUTHIER, 0. THOMAS AND R. B. BLUMSTEIN 39 This work was supported by NSF Grant DMR *8303989. The wide-line n.m.r. data were obtained at the Worcester Consortium NMR Facility. We thank Dr K. R. Wissbrun of the Celanese Research Co. for complex viscosity data on DDA-9. A. Blumstein, Macromolecules, 1977, 10, 872. H. Finkelmann, in Polymer Liquid Crystals, ed.A. Ciferri, W. R. Krigbaum and R. B. Meyer (Academic Press, New York, 1982). A. Blumstein, J. Asrar and R. B. Blumstein, in Liquid Crystals and Oriented Fluids, ed. A. C. Griffin and J. F. Johnson (Plenum Press, New York, 1984), vol. 4, p. 31 1. A. Blumstein and 0. Thomas, Macromolecules, 1982, 15, 1264. ’ A. Blumstein, S. Vilasagar, S. Ponrathnam, S. B. Clough, G. Maret and R. B. Blumstein, J. Polym. Sci., Polym. Phys. Ed., 1982, 20, 887. R. B. Blumstein, E. M. Stickles, M. M. Gauthier, A. Blumstein and F. Volino, Macromolecules, 1984, 17, 177. A. F. Martins, J. B. Ferreira, F. Volino, A. Blumstein and R. B. Blumstein, Macromolecules, 1983, 16, 279. A. Blumstein, 0. Thomas, J. Asrar, P. Makris, S. B. Clough and R. B. Blumstein, J. Polym. Sci., Polym. Lett. Ed., 1984, 22, 13. D. Coates and G. W. Gray, J. Phys. (Paris), 1975, 36, 365. l o A. C. Griffin and T. R. Britt, J. Am. Chem. SOC., 1981, 103, 4957. J. A. Bouglione, A. Roviello and A. Sirigu, Mol. Cryst. Liq. Cryst., 1984, 106, 169. l 2 R. B. Blumstein, M. D. Poliks, E. M. Stickles, A. Blumstein and F. Volino, Mol. Cryst. Liq. Cryst., in press. J. Asrar, H. Toriumi, J. Watanabe, W. R. Krigbaum, A. Ciferri and J. Preston, J. Polym. Sci., Polym. Phys. Ed., 1983, 21, 11 19. l4 See, among others: ( a ) D. Y. Yoon and S. Bruckner, ZBM Research Report 4330 (47278), 1984; (b) A. Abe, Macromolecules, 1984, 17, 2280. I’ R. S. Kumar, work in progress. R. B. Blumstein, 0. Thomas, M. M. Gauthier, J. Asrar and A. Blumstein, in Polymeric Liquid Crystals, ed. A. Blumstein (Plenum Press, New York, 1984). 13 16 l 7 A. de Vries, Mol. Cryst. Liq. Cryst., 1970, 10, 219. ’* J. Bergts and H. Pemn, Mol. Cryst. Liq. Cryst., 1984, 113, 1; 269. l9 A. Blumstein, 1st SPSJInt. Polym. Con$, Japan, 1984, in Polym. J. (Jpn), 1985, 17, 277. 2o E. T. Samulski, M. M. Gauthier, R. B. Blumstein and A. Blumstein, Macromolecules, 1984,17,479. 22 P. G. de Gennes, Mol. Cryst. Liq. Cryst., Lett., 1984, 102, 95. 23 F. Volino, J. M. Alloneau, A. M. Giroud-Godquin, R. B. Blumstein, E. M. Stickles and A. 24 Work in progress. F. Volino and R. B. Blumstein, Mol. Cryst. Liq. Cryst., in press. 21 Blumstein, MoZ. Cryst. Liq. Cryst., Lett., 1984, 102, 21.

ISSN:0301-7249    

DOI:10.1039/DC9857900033

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1985,79, 41-53 Configurational Characteristics and Nematic Order of Semiflexible Thermotropic Polymers BY Do Y. YOON," SERGIO BRUCKNER,~ WILLI VOLKSEN AND J. CAMPBELL SCOTT IBM Research Laboratory, San Jose, California 95193, U.S.A. AND ANSELM C. GRIFFIN Department of Chemistry, University of Southern Mississippi, Hattiesburg, Mississippi 39406, U.S.A. Received 2 1 st January, 1985 The stability and the molecular order of nematic states of thermotropic polymers compris- ing rigid groups connected by flexible spacers are found to be dominated primarily by the characteristics (configurational partition function) of highly extended conformers, which are favoured for packing. Moreover, both the macroscopic consideration of the enthalpies and the entropies of isotropic-nematic transitions and the microscopic probe by deuterium n.m.r.of labelled chains lead to the conclusion that the highly extended conformers are selected preferentially in polymeric nematogens. A number of unique results exhibited by polymer liquid crystals, i.e. strong effects of even-odd character of polymethylene spacers, abnor- malities associated with -C( =O)O- links [compared with -0- and -OC(=O)- links] between rigid (phenylene) groups and polymethylenes and drastic differences between poly- methylene and polyoxyethylene spacers, can all be attributed quantitatively to this conforma- tional ordering, which is the most prominent feature distinguishing polymer liquid crystals from their monomeric counterparts. The orientational order parameters of a nematic polymer measured from both D and H n.m.r.spectra of labelled chains are found to be quite high, ca. 0.8 throughout the nematic region. These findings on conformational order and orienta- tional order of polymer liquid crystals are compared with theoretical predictions based on ideal lattice chains and worm-like chains. The liquid-crystalline state of bulk polymers presents challenging scientific prob- lems as well as new technological applications. The molecular or structural factors controlling its stability over the isotropic state of random chains are directly related to the fundamental question of the packing of long-chain molecules to a high density in the condensed state. It is now well established that polymer chains with sufficient flexibility and negligible orientation-dependent (anisotropic) interactions assume unperturbed random-coil configurations in the undiluted amorphous state, unaffected by the intermolecular interactions.' In this regard, polymer liquid crystals are a clear reminder of the critical importance of intermolecular interactions for chains with incomplete flexibility and/ or appreciable anisotropic interactions. Liquid-crystalline polymers comprising rigid units connected by flexible spacer groups have been studied most prominently in recent years, mainly owing to the presence of well defined isotropic-liquid-crystalline transitions in these polymers.* The rigid groups which normally contain aromatic units introduce both limited flexibility and anisotropic interactions.Therefore these polymers may serve as model 7 IBM World Trade Postdoctoral Fellow.Permanent and present address: Dipartimento di Chimica, Politecnico di Milano, Milan, Italy. 4142 NEMATIC STATES OF THERMOTROPIC POLYMERS \o x o\ 00 v) c? CI - s I o=w 0 n > U WD. Y. YOON et al. 43 systems for trying to understand the packing of real polymer chains in the condensed state.394 For this class of thermotropic polymers it is now well established that the spacer groups play critical roles in determining the stability of the liquid-crystalline state over the isotropic state. This is demonstrated most clearly by the results listed in table 1, namely large oscillations of clearing temperatures, enthal y and entropy changes with the even-odd alternation of polymethylene spacers:-' the abnormal effect of the -C(=O)-0- group between the rigid (phenylene) units and poly- methylene spacers compared with the -0- or OC(=O)- linkages'?' and the rather drastic reduction in the enthalpy and entropy changes effected by polyoxy- ethylene spacers in comparison with polymethylenes." These examples show unambiguously that the spacers are not merely playing the role of solvents, but rather participate actively in the ordering process in the nematic state. Hence the nature of molecular order in nematic polymers and its dependence on chemical structures of polymers are the most pertinent questions.Recently we have investigated the molecular order in polymer liquid crystals by selecting conformations on the basis of chain-sequence extension to match the macroscopic thermodynamic properties of transition enthalpies and entropies as well as the microscopic deuterium n.m.r.spectra of labelled chains.12 The highlights of these studies are summarized in this paper and their detailed descrip- tions will be presented ~eparately.~"~-'* DISTRIBUTION OF CHAIN-SEQUENCE EXTENSION The total partition function of a bulk system of chain molecules may be factorized into three contributions by13 where Z,, denotes the steric repulsion, Zen is the orientation-dependent (anisotropic) intermolecular attractions and Zconf represents the configurational degrees of free- dom. Adopting the lattice treatment of Flory and Ronca, l4 the orientation-dependent part of the steric exclusion may be expressed by where X is the ratio between the contour length and the mean diameter of the chain, + is the angle between a chain segment (placed in a lattice cell) and the macroscopic orientation axis and the angle brackets denote averaging over all chain segments.The contribution of anisotropic attractions are likewise described by where T* denotes the magnitude of anisotropic interactions per chain segment of unit axial ratio and the angle brackets with subscript r refer to averaging over the rigid units only, which are the major source of anisotropic attractions. Hence the ordered state is driven by the favourable steric and anisotropic interactions, while sacrificing the configurational degrees of freedom, over the disordered state. Maximization of the steric partition function requires minimizing the value of (sin +), which is averaged over all the chain segments of rigid and flexible groups.This implies that highly anisotropic (extended) configurations will be favoured for the steric-packing consideration. The configurational anisotropy44 NEMATIC STATES OF THERMOTROPIC POLYMERS 0 5 10 15 20 25 dal A Fig. 1. (a) Distribution of chain-sequence extension calculated for polymer I at T = 500 K; the chain sequence refers to a rigid group and a spacer. (b) Orientational correlations of rigid units with the major extension axis and the average conformational energy plotted against the chain-sequence extension; the dashed line denotes the average energy of all conformers. 4.0 2 3.0 E '= 2.0. I h 0 : ' . O - - 0.0 200 - Ti* v 5 loo - 0 1 .o A 8 0.6 9 N mlN I - .0.2 5 10 15 20 25 d,/ 8, Fig. 2.Distributions of chain-sequence extension, orientational correlations of rigid units and the average energy as a function of sequence extension, calculated for polymer I1 at T = 500 K: see caption to fig. 1. here is defined with respect to the major (alignment) axis of chain sequences, which may be identified as the axis along which the total projection of the two rigid groups appended at the ends of a spacer group is maximized. The extension of chain sequence along this axis is therefore a proper measure of the anisotropy of spatial configurations, which is most critical to packing. On the other hand, the anisotropicD. Y. YOON et al. 45 Fig. 3. Distributions of chain-sequence extension, orientational correlations of rigid units and the average energy as a function of sequence extension, calculated for polymer 111 at T = 400 K.The interaction E, denoting the energy of gauche relative to trans state for the O-CH2&CH2-0 bond is taken to be -0.7 kcal mol-'; see caption to fig. 1. Fig. 4. Distributions of chain-sequence extension, orientational correlations of rigid units and the average energy as a function of sequence extension, calculated for polymer IV at T = 360 K; see caption to fig. I .46 NEMATIC STATES OF THERMOTROPIC POLYMERS 1.6 1 1 .o' CI 0.6 5 0 -IN 24 v 0.2 h i? G s 1.2 i;i' 0.8 - N I 0 5 10 15 20 25 da/A Fig. 5. Distributions of chain-sequence extension, orientational correlations of rigid units and the average energy as a function of sequence extension, calculated for polymer V at T = 400 K.The interaction Em denoting the energy of gauche relative to trans state for the O-CH2tCH2-CH, bond is taken to be -0.4 kcal mol-'; see caption to fig. 1. attractions can be maximized by aligning only the rigid groups; the detailed configur- ations of the intervening spacer groups are less critical in this regard. The distribution of chain-sequence extension, d,, calculated in this way employ- ing the rotational isomeric-state models'09" are presented in fig. 1-5 for the five different chain structures, denoted by I-V, as shown in table 1 ; each sequence comprises a rigid group and a spacer. They differ mainly in the even-odd character of the number of spacer bonds and the way the oxygen atoms are placed along the chain backbone; the differences in the rigid groups are inconsequential in these comparisons.Also shown in these figures are the average energies and the orienta- tional correlations of two successive rigid groups along the major extension axis for these conformers falling within a given range of d,. Polymer I exhibits a large fraction of highly extended conformers with the maximum allowed extension d, = 23 A. Orientational correlations of rigid groups exhibit a distinctively bimodal character with all the conformers with d, 3 18 A placing the rigid groups parallel to each other. Among these the conformers with da = 23 A are quite unique in that their (conformational) energy is much lower, by ca. l.Okcalmol-', than the average energy for all the conformers shown by the dotted line in fig.1. They are found to comprise a set of conformers that place every second bond starting from the 0-CH2 (attached to the phenylene oxygen) in the trans state. Owing to the nearly tetrahedral bond geometry involved, all the conformers of this type not only place the successive rigid groups aligned to each other, but also separate them at the same (maximum) extension along this alignment axis regardless of the conformations of the intervening bonds; see the schematic illustration in fig. 6. Polymer I1 with odd-numbered polymethylene spacers, on the other hand, exhibits a very small fraction of highly extended conformers. Furthermore, the relatively extended conformers place the two rigid groups at the ends of a spacer tilted by ca.30" from the major extension axis. Moreover, the relatively extended conformers making up significant enough fractions exhibit energies which are only slightly lower than the average energy. The contrast between the even- andD. Y . YOON et al. 47 'c-c, c-c, t' Fig. 6. Schematic drawings of the spacer conformers exhibiting the maximum extension along the major axis defined by the two rigid units at the ends of the spacer group. The bold lines denote the bonds that assume exclusively ?runs conformations, and the remaining bonds assume the normal conformations according to their statistical weights. The bonds denoted by t" refer to the trans states which are of higher energy than the alternative gauche states. the odd-numbered polymethylene spacers is therefore rather dramatic in these considerations.Polymer 111, which has even-numbered (8) skeletal atoms between linear rigid groups, also exhibits a peak of highly extended conformers and the bimodal distribu- tion of orientational correlations of rigid groups that resemble very closely to those of polymer I. However, a very important difference is noted in the energy of the most extended conformers, which is nearly identical to the average energy in contrast to the large decrease in polymer I. This difference can be traced to the unique feature of the O-CH2+CH2-0 rotation, which favours the gauche state over the trans by ca. 0.4-0.7 kcal m ~ l - ' . ' ~ ~ ' ~ As shown in fig. 6, the highly extended conformers place the O-CH2-&-CH2-0 rotation marked by t* in the trans state exclusively, thereby increasing the energy of these extended conformers.The characteristics of polymer IV are very much the same as those of polymer I. Therefore, the effect of the -OC(=O)- linkage between the phenylene unit and polymethylene spacers is nearly identical to that of the -0- linkage. On the other hand, the -C(=O)O- linkage in polymer V leads to drastically different characteristics, as the results of fig. 5 demonstrate. The population of highly extended conformers is much smaller than that of polymer IV; furthermore, the distribution is skewed toward small values of d,, which is even smaller than that of polymer I1 despite the even-odd difference in polymethylenes. The origin of this drastic change is illustrated in fig. 6; the highly extended conformers of polymer V place the two O-CH2+CH2-CH2 rotations in the trans state, which is of higher energy than the gauche by CQ.0-0.4 kcal mol-1.'6 This situation is completely avoided in polymer IV. This situation is also avoided upon replacing the even-numbered polymethylenes with odd-numbered ones, and hence the polymers with odd- numbered polymethylene spacers do not exhibit this drastic effect."48 NEMATIC STATES OF THERMOTROPIC POLYMERS Comparisons of the characteristics of chain-sequence distributions discussed above with the experimental results of isotropic-nematic transitions of these poly- mers in table 1 show unambiguously the critical importance of highly extended conformers in determining the stability of the liquid-crystalline state of polymers.Furthermore, the enthalpies of the isotropic-nematic transitions seem to reflect directly thn energy of the highly extended conformers, as the comparison of polymer I with polymer I11 demonstrates most strikingly. Therefore, it is obvious that highly extended conformers are selected preferentially in the nematic state. CONFORMATIONAL ORDER The detailed conformational order in the nematic state may then be determined by selecting the conformers on the basis of chain-sequence extension to match the available experimental results. For this purpose we have considered both the macroscopic thermodynamic properties of the enthalpies and the entropies of isotropic-nematic transitions and the microscopic spectroscopic results of deuterium n.m.r. of labelled polymer I.ENTHALPY AND ENTROPY OF THE ISOTROPIC-NEMATIC TRANSITION The enthalpy of the isotropic-nematic transition arises from both orientational order and conformational order, as is apparent from eqn ( 1 ) and ( 3 ) . The orienta- tional contribution can be estimated according to eqn (3) from the enthalpy of the corresponding monomer and the ratio of the orientational-order parameter of the polymer to that of the monomer, assuming that the conformational ordering is negligible in monomeric nematogens. For the corresponding monomer of polymer I, AHNI = 0.18 kcal mol-' and s == 0.37 at the transition.6 The order parameter, sN1=O.75, of polymer I at the transition12 (see below) then leads to an estimate of ca. 0.74 kcal per mole of repeat unit (mru) for the enthalpy due to orientational ordering; the repeat unit here refers to a rigid group and a spacer.Since this procedure attributes the enthalpy change ofthe monomer entirely to the orientational order and thus neglects any contribution from conformational changes, this estimate represents an upper bound. Comparison with the experimental results then requires the conformational contribution to the enthalpy to be at least 0.8 kcal (mru)-'. From the results of average energy as a function of the sequence extension d, shown in fig. 1 it is obvious that this conformational enthalpy is obtainable, provided that only those conformers with the sequence extension d, = 23 A are selected in the nematic state: see table 2. The entropy change has contributions from the conformational order and the steric (packing) interactions.The conformational contribution can be estimated from Ah AS,=-k lnfN+- TN I (4) where fN is the fractional configurational partition function of the conformers selected in the nematic state, k is Boltzmann's constant, Ah denotes the decrease of the energy of these conformers relative to the average and TNI is the temperature of the isotropic-nematic transition. The steric packing contribution to the entropy is obtained from eqn (2) by averaging the value of sin t,b over the rigid and spacer groups in proportion to the contour length. For this purpose the spacer group isD. Y. YOON et al. 49 Table 2. Calculated and experimental values of enthalpy and entropy changes at the isotropic-nematic transition of polymer I AH/kcal (mru)-' AS/cal (mru)-' K-' ~~ steric interactions - anisotropic interactions 0.74 conformational ordering 0.98 total calculated" 1.72 experimental 1.56 ~~ -3.3 6.3 3 .O 3.2 - All the calculations are carried out at 500 K, taking the axial ratio of the repeat unit to be ca.5.3. Ref. (6). Table 3. Calculated and experimental values of enthalpy and entropy changes at the isotropic-nematic transition of polymer I11 AH/kcal (mru)-' AS/cal (mru)-' K-' steric interactions - anistropic interactions 0.74 total calculated" 0.74 experimental 0.46 conformational ordering 0.0 -3.35 4.76 1.41 1.13 - " All the calculations are carried out at 400 K, taking the axial ratio of the repeat unit to be ca. 4.6. Ref. (10). represented by lattice cells, the length of each approximating the average chain diameter." The total entropy change calculated in this manner for polymer I by selecting only those conformers with the sequence extension d, = 23 A matches very closely the experimental result, as shown by the comparison in table 2.For polymer I11 the orientation-order parameter in the nematic state has not been determined, but the order parameter of the corresponding dimer" is nearly identical to the value, ~ ~ ~ ~ 0 . 4 9 , reported for the dimer of polymer I.6 Hence the orientational contribution to the enthalpy may be taken to be identical to that of polymer I. The transition enthalpy and entropy thus calculated by selecting only those conformers with the maximum sequence extension d, B 20 I$ also match closely the experimental results, as shown in table 3.For this polymer the conforma- tional ordering contributes little to the enthalpy change, whereas this conformational contribution is dominant for polymer I. The large difference in enthalpy changes between polymer I and polymer I11 can therefore be attributed almost entirely to the energies of their extended conformers illustrated in fig. 6. The fact that the entropy change for polymer I1 is much smaller than that for polymer I indicates that the conformational restriction is not so severe for the odd-numbered polymethylene spacers. Good agreement with experimental results for enthalpy and entropy changes is obtained by selecting all the conformers with their sequence extensions d , ~ 18 A. Since all these conformers place the rigid groups tilted by ca.30" from the major extension axis, with their orientational correlations of ca.0.6, the order parameter of the rigid units is expected to decrease accordingly. The estimate of the orientational contribution to the transition enthalpy has thus been reduced from that of polymer I according to eqn ( 3 ) .50 NEMATIC STATES OF THERMOTROPIC POLYMERS I " l 1 " " l l -60 -30 0 30 60 f l k H z Fig. 7. Traces of the deuterium n.m.r. spectra of the labelled polymer I, with C,0D20 spacers, at various temperatures in the nematic region reached by heating the sample through the crystalline-nematic transition. DEUTERIUM N.M.R. SPECTRA OF LABELLED POLYMER I The chain conformations in the nematic state have also been investigated with the microscopic probe of deuterium n.m.r.spectra of polymer I in which the protons of the spacer groups are replaced by deuterium.12 The deuterium n.m.r. quadrupole splitting for each C-D bond in the aligned nematic state can be approximated taking the fast motion limit:I8 where e2qQ/h ( = 174 kHz) is the quadrupolar coupling coiistant, s denotes the (orientational) order parameter of chain segments with respect to the director of the nematic domain, 4 represents the angle between the C-D bond and the alignment axis of chain segments and the angle brackets denote averaging over all the allowed conformations. The deuterium n.m.r. spectra of the labelled polymer I, (C10D200C6H4- COOC6H40C10D200C6H400CC6H40)x, in the nematic temperature region are shown in fig. 7. The central peak, whose intensity increases with temperature, reflects the presence of the isotropic phase coexisting with the nematic phase, owing to the relatively low average molecular weight and the polydispersity of the sample.Aside from this central peak, the five distinguishable CD, groups of the polymer exhibit only one quadrupole splitting throughout the nematic range, thereby indicating severe restrictions on the chain conformations for the ordinarily flexible C10D20 group. This deuterium n.m.r. result may also be analysed by selecting the conformers on the basis of chain-sequence extension. All the conformers with d, b 18 8, place the two successive rigid groups parallel to each other, so that the alignment axis of chain segments virtually coincides with the phenylene-0 bond in fig.6.The deuterium n.m.r. spectra thus calculated according to eqn (5) for the three groups of conformers, which differ in the minimum allowed sequence extension, are plotted in fig. 8. Only those conformers with d, = 23 8, exhibit one quadrupole splitting, in agreement with experimental results. Adding the conformers with d, = 21 8, leads to three quadrupole splittings, and further addition of less extended conformersD. Y. YOON et al. 51 - f l k H z Fig. 8. The deuterium n.m.r. spectra calculated for the labelled polymer I by including only those conformers whose sequence extensions d, fall in the range indicated: ( a ) 23, ( b ) 221 18 A. The orientational-order parameter is taken to be 1 in these calculations. -60 -30 0 30 60 and (c) with d, 2 18 A leads to larger separation of two external splittings.Therefore, the chain conformations consistent with the deuterium n.m.r. spectra of labelled polymer I is in good agreement with the conclusion drawn above on the basis of the enthalpy and entropy changes at the isotropic-nematic transition. This excellent agreement may also be taken as evidence confirming our procedure of estimating the enthalpy and entropy changes. ORIENTATIONAL ORDER The orientational-order parameter of polymer I can be determined from the magnitude of the deuterium n.m.r. quadrupole splittings according to eqn ( 5 ) , once the general profile is matched by the specific conformations as described above. The order parameters thus obtained at different temperatures while the sample is heated (filled circles) or cooled (open circles) are plotted in fig.9. Since the alignment axis of these conformers are taken to coincide with the phenylene-0 bond, the proton n.m.r. dipolar splittings of the same labelled polymer, which are dominated by the interactions of the vicinal protons of the phenylene group, can also be used to determine the order parameters fromlg 3 y2h ~ I T rL-H 28, =-- S where y is the gyromagnetic ratio of the proton, h is Planck’s constant and TH-H denotes the distance between two vicinal protons of the phenylene group. Taking TH-H to be 2.45 A leads to a value of 24.5 kHz for the case of perfect alignment. The experimental results of the proton n.m.r. dipolar splittings in the nematic state of labelled polymer I, obtained while the sample is heated through the crystalline- nematic transition, are shown in fig.10, and the order parameters determined from these spectra are plotted in fig. 9 as filled triangles. Excellent agreement between the results deduced from deuterium quadrupole splittiilgs and proton dipolar splittings further confirms our interpretation of deuterium n.m.r. spectra in terms of the specific nematic conformations described above.52 NEMATIC STATES OF THERMOTROPIC POLYMERS 0.9 0.8 S 0.7 0.6 170 180 190 200 210 220 T / "C Fig. 9. Orientational-order parameter of the labelled polymer I in the nematic state plotted against temperature. 0, Results determined from the deuterium n.m.r. quadrupole splittings in the nematic state reached by heating through the crystalline-nematic transition; 0, deuterium n.m.r.results in the nematic state cooled from the isotropic state; A, results from the proton n.m.r. dipolar splittings for the sample heated through the crystalline-nematic transition. The solid line is drawn through the experimental points to extrapolate to the clearing temperature. L 202 197 187 172 I I I 1 I I 1 f / k H z Fig. 10. Traces of the proton n.m.r. spectra from the labelled polymer I with C,0D20 spacers obtained at the nematic temperatures indicated when the sample is heated through the crystalline-nematic transition. -30 -20 -10 0 10 20 30 The orientational-order parameter of the nematic state of polymer I is ca. 0.8 throughout the nematic range and extrapolates to ca. 0.75 at the isotropic-nematic transition. This is in close agreement with the results of Martins et aL2' on polymer IV, determined from the proton n.m.r.dipolar splittings of unlabelled polymer. These results from n.m.r. measurements represent the order parameters of the nematic phase aligned in the applied field. In this regard the n.m.r. methods are preferable to other methods such as diamagnetic anisotropy, i.r. dichroism etc. that average over the entire sample, since these methods may overlook the possibility of coexisting isotropic phase and/or the nematic domains which have not been aligned properly, leading to lower estimates of nematic-order parameters.D. Y. YOON et al. 53 Table 4. Calculated and experimental values of enthalpy and entropy changes at the isotropic-nematic transition of polymer I1 AH/kcal (mru)-' AS/cal (mru)-' K-' steric interactions - anisotropic interactions 0.27 conformational ordering 0.40 total calculated" 0.67 experimental 0.92 -Oh3 - 2.15 1.52 2.0 " All the calculations are carried out at 500 K, taking the axial ratio of the repeat unit to be CQ.5.0. Ref. (7). DISCUSSION The conformational order in nematic polymers is found to be severely restricted to the highly extended configurations, indicating the dominant effect of steric packing interactions. This finding is also consistent, qualitatively at least, with the result of Monte Carlo simulations on cubic-lattice chains which show the ordered state to exhibit nearly perfect order both in conformation and orientation. In this regard, the presence of significant fractions of gauche bonds in the nematiqstate of polymer I may be attributed to the different geometrical features of polymethylene chains and thus points to the shortcomings of the cubic-lattice model in representing the diverse configurations of real chains.The orientational-order parameter, sNI == 0.75, at the isotropic-nematic transition of polymer I is rather high, falling in the upper limit of the predictions of worm-like chains.13 However, it is still substantially lower than the perfect alignment predicted for the lattice chains. This deviation may be related to the fact that real chain conformations exhibit departures from the strictly lattice-like characteristics owing to the appreciable range, ca. 20°, in the allowed torsional' angles around each rotational isomeric state of polymethylene chains.I6 Therefore both conformational order and orientational order of polymer liquid crystals are likely to depend on subtle details of chain configurations of real polymers. P. J. Flory, Faraday Discuss. Chem. SOC., 1979, 68, 14. P. J. Flory, Proc. R. SOC. London, Ser. A, 1984,234,73; Proc. Natl Acad. Sci. USA, 1982,79,4510. D. Y. Yoon and A. Baumgartner, Macromolecules, 1984, 17, 2864. A. C. Griffin and S. J. Havens, J. Polym. Sci., Polym. Phys. Ed., 1981, 19, 951. G. Sigaund, D. Y. Yoon and A. C. Griffin, Macromolecules, 1983, 16, 875. A. Blumstein and 0. Thomas, Macromolecules, 1982, 15, 1264. A. Blumstein, S. Vilasager, S. Ponrathnam, S. B. Clough and G. Maret, J. Polym. Sci., Polym. Phys. Ed., 1982, 20, 877. W. Volksen, D. Y. Yoon, S. Bruckner and J. C. Scott, work in preparation. S. Bruckner, J. C. Scott, D. Y. Yoon and A. C. Griffin, Macromolecules, in press. * See, for example, C. K. Ober, J-I. Jin and R. W. Lenz, Adv. Polym. Sci., 1984, 59, 103. ' A. C. Griffin and D. Y. Yoon, work in preparation. 10 l 1 D. Y. Yoon and S. Bruckner, Macromolecules, 1958, 18, 651. l 3 G. Ronca and D. Y. Yoon, J. Chem. Phys., 1982,76, 3295. l4 P. J. Flory and G. Ronca, Mol. Crysf. Liq. Cryst., 1979, 54, 289. I s P. J. Flory and G. Ronca, Mol. Cryst. Liq. Cyst., 1979, 54, 311. l 6 P. J. Flory, Statistical Mechanics of Chain Molecules (Interscience, New York, 1969), chap. V. '' E. Riande, J. Guzman and M. A. Llorente, Macromolecules, 1982, 52, 298. A. D. Buckingham and K. A. McLauchlan, Prog. Nucl. Magn. Reson. Spectrosc., 1967, 2, 63. l9 F. Volino, A. F. Martins and A. J. Dianoux, Mol. Cryst. Liq. Cryst., 1981, 66, 37. *' A. F. Martins, J. B. Ferreira, F. Volino, A. Blumstein and R. B. Blumstein, Macromolecules 1983, 12 16, 279.

ISSN:0301-7249    

DOI:10.1039/DC9857900041

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1985, 79, 55-72 Molecular Correlation in Thermotropic Copolyesters BY ALAN H. WINDLE," CHRISTOPHER VINEY, RUTH GOLOMBOK, ATHENE M. DONALD? AND GEOFFREY R. MITCHELL$ Department of Metallurgy and Materials Science, University of Cambridge, Pembroke Street, Cambridge CB2 342 Received 14th December, 1984 Uniaxially oriented samples of thermotropic random copolyesters of hydroxybenzoic and hydroxynaphthoic acids, and of hydroxybenzoic acid and ethylene terephthalate, have been characterised in detail using wide-angle X-ray diffraction. The technique was used to measure the global chain orientation, and in the case of the first polymer the meridional scattering has been analysed in terms of diffraction from an isolated straight random chain. Optical microstructures were observed between crossed polars, with circularly polarized light and in plane-polarised light with no analyser present.Interpretation of the contrast seen shows that the material is optically biaxial and leads to the conclusion that, in the liquid-crystalline phase, there is long-range correlation of the rotations of the molecules about their chain axes. The polymers are examples of 'biaxial nematics'. Thermal analysis indicates that the solid phase has a higher level of conformational order than the melt and that the melting range is very broad. Annealing below the melting point leads to the development of localised regions of enhanced order, especially in specimens with a high degree of overall orientation. The combination of electron diffraction and electron microscopy provides evidence that the lateral growth of ordered entities does not necessarily require regular sequences within the otherwise random copolymer molecules.Small crystals based on identical but non-periodic sequences within the molecules are (NPL crystals) proposed as being consistent with the experimental evidence and the implications of a brief statistical analysis. Building polymer chains by connecting small mesogenic molecules leads to considerable increases in both the crystalline melting point (T,) and the liquid- crystalline to isotropic transition temperature. In fact, for any rigid homopolymer molecule, T, tends to be in the temperature range where chemical decomposition is beginning to be a problem, while the upper transition is often not seen at all.The development of random copolymers of such rigid units has enabled substantial reductions in T, to be achieved, with the stabilization of the liquid-crystalline phase in a temperature range where conventional processing is pra~ticable.'-~ The lack of periodicity along the rigid but random chains also greatly reduces the ability of the material to crystallise in any conventional sense, although there is evidence for the presence of some type of ordered entities which melt out over a wide temperature range up to T,. Investigation of a number of rigid-chain random copolyesters has raised the basic structural issues of the local organization in the liquid-crystalline phase itself and the nature of the additional order which appears on solidification. In general, the optical microstructures seen in thermotropic polymers are smaller in scale than those characteristic of small-molecule materials and are sometimes at the limit of resolution of light microscopy.They do, however, show many of the same and in conjunction with a hot stage may give useful indications of t Present address: Cavendish Laboratory, Madingley Road, Cambridge. $ Present address: J. J. Thomson Laboratory, Reading University, Whiteknights, Reading RG6 2AF. 5556 MOLECULAR CORRELATION IN THERMOTROPIC COPOLYESTERS the appropriate Friedelian classification. The ready orientation of polymeric mesophases as a consequence of flow fields,6 while opening up a range of potential practical applications, also enables X-ray diffraction measurements to be carried out on well aligned samples.The information from diffraction patterns is not especially easy to interpret, although some progress has been made by analysis of the scattering from isolated random copolymer chains7** and through the calculation of cylindrical distribution functions.' Electron diffraction and microscopy of necessarily thin samples has proved very valuable, not only in understanding the origin of banded textures seen in the light microscope,'o*l' but also in observing the changes in lacal molecular order and orientation as a consequence of heat treatments. This paper gathers together the results of a number of different techniques used to assess the nature of the local order in three copolyesters based on rigid aromatic units.The experimental variety means that some features of the methods are not described fully, although in most cases reference is made to additional detail elsewhere. MATERIALS Three main-chain thermotropic copolyesters feature in this work. The first is a random copolymer of hydroxybenzoic and hydroxynaphthoic acids', in number fractions of 0.7 and 0.3, respectively; it is referrred to as B-N [structure (I)]. The molecule is comparatively rigid, although some conformational changes are possible, especially those involving 'crankshaft'-type rotations about bonds nearly parallel to the chain axis. The melting temperature of the 0.7/0.3 composition is 280 "C. There is no indication of any upper transition to an isotropic melt before the polymer decomposes.The scarcity of suitable solvents makes molecular weight determination difficult; however, current estimates of the degree of polymerization are between 150 and 250. The polymer was kindly supplied by Celanese Corporation and ICT (P and P). The second system examined is a random copolyester built from hydroxybenzoic acid and ethylene tere~hthalate'~ in number fractions of 0.6 and 0.4, respectively; it is referred to as B-ET [structure (II)]. The polymer was manufactured by Tennessee Eastman and was one of the first to be made widely available. Its main melting endotherm is at 190 "C, although as can be seen from fig. 1 there are two additional peaks at 250 and 340 "C before the significant peak associated with the mesophase to isotropic transition at 400°C. The implications of this endotherm have been discussed before in relation to the observed optical microstructures; however, it is significant to note here that the polymer does not flow freely until above the small 250 "C transition.The presence of the relatively flexible (CH,), units can be seen as responsible for the lower melting temperature (compared with B-N) and the fact that the isotropic phase is observed. Again the degree of polymerization is considered to be between 150 and 250. (11)WINDLE, VINEY, GOLOMBOK, DONALD AND MITCHELL 57 190 120 Fig. 1. Thermal analysis plot of an unoriented sample of B-ET. Heating rate 20 "C min-'. Peak temperatures in "C. A third random copolyester which features briefly in this work is designated- ClQT-QG [structure (III)].It melts at 300 "C and has a lower molecular weight than either B-N or B-ET, corresponding to a degree of polymerization of cu. 100. CI X-RAY CHARACTERIZATION OF THE COPOLYMERS The polymers B-N and B-ET were obtained in the form of extruded pellets. Preliminary X-ray diffraction patterns showed that there was marked alignment of the molecular-chain axes with the extrusion axis, that the orientation distribution was symmetrical about this axis and that the degree of molecular orientation was uniform across the section normal to the axis, there being no suggestion of a skin-core-type texture. Fig. 2 ( u ) shows the two-dimensional scattering patterns for the two poly- mers. The extrusion axis is vertical. The patterns were recorded in transmission using symmetrical geometry and are plotted as the s- weighted interference function si(s, a) [where s = 47r sin 8/A, a is the angle to the unique axis of the sample and i( s, a ) = I,,,,( s, a) - C f2( s), where I,,,, is the fully corrected intensity function and f2( s) is the independent coherent scattering]. Meridional and equatorial sections are shown in fig.2(6). By using a method based on spherical-harmonic analysis it has proved possible to utilize the azimuthal spread of the most intense meridional peak to measure the orientation distribution function for the two specimens. In effect, the arcing of the diffraction peak is recorded, expressed as harmonic coefficients and corrected for the spread of the peak due to the comparatively short-range correlations typical of these structures.The method is already documented, l4 so here the orientation of58 MOLECULAR CORRELATION IN THERMOTROPIC COPOLYESTERS 0 1 2 3 4 5 6 s/A-' 0 1 2 3 4 5 6 S / k ' 0 1 2 3 4 5 6 s/A-' 0 1 2 3 4 5 6 s/A-' Fig. 2. Wide-angle X-ray scattering data from samples of B-N (top) and B-ET (bottom). The samples are oriented and have uniaxial symmetry about the vertical axis. The diffracted intensity is plotted as interference functions and represented by contours in ( a ) and equatorial (dashed line) and meridional (full line) sections in (b). Table 1. Amplitude coefficients for B-Nand B-ET ( P n ) B-N B-ET Po 1 1 P6 0.006 0.12 PS 0 0 PI0 0 0 p2 0.53 0.58 p4 0.17 0.37 the two specimens will be simply recorded as amplitude coefficients, (P,,), of the spherical-harmonic series.t These are shown in table 1.It must be emphasised that the measured orientations are global averages for the local chain axes with respect to the external reference (extrusion) axis. The local orientation of the chain axes t Th? even-order terms of the spherical-harmonic series are, for x = cos a: Po = 1, P2 = $(3x2 - l), p4=$(35x4-30x2 + 3), P6 = &j(231x6-315x4+ 1 0 5 ~ ~ - 5 ) , Pg = etc.WINDLE, VINEY, GOLOMBOK, DONALD AND MITCHELL 1 0 1 2 3 4 5 6 s/A-’ 59 Fig. 3. Comparison of the meridional section of the two-dimensional weighted interference function measured from a 70/30 unannealed fibre of B-N ( b ) with the calculated meridional scattering intensity for linear lattices having nearest-neighbour spacings of 6.3 and 8.3 A, sequenced at random for ( a ) and in blocks of 10 similar units for (c).60 MOLECULAR CORRELATION IN THERMOTROPIC COPOLYESTERS with respect to their neighbours is likely to be very much more regular.Comparison of the orientation measured from the azimuthal distribution of the equatorial peak with that obtained from the intense meridional maximum at s = 3 8,-‘ indicates that the orientation within the correlation distance associated with interchain diffraction, the local orientation, is described by a (PJ of very close to unity for B-N,” although there is some suggestion that the local orientation is not quite as high in B-ET. Accurate knowledge of the global molecular orientation is important for the consider- ation of optical properties which follows. As received, the copolymers were nominally random.N.m.r. measurements on B-ET as polymerized underline the randomness of the chain.16 In the case of B-N the essential randomness of the molecule has been confirmed by analysis of the meridional X-ray scattering from the axially oriented sample [fig. ? ( a ) ] . Following B ~ n a r t , ’ ~ it is possible to apply the equation derived by Hosemann and Bagchi18 to calculate the scattering from a random copolymer chain with two components. The equation is where F( s) is the transform of the nearest-neighbour distribution function. Fig. 3 ( a ) shows the scattering intensity along the meridian calculated for a chain of points of nearest-neighbour distances 6.3 and 8.3 8, sequenced at random and in the relative proportions 0.7/0.3.The distances are those encountered in the polymer B-N (hydroxybenzoic 6.3 A, hydroxynaphthoic 8.3 A). The simplicity of the linear point model means that the relative magnitudes of the calculated peaks will be approximate. However, this first-order approach predicts the number and positions of the peaks, as can be seen by comparison with the experimental scan of fig. 3 ( b ) obtained from a fibre of 0.7/0.3 B-N. Further development of the model* to account for finite persistence lengths and the scattering represented by the transform of the chemical unit improves the agreement with experiment. Fig. 3 ( c ) is the scattering calculated for the same model except that the different units are segregated into unit blocks of 10. The scattering is profoundly different with many more peaks present.The observed meridional scattering can thus be considered as the fingerprint of a random copolymer. There is no evidence for blockiness in the B-N sample. In addition to confirming the polymer as being essentially random, the develop- ment of an understanding of the origins of the meridional scattering also provides a basis for the treatment of annealing effects considered below. OPTICAL MICROSTRUCTURE Initially sections were microtomed from the extruded samples of B-N and B-ET parallel to the extrusion axis, as in fig. 4. Great care was taken in order to avoid introducing cutting artefacts,” and any specimens showing knife or judder marks were discarded. Plate 1 (top) shows the microstructures observed between crossed polars for B-N and B-ET.As noted previously,20 the striking feature is that the optical textures do not reflect any of the preferred molecular orientation so evident in the diffraction pattern (fig. 2). The diffraction patterns in plate 1 (bottom) are obtained from the actual optical specimens, using a microbeam X-ray camera. They demonstrate that the preferred molecular orientation has not been changed or destroyed by the sectioning. As the crossed polars are rotated in relation to the sample, the contrast within individual domains? changes through the expected t The even-order terms of the spherical-harmonic series are, for x = cos a: Po = 1, P2 = 1/2(3x2 - l), P4=i(35x4-30x2+3), P6=&(231x6-315x4+ 104x2-5), P, = etc.WINDLE, VINEY, GOLOMBOK, DONALD AND MITCHELL 61 draw axis , Fig.4. Illustration of the relationship of the section plane to the extrusion (symmetry) axis of the sample. light-dark-light sequence every 90" and yet there is no change in the transmitted intensity integrated over the field of view. Plate 2 shows the microstructure for different polar rotations for the B-N sample and also the absence of contrast with no polars present. The B-ET sections appeared the same. The samples of each polymer were 3-5 pm thick, which is of the same order as the domain size. In order to check the influence of sample thickness, a series was prepared ranging from 2 to 15 pm. As the thickness increased, the general appearance of the domain structure did not change, although above I 0 pm the contrast became very weak. The micro- structure appears to be a reliable indication of the variation in local optical properties for the experimental conditions used, and thus can give a fairly direct measure of the orientation of the principal axes of various optical tensors as a function of position within the specimen plane.The fact that there is no preferred orientation of the axes describing the local optical anisotropy within the plane of the sample, despite the obvious preferred orientation of the molecular-chain axes, suggests that the polymer may be optically biaxial. If this is the case, the projection of the mean chain axis within a domain onto the specimen plane need no longer be parallel to either of the principal axes of the section of the optical indicatrix, which are the observed extinction directions.21 In order to confirm the biaxiality of the indicatrix, a series of sections was cut at different angles 4 to the extrusion axis ( 4 being measured from the normal to the section).Each of these sections was examined between (crossed) circular polarizers. Dark contrast seen in this mode indicates that the microscope axis is parallel (or nearly parallel) to one of the optic axes of the polymer (an optic axis is defined as being an axis perpendicular to a circular section of the indicatrix, along which the birefringence is zero; there are two in the biaxial case). Micrographs for different values of the section angle 4 are shown in plate 3 ( a ) for the polymer B-N. The proportion of dark areas, as estimated by reducing each image to 10 grey levels and defining the blackest of these as 'dark', is plotted as a function of 4 in plate 3( b ) .Consider a domain such as is apparent as a dark region in the micrograph and assume that one of the three principal axes (different from optic axes) of the indicatrix is parallel to the chain director for that domain. The probability of a particular domain being 'dark' is related to both the' distribution function describing the preferred orientation of the molecular chains about the specimen symmetry axis and the possible rotations of the optic axes about the chain director within a domain, all of which are equally probable. Then, as the global chain orientation (as measured62 MOLECULAR CORRELATION IN THERMOTROPIC COPOLYESTERS by X-rays) is defined by an even-order spherical-harmonic series with coefficients (PL), the distribution of optic axes about the external reference axis is given by a new series of harmonic coefficients given by the product (PL)(P:), where ( P : ) is the magnitude of the harmonic function at the angle p between the chosen principal axis of the indicatrix and the two optic axes (2p is known as the optic axial angle).The probability, P, that an optic axis will be parallel to the normal to a given section will be given by the magnitude of the distribution function at that particular angle, 4. Hence P'C (2n+l)(P',)(P;)(Pn cos 4). n This function is normalized such that it would equal unity for all values of the section angle, 4, for the case where any orientation of the indicatrix would be equally likely.It is plotted against 4 for a number of possible values of the semi-optic axial angle, p, in plate 3 ( c ) for B-N, using the appropriate global chain orientation function as measured by X-rays and expressed as harmonic coefficients (table 1). /3 = 0 corresponds to the case of a uniaxial indicatrix with its unique axis parallel to the chain director of the region under consideration. Similar results are obtained for B-ET. Comparison of the form of the plots determined from the known orienta- tion distribution of chain axes with that representing the proportion of 'dark' areas seen in circularly polarized light [ (6) with ( c ) in plate 3 1, leads to two conclusions: ( a ) the two polymers are emphatically biaxial and ( 6 ) in both cases the optic axial angle is greater than 60".Knowledge of the optic axial angle is not sufficient to determine the shape of the indicatrix and thus the relative magnitudes of the principal refractive indices, but the fact that it is much closer to 90" than to 0" indicates that one of the two principal refractive indices normal to the chain director will be either considerably larger or smaller than both of the other principal indices, one of which is in the chain direction. Another indication of optical biaxiality is provided by the examination of thin samples of polymer ClQT-QG prepared directly from the melt. The microstructure of such samples shows domains clearly delineated by walls. Between crossed polars (without wave plates) the normal extinction contrast is evident, as in plate 4 (top).However, if the analyser is removed some contrast remains which is due to dichroism [plate 4 (bottom)], the transmitted light following a dark-light-dark cycle for every 180" rotation of the polarizer.22 While the dichroism may be useful in circumventing the 0"-90" degeneracy associated with extinction contrast, there is, in this case, another important feature. Fig. 5 shows a plot of the transmitted intensity, estimated from grey levels, as a function of polar orientation for the two systems (one with the analyser and one without). It is clearly apparent that the polar orientations for maximum or minimum transmitted intensity are not the same, the discrepancy being ca. 25". If both the indicatrix and the absorption tensor were uniaxial, then the contrast variations would be 'in phase'.The observation that they are not, coupled with the fact that the greater absorption for light polarized parallel to the chain axis is associated with conjugation along the chain which is unlikely to be significant in any other direction, is circumstantial evidence for a biaxial indicatrix. OPTICAL PROPERTIES OF THE MESOPHASE MELT The arguments in favour of optical biaxiality have been organized on the basis of optical and X-ray observations made at room temperature. It is also important to determine whether the polymers examined retain this property in the mesophaseWINDLE, VINEY, GOLOMBOK, DONALD AND MITCHELL n Y .M c 0 30 60 90 rotation/' 63 -90 5 -60 -30 0 30 60 90 rotation/' Fig. 5. Measurements of transmitted intensity made at the point x on plate 4, for different rotation angles (4 is the clockwise angle between the vertical and the polarizer direction): ( a ) crossed polars and ( b ) polarizer only.melt. However, a difficulty in repeating the experiments already described above the melting point is that the global orientation of the molecular chains tends to be lost and an unoriented diffraction pattern is seen. There are, however, several observations at elevated temperatures which appear significant. The first is that the microstructure seen in sections of the oriented pellets does not change in scale or form when the specimen is taken through the melting point, save for the onset of mobility, the domains of light and dark contrast constantly changing position. It does appear that in the B-N sample the loss of diffraction orientation takes ca.1 min at 3 10 "C whereas the microstructural mobility occurs with a characteristic time of ca. 1 s, so if there was a loss of biaxiality associated with mobility, then transient evidence of the preferred orientation in the optical microstructure would be expected. There was no evidence of any such effect, the nature and scale of the microstructure not changing on melting. In the case of the polymer B-ET, the temperature range between the main melting endotherm at 190 "C64 MOLECULAR CORRELATION IN THERMOTROPIC COPOLYESTERS and the small transition at 250 "C is especially interesting. For the microstructural mobility begins at 190 "C whereas the global molecular orientation does not begin to decay significantly with time until the temperature is in excess of 250°C.The microstructure within this temperature band remains characteristic of optical biaxiality in that it does not reflect the prefixred molecular orientation, and yet it is mobile. It is possible that the optical mobility is due to the variations of the orientation of the indicatrix by rotation about the local chain axes. The small 250 "C transition increases in strength with increasing hydroxybenzoic acid content of the copolymer and is possibly associated with very small crystallites of the homopolymer of that component only. There is also another aspect of the behaviour of B-ET which has been previously documented.' The microstructure of a sample held between glass slides in the melt undergoes a pronounced change at 350 "C, ca.50 "C below the transition to the isotropic phase. It is possible that the new texture, which is characteristic of nematic small-molecule liquid crystals, represents the loss of biaxiality, although the extent to which ordering is dictated by the glass-slide surfaces is not clear. A further approach to the question of retained biaxiality in the melt can be made by monitoring the microstructural changes in a series of specimens cut at different angles to the pellet axis, as a function of temperature, when observed in circularly polarized light. When specimens of B-N are heated above 280 "C, the microstructure has mobility on a timescale of ca. 1 s, as described above. There is also a more gradual decrease in the proportion of dark regions in specimens cut at large angles, 4, to the pellet axis, and an increase in the proportion of such areas in specimens cut at small angles to the pellet axis, to a common value on a timescale of a few minutes; this can again be related to the loss of preferred molecular orientation in the molten phase.In B-ET changes in the proportions of light and dark regions only become apparent above 250 "C. STRUCTURAL IMPLICATIONS OF BIAXIAL OPTICAL PROPERTIES A single copolyester molecule does not have axial symmetry; in fact if it were possible for the aromatic groups to be coplanar with the esters then it would be clearly 'lath' like. Conformational-energy calculation^^^ suggest that steric hindrance between the doubly bonded oxygen of the ester group and a hydrogen atom of the opposing aromatic ring will twist the aromatic ring 30" away from the plane of the ester.However, it is possible that sequences of ester groups along one molecule remain within the same plane. The fact that the polymers are optically biaxial, a bulk rather than a molecular property, implies that the molecules must be rotationally correlated about their chain axes over distances that are substantial compared with their diameters. Hence the non-axial symmetry of the individual molecules must be transmitted to the material over distances of a micrometre or more. A possible structural arrangement is represented in fig. 6 . The structural classification of a liquid crystal with this additional level of long-range orientational order is commonly referred to as biaxial nematic in deference to its optical characteristic.Biaxial nematics have been pro- posed on theoretical grounds a number of time^,^^-^^ although they are not normally seen in small molecule liquid crystals, as the uniaxial-biaxial transition temperature (on cooling) is likely to be below the crystallization temperature. In polymers,Plate 1. Micrographs of 3-5 pm sections of the two polymers [ ( a ) B-N and (6) B-ET) viewed between crossed polars. The X-ray microbeam diffraction patterns were obtained from the same thin sections and show the same preferred orientation apparent in the dihction patterns of the unseaioned sample (fig. 2).Plate 2. Microstructure of the section of the B-N sample for different rotations of the crossed polars.There is no indication of the preferred orientation shown by the diffraction patterns and for the experimental conditions used there is no contrast visible with the polars absent.Plate 3. ( a ) Micrographs of sections cut at different angles, 4, to the symmetry axis of the B-N sample prepared using circularly polarized light. For the section angle +=O, the microscope axis is parallel to the extrusion axis of the sample. (6) Relative proportions of dark areas estimated by comparison with grey levels. (c) Calculations of the relative propor- tions of dark areas as a function of section angle for different values of the semi optic axial angle, B : (1) 0, (2) 30, (3) 60 and (4) 90".Plate 4. Micrographs of the polymer ClQT-QG showing contrast associated with birefrin- gence seen with crossed polars (top), and dichroism apparent when the polariser alone is present (bottom).The angles are measured clockwise from the vertical, to the polariser direction. The point x defines the position at which the data plotted in fig. 5 are measured.Plate 5. Transmission electron ditfraction patterns of very thin samples of B-N: (a) oriented by shearing at 300 "C onto rocksalt and (6) after shearing and annealing at 200 "C for 20 min.Plate 6. Dark-field electron micrograph of a very thin sample after annealing for 20 min at 200 "C imaged in the equatorial reflection. The small entities of bright contrast are extended normal to the shear axis, which is vertical. They are not apparent in unannealed samples.WINDLE, VINEY, GOLOMBOK, DONALD AND MITCHELL 65 Fig.6. Schematic diagram illustrating the rotational correlation about the chain axes in a nematic which is indicated by the biaxial optical properties. Although the molecules are drawn as laths, it is not implied that the additional correlation is a result of their anisotropic cross-sections rather than asymmetry in the attractive bonds between the molecules. however, where the melting temperature has been drastically reduced by copoly- mefization, it appears that the biaxial nematic phase can be readily accessed. THE MELTING TRANSITION The melting endotherm of these random copolymers, as observed by differential scanning calorimetry, is difficult to reproduce accurately from run to run. Its size and shape are especially sensitive to the thermal history of the sample, and the determination of a reliable base line for the estimation of the heat of melting is a particular problem.Fig. 7 shows a series of thermal traces for the polymer B-N. Runs on as-received samples produce traces such as that of fig. 7 ( a ) . The main melting peak at 290°C appears to be superimposed on a much broader endotherm ranging from ca. 150 "C to above the sharp peak. Reruns on samples previously quenched from 340"C, a temperature below that at which any decomposition is evident but clearly in the melt, give traces which are essentially the same as those for the as-received material except that in the region above the main peak the exothermic trend is much less marked [fig. 7(b)]. It is possible that this difference is associated with the contribution during the first heating run from reactions which are completing the polymerization process.Faster cooling rates from the liquid- crystalline melt (up to lOOO"Cmin-') did not have any significant influence on subsequent thermal traces. It is important to be convinced that the perceived endotherm does indeed represent a melting process and is not instead a glass-transition temperature associ- ated with an inhomogeneous material, as has been suggested for B-ET.28 Several aspects of the data appear to confirm that the transition is associated with some form of crystalline melting. First, if the actual endothermic peak is to be interpreted as the 'overshoot' peak sometimes associated with the glass transition, then it should substantially increase in size for faster heating rates.Secondly, if a sample is given a solid-state anneal, quenched and then thermally analysed, the anneal temperature is marked by a small exotherm followed by an endotherm. This behaviour is apparent in fig. 7 ( c ) . It is characteristic of a modification of the distribution in crystallite66 MOLECULAR CORRELATION IN THERMOTROPIC COPOLYESTERS T/ "C 100 2 00 3 00 L 00 I I I I 350 Fig. 7. Thermal-analysis traces for the polymer B-N with a heating rate of 20 "C min-': ( a ) As received, ( b ) after quenching from 340 "C, (c) after annealing at 220 "C and ( d ) after shearing onto a substrate at 3 10 "C, quenching and removing mechanically prior to analysis. Peak temperatures in "C.size and perfection associated with an improvement in order of those elements not quite molten at the anneal temperature. Annealing was seen to affect the thermal traces in this way down to 180 "C and demonstrates the large temperature range of the melting process. Fig. 7 ( d ) is a thermal trace of a B-N sample oriented by shear onto a glass substrate. The endotherm, obtained after removal of the sample from the substrate, is enhanced by the orientation, and the apparent melting temperature increased by 60°C. The enhanced peak does not recur when the sample is quenched from the melt and reanalysed, although the melt temperature (360 "C) is now approaching the range where degradation may be a factor. The explanation of such behaviour may parallel that accepted for similar effects seen in conventional crystalline poly- mers, namely that the higher melting temperature is due to the increased order in the oriented melt reducing the magnitude of the entropy increase on melting.The relaxation on final melting of particularly extended conformations, locked into the non-crystalline component of the polymer by the crystallites, will leave its imprint as a substantial endothermic contribution. Such an explanation, however, does not rule out the possibility that crystallites formed from an oriented melt may be larger or better ordered and thus have an intrinsically higher melting point. Annealing at 300 "C (10 "C above the melting point of the unoriented material) increased the melting peak by a further 30°C.WINDLE, VINEY, GOLOMBOK, DONALD AND MITCHELL 67 x Y .e m E - c .- 0 40 2 e / o 20 60 40 60 Fig.8. X-Ray diffractometer scans of an unoriented sample of B-N at: ( a ) 'O 2e/o 0 3 and ( - ) 300 "C. ORDER IN THE SOLID PHASE Whereas thermal analysis suggests some form of crystallinity in the solid phase, there is little evidence for sharp crystalline-type diff raqtion maxima in the wide-angle patterns, at least for the polymers examined. Given that the molecules are random copolymers this is not surprising: crystals of sufficient size to give sharp diffraction maxima would appear to be ruled Fig. 8 shows preliminary diffractometer traces of unoriented B-N both above and below the final melting temperature. On melting there is an increase in breadth of both the main interchain peak (at 20°, 28) and the most prominent meridional maximum at 42", 28.The half-width of the main peak can be used to give a lower limit to the lateral extent of the ordered68 MOLECULAR CORRELATION IN THERMOTROPIC COPOLYESTERS regions in the solid phase. Such an estimate is 30 A, although as there is likely to be a contribution from type I1 (paracrystalline) disorder the actual dimension will almost certainly be larger. The evidence that optical biaxiality and thus rotational correlation about the chain axes persists into the melt raises the question as to the nature of the solid-state order which melts out at T,. If, in accord with normal polymeric behaviour, the melting endotherm is primarily associated with an increase in conformational di~order,~' then the improved molecular packing below the melting point may be seen as a corollary of the matching of conformations between neighbouring chains.Additional clues as to the nature of the ordered entities are provided by the structural changes introduced by annealing in the solid state. ANNEALING BELOW THE MELTING POINT In the current work the modification of diffraction patterns as a consequence of solid-state annealing has only been seen in the case of well oriented samples, although this may be because changes are more apparent in oriented diffraction patterns. There are two recognizable modifications to the diffraction patterns on annealing: the lateral spread of the meridional maxima is markedly reduced and the high-angle shoulder of the main equatorial component becomes more clearly divided into two components displaced above and below the equator by ca.0.5s. It is significant that for the polymers B-N and B-ET there is no change in the positions of the meridional components, indicating that the material remains a random copolymer. Some of the strongest annealing effects have been seen by electron diffraction in the transmission microscope. Plate 5 shows diffraction pat- terns of B-N before and after annealing at 200 "C. The first meridional maximum, that at s = 0.95 A-', has been changed from a diffuse lateral line into a compact peak on the meridian. On the other hand the meridional maximum at s = 3 A-' is comparatively unaffected. The implication of the lateral confinement of the first meridional peak on annealing is that there is considerable enhancement in the extent of lateral order in the solid state.The fact that the effect is confined to the first peak suggests that the development of longitudinal register between the neighbouring molecules is sufficiently precise to bring ester groups or aromatic rings into register, but not precise enough to maintain registry between corresponding atoms in the chains over any appreciable lateral distance. There is no evidence in B-N that the increase in order is associated with the packing of pre-existing homopolymeric sequences in the otherwise random chains or with regular sequences produced by transesterification during the anneal (i e. the development of blockiness). The increase in the intensity of the off -equatorial components of the high-angle shoulder of the main interchain peak at s = 1.55 A-' is not properly understood.However, it is significant that the vertical displacement (+ and -) of the components is not sufficient to place them on a 'layer line' corresponding to the first meridional maximum. Note also that a high-angle shoulder is a common feature of inter- molecular peaks in main-chain aromatic polymers3' and also occurs in benzene,32 where it is associated with face-to-face contacts of the ring groups, and that the components are at an azimuthal angle *75", similar to that between the normals to the planes of the aromatic groups and the axis of a polyester molecule in extended conformation. Equatorial dark-field images of B-N specimens annealed at 200°C have been obtained using the transmission electron microscope.These images show a fine distribution of regions with bright-diffraction contrast superimposed on the lower-WINDLE, VINEY, GOLOMBOK, DONALD AND MITCHELL 69 level contrast of the banded structure. Such regions can be seen in plate 6. They are thin in the direction of the molecular axes, d 200 A, and for this particular sample extend up to approximately ten times this distance in the lateral direction, although such pronounced anisotropy of shape is not always apparent. We interpret these entities as the ‘crystallites’ which have grown, as a consequence of the anneal, to dimensions sufficient to be seen in the transmission electron microscope, the resolution available being significantly limited by specimen beam damage, which precludes parallel observations on the more beam-sensitive B-ET.It was also observed that the development of ‘crystallites’ during annealing at 200°C was suppressed if the initial preferred molecular orientation in the sample was not very high. SOLID-STATE ORDER A MODEL The evidence of thermal analysis is that the transition from solid to liquid in these polymers is associated with some loss of order. The absence of sharp diffraction peaks and multiply sampled ‘layer lines’ in the oriented patterns means that the additional order characteristic of the solid state cannot be associated with three- dimensional crystals larger than ca. 50 A. Additionally, the random nature of the copolymers, as confirmed by analysis of the meridional scattering, precludes the formation of large three-dimensional crystals.Indeed one must presume that this is the mechanism by which random copolymerization decreases the melting point so effectively, as any small crystals which can form will have large surface energies in relation to their volume free energies of crystallization. However, solid-state annealing of the polymers appears to give rise to a distinct increase in lateral order, as indicated by the concentration of the first meridional maximum onto the peridian and the ability to image entities in dark-field electron microscopy, without any apparent development of blocky runs in the copolymer molecules. Such behaviour is also consistent with the appearance of the d.s.c. traces of annealed material [fig. These observations are reminiscent of those reported for atactic PVC gels,33 in which measurements of crystallite size were larger than expected for atactic molecules.However, in the case of B-N the absence of sampling on the first ‘layer line’ except on the meridian is an additional factor. In the light of this we suggest that the small crystallites which do develop are conformationally ordered and consist of laterally matched sequences of the random chain. They could be considered perfect and yet only one ‘unit cell’ thick in the direction of the chain axes or alternatively as a non-periodic sequence of layers of relatively large lateral extent compared with the intermolecular spacing. We refer to them as non-periodic layer (NPL) crystallites. Fig. 9 illustrates one. The conformation chosen [fig.9 ( b ) ] is the extended chain, partly because of the form of the interchain diffraction peak but also because this conformation is stabilized by extension and could thus account for the observation that orientation appears to enhance the rate of the annealing effects. Such a proposal immediately raises questions of a statistical nature. In particular, what is the probability that a particular sequence of length rn‘ might be found somewhere within a given search length rn along each of N adjacent molecules? If the probability is high then NPL crystals of substantial lateral extent can be expected to occur. The statistical exploration required is extensive and is still in progress; however, some results for the simple case of an A-B random copolymer with the components present in equal proportions are presented.7 ~ .70 MOLECULAR CORRELATION IN THERMOTROPIC COPOLYESTERS I I I I I I I A B B A A B A A A A B A B 8 A A A A A A A A A A A A A A B S S B S P B A A A A A A A B B B B B B B g B B B 8 @ A B B B A B A B B A A B B A - B B B A B A B B B B B B B B - A A A A A A A B B A B A A A A B A A B A A B A B B A B B Fig. 9. Sketch indicating (a) the matching of random sequences in a non-periodic layer crystallite and (b) part of such a crystallite with the chains in the extended conformation. 1 P 0 1 5 m' 10 15 Fig. 10. Plots of the probability (P) of (a) a particular, but not necessarily regular, sequence of length rn' units being present in a chain 100 units long and (b) any sequence of length rn' being present in two 100 unit chains.The probability of one or more pre-defined sequences occurring in such a chain (neglecting end effects) is given by P = I -exp[-(1/2)"'(m-m'+1)]. As can be seen from fig. 10( a ) , drawn for a search length of 100 units, the probability rapidly decreases for lengths > 5 . Thus for an NPL crystallite of thickness 7 units,WINDLE, VINEY, GOLOMBOK, DONALD AND MITCHELL 71 which would be ca. 50 A for B-N, 50% of the chains would contain the required sequence and be able to contribute to lateral growth which would otherwise be restricted only by encounters with other crystallites. Such a proposition requires not only a measure of lateral sorting in addition to the relative longitudinal motion implied by the ‘search length’ but also opens up the possibility, through longer-range sorting and fractionation, of the formation of NPL crystals of greater thicknesses and volume fractions.This may be the mechanism occurring during solid-state annealing. The probabilities refer to any one particular sequence of length rn’ and thus must apply equally to regular blocks such as AAAAAAA or ABABABAB which occur by chance in the random chains. NPL crystals have been proposed for two reasons. First, the diffraction evidence from the annealed samples indicates the presence of random sequences with fairly extensive lateral correlation, for if the lateral correlation was the prerogative of regular sequences then the annealing would be expected to change the position and number of meridional sequences and also produce off-axial (hkl) peaks in the oriented pattern.Secondly, at the nucleation stage of a crystal, where a number of neighbouring chains are moving relative to each other in order to find a match over sufficient length, the statistical condition is more relaxed as any sequence in common will suffice. The probability of two chains (search length 100 units) having any sequence of length rn’ in common is given by the above equation but with the exponent multiplied by the factor (rn - m’+ 1). The probability in the case of a nucleus with two chains is plotted in fig. lO(6). There is a 50% probability of the chains having at least one sequence of 13 units in common or virtual certainty for any sequence of 7 units, as long as there is no restriction on the make up of the sequence.However, the possible presence of chemical blockiness in the molecules, as perhaps in B-ET, may well make the nucleation of crystallites based on regular sequences much more likely. This brief excursion into the statistical properties of random chains has shown that small crystallites, as suggested by the diffraction evidence, the transmission electron micrographs and the thermal-analysis data, can form without the necessity of blocks of regularly sequenced units either being introduced into the molecules at polymerization or created by subsequent processes such as transesterification. There is, however, no implication that regular blocks may not sometimes be present in polymers of this type. We thank Prof. Manfred Gordon for helpful advice in the formulation of the statistical approach.We also thank the S.E.R.C. for the provision of funding through a grant, a fellowship and a studentship, and Prof. R. W. K. Honeycombe and Prof. D. Hull for the provision of laboratory facilities. ’ J. Preston, in Liquid Crystalline Order in Polymers, ed. A. Blumstein (Academic Press, New York, * W. J. Jackson, Br. Polym. J., 1980, 12, 154. 1978), p. 141. J-I. Jin, S. Antoun, C. Ober and R. W. Lenz, Br. Polym. J., 1980, 12, 132. M. R. Mackley, F. Pinaud and G. Siekmann, Polymer, 1981, 22, 437. C. Viney and A. H. Windle, J. Muter. Sci., 1982, 17, 2661. ‘ J. R. Schaefgen, T. I. Bair, J. W. Ballou, S. L. Kwolek, P. W. Morgan, M. Planar and J. Zimmermann, in Ultra High Modulus Polymers, ed. A. Ciferri and I. M. Ward (Applied Science, London, 1979), p. 173. G. A. Gutierrez, R. A. Chivers, J. Blackwell, J. B. Stamatoff and H. Yoon, Polymer, 1983,24,937. G. R. Mitchell and A. H. Windle, Colloid Polym. Sci., 1985, 263, 230. G. R. Mitchell and A. H. Windle, Polymer, 1982, 23, 1269. lo A. M. Donald, C. Viney and A. H. Windle, Polymer, 1983, 24, 155.72 MOLECULAR CORRELATION IN THERMOTROPIC COPOLYESTERS ‘ I A. M. Donald and A. H. Windle, J. Muter. Sci., 1983, 18, 1143. l2 G. W. Calundann, US. Patent 4161470, 1979. l 3 W. J. Jackson and H. F. Kuhfuss, J. Polym. Sci., Polym. Chem. Ed., 1976, 14, 2043. l4 G. R. Mitchell and A. H. Windle, Polymer, 1983, 24, 1513. l5 G. R. Mitchell, unpublished work. l6 F. E. McFarlane, V. A. Nicely and T. G. Davis, Contemp. Top. Polym. Sci., 1977, 2, 109. l7 R. Bonart, Bog. Colloid Polym. Sci., 1975, 58, 36. l8 R. Hosemann and S. N. Bagchi, Direct Analysis of Diflruction by Matter (North Holland, l9 C. Viney, Lab. Pracr., 1983,32, 87. 2o C. Viney, G. R. Mitchell and A. H. Windle, Polym. Commun., 1983,24, 145. 21 P. Gay, Introduction to Crystal Optics (Longman, London, 1984). 22 A. M. Donald, C. Viney and A. H. Windle, Philos. Mag., 1985, 852, 925. 23 J. P. Hummel and P. J. Flory, Macromolecules, 1980, 13, 479. 24 M. J. Freiser, Phys. Rev. Lett., 19?0, 24, 1041. 25 C. S. Shih and R. Alben, J. Chem. Phys., 1972, 57, 3055. 26 J. P. Straley, Phys. Riu. A , 1974, 10, 1881. 27 G. R. Luckhurst and S. Romano, Mol. Phys., 1980,40, 129. ** W. Meesiri, J. Menczel, U. Gaur and B. Wunderlich, J. Polym. Sci., Polym. Phys. Ed., 1982,20,719. 29 D. J. Blundell, Polymer, 1982, 23, 359. 30 B. Wunderlich and J. Grebowicz, in Advances in Polymer Science (Springer-Verlag, Berlin, 1984), 31 T. P. H. Jones, G. R, Mitchell and A. H. Windle, Colloid Polym. Sci., 1983, 261, 110. 32 A. H. Narten, J. Chem. Phys., 1977, 67, 2102. 33 S. J. Guerrero, A. Keller, P. L. Soni and P. H. Geil, J. PoZym. Sci., Polym. Phys. Ed., 1980,18,1533. Amsterdam, 1962). p. 1.

ISSN:0301-7249    

DOI:10.1039/DC9857900055

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1985,79, 73-84 X-Ray Analysis of the Structure of Liquid-crystalline Copolyesters BY JOHN BLACKWELL,* AMIT BISWAS, GENARO A. GUTIERREZ AND ROBIN A. CHIVERS Department of Macromolecular Science, Case Western Reserve University, Cleveland, Ohio 44106, U.S.A. Received 10 th December, 1984 The X-ray analyses of the structures of a group of aromatic copolyesters that form liquid-crystalline melts is described. X-ray patterns of melt-spun fibres of wholly aromatic copolymers prepared from p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) show a high degree of axial orientation and there is also evidence for three-dimensional order. The meridional maxima are aperiodic, and it is shown that their positions and intensities are predicted by a model consisting of an assembly of parallel chains of completely random monomer sequences.Analyses of the scattering by aperiodic chains are summarized, first for point monomers and subsequently for an atomic model. From the breadth of some of the meridional maxima it is.possible to estimate the persistence or correlation length for the stiff-chain conformation, which is found to be between 9 and 13 monomer units, depending on the HBA/ HNA ratio. Determination of the three-dimensional structure is initiated via calculation of the cylindrically averaged transform for the random chain, followed by consideration of the interference effects caused by chain packing. The patent literature describes a number of aromatic copolyesters [see ref. (1) for a review] prepared from monomers such as p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA), that form mesomorphic melts and have poten- tial applications as, for example, high-strength fibres and novel moulded plastics.Homopolymers of HBA and HNA are crystalline, infusible materials, but copoly- merization breaks up the ordered structure and nematic melts occur as a result of the extended conformation. The mechanical properties of the different copolymers2 are very dependent on the chemistry of the monomers and this has prompted the study of their physical structure described below. Analyses of their morphology3 and thermal properties4 have been reported recently by other groups. X-ray diffraction patterns of melt-spun fibres of copoly( HBA/ HNA) at three comonomer ratios’ are shown schematically in fig.1. The polymer specimens were prepared6 at Celanese Research Co., Summit, N.J. The diffraction patterns reveal a high degree of axial orientation of the molecules, and the observation of sharp equatorial and off-equatorial Bragg maxima point to the existence of some three- dimensional order. Blundell’ used the intensity of these reflections to estimate a ‘degree of crystallinity’ of ca. 21% for a preparation of the 40/60 copolymer. Our attention was first focused on the meridional maxima. The d spacings of these maxima for five comonomer ratios’ are listed in table 1, where it can be seen that they are aperiodic, i.e. they are not orders of a simple repeat, and also shift progressively with the monomer ratio. There are no analytical data presently available on the monomer sequence distribution: n.m.r.methods, for example, have not been successful because of the low solubility and the chemical similarity of the 7374 X-RAY ANALYSIS OF LIQUID-CRYSTALLINE COPOLYESTERS Fig. 1. Schematic representations of the X-ray fibre diagrams of melt-spun fibres of copoly(HBA/HNA) for three comonomer ratios: (a) 30/70, (b) 58/42 and (c) 75/25 [from ref. ( 5 ) ] . monomers. However,- these X-ray data argue against the existence of extensive block-copolymer structure, which would lead to periodic meridional maxima charac- teristic of one or both types of block. We have therefore analysed the data in terms of a completely random monomer sequence. DIFFRACTION BY APERIODIC POLYMER CHAINS Fig.2 ( a ) shows a projection of a model of a typical random sequence of copoly( HBA/ HNA). The monomers were constructed' using standard bond lengths and angles, with planar aromatic and carboxy groups. The only conformational freedom is due to torsional rotation about the aromatic-carboxy linkage bonds; these angles were set so that the mutual inclination of the planes of the aromatic and carboxy groups was 30°, consistent with the results of conformational analysis.' The meridional intensity I ( 2 ) depends on the Fourier transform of the projection of the structure onto the chain axis, z. Since the aromatic-carboxy bonds areJ. BLACKWELL, A. BISWAS, G. A. GUTIERREZ AND R. A. CHIVERS 75 Table 1. Observed and calculated meridional intensity maxima for copoly( HBA/ HNA)" monomer experimental d spacings/A calculated d spacings/A mole ratio, HBA/HNA film diffractometer atomic model point model 25/75 8.09 f 0.07 4.08 f 0.04 2.77 f 0.03 2.05 f 0.03 30/70 7.95 4.1 1 2.83 2.06 50/50 7.43 2.84 2.02 58/42 7.35 2.98 2.05 75/25 6.78 3.03 2.03 8.1 1 f0.07 4.15 f 0.02 2.85 f 0.01 2.09 f 0.01 7.89 4.09 2.87 2.09 7.49 2.95 2.09 7.19 2.96 2.08 6.70 3.09 2.09 8.09 4.20 2.85 2.10 8.01 4.2 1 2.86 2.10 7.6 1 4.3 1 2.95 2.10 7.45 4.40 2.99 2.1 1 7.04 3.09 2.1 1 7.98 4.17 2.84 2.10 7.88 4.17 2.85 2.10 7.41 4.1 1 2.93 2.10 7.19 4.0 1 2.98 2.10 6.75 3.09 2.1 1 a From ref.( 5 ) . ( b ) * B o B * N - N * B * N . B * N * B * B * Fig. 2. ( a ) Projection of a model of a typical random sequence of copoly(HBA/HNA) and (b) point model for the same sequence.approximately parallel to the fibre axis, this z projection will be largely independent of the conformation. In particular the axial advance will be approximately constant for each monomer type, Le. approximately equal to the residue length. Based on the molecular models, the lengths of the HBA and HNA residues were taken as 6.35 and 8.37 A, respectively. As a first approximation to the structure of wholly aromatic copolyester chains" we represented each residue by a point placed for convenience at the ester oxygen and separated from its neighbours by the appropriate residue lengths, as in fig. 2( 6 ) . The Fourier transform F,(Z) of a chain of N point monomers is given by N F J Z ) = C exp 2niZzj j = 176 X-RAY ANALYSIS OF LIQUID-CRYSTALLINE COPOLYESTERS where zj is the coordinate of the jth point.If we assume there is no axial register of the chains and neglect for the moment the Lorentz and polarization effects, then the intensity is obtained by averaging the square of the modulus of Fc(Z) over all n possible monomer sequences: where pc is the probability of the cth chain and depends only on the monomer ratio in the case of a completely random copolymer. I ( 2 ) was first evaluated approxi- mately by use of Monte Carlo methods to set up random chain sequence^.^ Sub- sequently this was replaced' ' by an exact calculation via the autocorrelation function of the chain, Q ( z ) : I ( 2 ) = 1 Q(z,) exp 2.rriZz1. 1 (3) Q ( z ) is the probability of first, second, third etc. nearest neighbours along the random chain and is zero except at specific values of z = zl.Q ( z ) for the 58/42 copoly(HBA/HNA) is shown in fig. 3 out to the 14th (positive) nearest neighbour in an infinite chain. The summation in eqn (3) has a closed solution'* which can be derived by treatment of the chain as a one-dimensional paracry~tal'~ with bimodal statistics. In fig. 3 the positive terms in Q ( z ) are separated into components labeled H,, HI, H2, H3 etc. for the zeroth, first, second, third etc. nearest neighbours. It can be seen that H2(z) is the self-convolution of Hl(z): and in general H n ( z ) = H n - , ( z ) * H d z ) . Thus I ( Z ) , the Fourier transform (a) of Q(z), can be written as +W +W I ( z ) = flQ(z)I = C F[Hrn(z)l= C Frn(Z) -W -W where The summation of the power series in eqn (5) can be written as F ( Z ) + F*(Z) I ( 2 ) = 1 + 1 - F ( 2 ) 1 - F * ( Z ) 1 + F ( 2 ) = R e ( l - F ( z ) ) for an infinite chain.F*(Z) is the complex conjugate of F ( Z ) . For a limited chain of N monomers, I ( Z ) is given by l - F ( Z ) - "1-F(Z)I2 1 + F ( Z ) 2F(Z)[l- FN(Z)] I ( 2 ) =Re (7)J. BLACKWELL, A. BISWAS, G. A. GUTIERREZ AND R. A. CHIVERS h 0.8- 0, t.3 v 77 p Y 0.4 -s 2 0.2 0 c M ._ 0 0 20 40 6-0 SO 100 120 z l A Fig. 3. Autocorrelation function Q( z) for 58/42 random copoly( HBA/HNA) plotted against z out to the 14th nearest neighbour of an infinite chain. H,, H I , H2 etc. indicate the terms for the zeroth, first, second etc. nearest neighbours [from ref. (12)]. It will be shown below that the above analysis predicts the positions of the aperiodic maxima for copoly(HBA/ HNA).This approach was derived indepen- dently, but note that similar analyses have been reported previously for inorganic layer structure^'^ and deformed a-keratin fibres.15 However, to our knowledge this has not been done before for polyatomic monomers, which is essential if we are to compare the intensities as well as the positions of the meridional maxima. Conversion into atomic monomers is achieved by separating Q(z) into its components : (9) Q(z) = Q(0) + C C QAB(z) A B where QAB( Z) describes the probability of sequences beginning with monomer A and ending in monomer B. [There will be four such QAB(z) series for copoly- (HBA/HNA).] Since Q ( O ) =C (10) A where PA is the mole fraction of monomer A, we can derive the I ( 2 ) for the atomic model by analogy with eqn (3): I ( z ) = C P A E u ( z ) + C C C QAB(zi ) h B ( Z ) ~ X P 2rizzi (11) A A B 1 where F A B ( 2 ) is the Fourier transform of the convolution of residue A with residue B: F A B ( 2 ) = C C &,,kfsB ~ X P 2 ~ i z ( z k , - zj.A) (12) j k where f is the atomic scattering factor, z is the axial atomic coordinate and the subscripts j,A and k,B denote the jth atom of residue A and the kth atom of residue78 X-RAY ANALYSIS OF LIQUID-CRYSTALLINE COPOLYESTERS B, respectively.Closed forms analogous to eqn (7) for an infinite atomic model are derivedI6 by separating H,(z) into its AB components: QAA(z) can be written as for which the Fourier transform is and so on for the other components of Q ( z ) . These expressions are then multiplied by the respective F A B ( 2 ) terms in eqn (1 1). Expressions for finite chains analogous to eqn (8) can be derived in the same way.The above analyses have been for the meridional data. Extension to the entire fibre diagram requires calculation of I( R, Z), where R is the radial polar coordinate in reciprocal space. The meridional intensity derived above now becomes I ( 0 , Z ) . We have approached this problemi7 by calculation of the cylindrically averaged intensity transforms for the random chain. This requires definition of the conforma- tion, and for this we have considered two extremes: an extended random conforma- tion, Le. a straight chain with random torsion angles, and a rigid conformation in which all the planes of the aromatic rings are parallel to each other.I(R,Z) is derived by modification of eqn (1 1) to consider a three-dimensional structure. For the random conformation where AAB(R, z, =C C J ; , ~ ~ B J O ( 2 7 T ~ r A j ) J O ( 2 7 T R ~ B , k ) cos 27Tz(zk,B-zj,A) (17) j k and BAB(R, 2) is the equivalent sine term. For the rigid conformation, I ( R , 2) is calculated via eqn (16) except that AAB(& z, C CJ;,kfiSBJn(27TR~,A)Jn(27T~rk,B) n j k xcos r n ( 4 j , A - 4k,B)+2n2(2j,A- 2k,B)1 (18) and BAB(R, 2) in the equivalent sine term. r, 4 and z are the atomic polar coordinates. Application of the above analyses in investigations of the structure of copoly( HBA/HNA) are described below. RESULTS AND DISCUSSION Fig. 4 and 5 compare the observed and calculated meridional intensity for 30/70 and 75/25 copoly(HBA/HNA).* The positions of the observed and calcu- lated maxima are given in table 1.The observed data, shown as the dashed lines,J. BLACKWELL, A. BISWAS, G. A. GUTIERREZ AND R. A. CHIVERS 79 , 10 20 30 40 : 2 e p 0 Fig. 4. Observed and calculated meridional intensity data for 30/70 copoly(HBA/HNA). Dashed line, observed 8/28 diffractometer scan; upper solid line, calculated data for atomic model; lower solid line, calculated data for point model [from ref. (S)]. 3 Fig. 5. Observed and calculated meridional intensities for 75/25 copoly( HBA/ HNA). Dashed line, observed 8/28 diffractometer scan; upper solid line, calculated data for atomic model: lower solid line, calculated data for point model [from ref. (S)].80 X-RAY ANALYSIS OF LIQUID-CRYSTALLINE COPOLYESTERS were recorded by Dr J.B. Stamatoff of Celanese Research Co., as 8/28 diffractometer scans.' The solid lines are calculated data for point and atomic models using eqn (3) and (1 1). The models were for chains of limited length, and in order to minimize the subsidiary maxima the data were averaged over a normal distribution of chain lengths centred on 11 monomers (with (T I= 1 monomer). The calculated data have also been corrected for the Lorentz and polarization effects. It can be seen that the point model predicts the positions of the observed maxima to within CQ. 0.1 A; this agreement could be improved by refinement of the residue lengths. At high proportions of HNA, four aperiodic maxima are predicted. As the HBA content increases, the first maximum moves outwards (to lower d spacings), the third moves inwards and the second gets progressively weaker until it disappears at about the 50/50 residue ratio.Meanwhile the strong maximum at d = 2. 1 A is unchanged in position across the entire range of composition. The latter effect arises because the 2.1 A maximum corresponds to the third order of the HBA length (6.35 A) and the fourth order of the HNA length (8.37 A). The point model can be seen as a lattice with a repeat of ca. 2.1 A, but with low occupancy, and hence the 2.1 A maximum is a Bragg peak occurring at all compositions. However, the occupancy is not random but is as defined by Q ( z ) ] , with the result that aperiodic The point model predicts the positions of the maxima but cannot be expected to give good agreement for the intensities because intraresidue interferences have been ignored.Conversion into an atomic model leads to prediction of maxima in approximately the same positions: the F A B ( 2 ) terms in eqn (1 1) vary relatively slowly with 2 and are sampled by interference functions analogous with that for the point model. However, the intensity agreement is now reasonably good. In fig. 4 and 5 the peak heights at d = 2.1 A have been set equal, and it can be seen that there is good agreement for the other peaks except that the first peak is predicted to be too weak. This is a general feature across the entire composition range and points to several defects in the model. In particular, the model for the copolymer is defined as a straight chain in which all residues have their ester oxygen-ester oxygen vectors parallel to the z axis. This can only be an approximation, as is apparent from fig.2(a): rather there must be a distribution of these vectors about the z axis. The F A B ( 2 ) terms' have minima in the region d = 6-8 A, which are smoothed when a distribution of residue orientations is considered, with the effect that the intensity of the first maxima for the atomic model is increased. In separate work'* on copolymers of HBA, 2,6-dihydroxynaphthalene (DHN) and terephthalic acid (TPA) we have refined the residue orientations to obtain a match between the observed and calculated maxima. A second factor that can be expected to change the meridional intensities is interference caused by preferred axial stagger of the chains.Our calculations so far have been for nematic structures with random axial stagger, but the presence of off-equatorial Bragg maxima points to the existence of some three-dimensional order, which will be expected to affect the meridional intensities. Calculations incorporating preferred axial stagger for the chains are now in progress and will be described in a future paper. Thus it can be seen that we can reproduce the meridional scattering for copoly( HBA/ HNA) using a completely random monomer sequence. This approach has also been applied to copoly(HBA/DHN/TPA) "J' and to several other copolyesters with equal success. This leads to the question of the sensitivity of this type of X-ray data to non-random sequence distribution.This can be modelled by variation of Q ( z ) to take account of different neighbour probability in blocky structures. We have studied this in detail for copoly( HBA/DHN/TPA) and have maxima occur at d > 2.1 d .J. BLACKWELL, A. BISWAS, G. A. GUTIERREZ AND R. 6 8 10 15 A I I 10 20 30 LO E 2ei0 4. CHIVERS 81 2e/c Fig. 6. ( a ) Calculated meridional intensities for 58/42 copoly( HBA/HNA) for atomic models of different chain lengths (marked on the curves). ( b ) Observed meridional intensity data for 58/42 copoly(HBA/HNA). been able to show that all but minimal blockiness can be ruled out for these copolymers in favour of the completely random m0de1.I~ Thus, in this relatively special case of a stiff copolymer of monomer units of different lengths, we can use X-ray methods to investigate sequence distribution, i.e. microstructure. The calculated data shown in fig. 4 and 5 were obtained using models of chains with an average degree of polymerization (d.p.) of 11. The actual d.p. for these polymers is ca. 150, based on a reported molecular weight of 25 000.6 Nevertheless, the main features of the meridional data can be predicted for chains of 6 or more monomers. Fig. 6 shows the meridional intensity for 58/42 copoly( HBA/HNA) calculated for chains of 6, 8, 10 and 15 monomers.8 The data have been normalized to have the same peak height at d == 2.1 A, for ease of comparison. This copolymer ratio gives three maxima at d = 7.2, 3.0 and 2.1 A; the other peaks are subsidiary maxima caused by the monodisperse short chain length.It can be seen that the82 X-RAY ANALYSIS OF LIQUID-CRYSTALLINE COPOLYESTERS first two peaks are approximately independent of chain length. However, the peak at d == 2.1 A gets progressively sharper as the chain length incremes, as expected for a Bragg peak. Fig. 6( 6) shows the observed data for this monomer ratio: following correction for instrumental broadening the peak width at d = 2.1 A is reproduced by a chain of 11 monomers. Given the higher d.p. for the actual molecules, this ‘chain length’ must correspond to a persistence or correlation length for the stiff-chain conformation: it is the distance beyond which our approximation of a straight chain with residues parallel to the chain axis is no longer adequate. Comparison of the linewidth data for other HBA/HNA ratios from 30/70 to 75/25 shows that the correlation length increases from 9 to 13 residues as the HBA content increases.* Non-linearity of the chain is due in large part to the offsetting 2,6-saphthylene linkages, and hence it i s to be expected that the chains will tend to be straighter as the HBA content increases.The results in fig. 6 were obtained for finite chains using eqn (1 1). In more recent work we have approached this via the infinite chain with H , ( z ) defined as the sum of distribution functions for the possible HBA and HNA lengths. The above calculations yield a model for the chain with which it is possible to proceed to consider the three-dimensional structure. Our first step in this direction has been to calculate the cylindrically averaged transforms for (extended) random and rigid chain^.'^ Fig.7 shows these data for 58/42 copoly(HBA/HNA), together with a schematic representation of the observed data. The calculated data are for a single (average) random chain, i.e. no allowance is made for interchain interference effects. Note also that fibre disorientation causes arcing of the observed data. The two models each have the same axial projection and thus predict the meridional data I( 0 , Z ) as described above. There is high intensity along the equator that declines steadily with R. No maxima are seen corresponding to the equatorials at d = 4.6, 2.6 and 2.3 A, but these can be expected to arise when the chains are packed on a hexagonal lattice with a = 5.2 A.There is intensity in the region of the off-equatorial at d = 3.3 A. This is weaker than the lowest intensity contour shown in fig. 7 (a) and ( 6 ) but is probably sufficient to account for the observed sampling. The ‘layer line’ at 2 = 1/2.1 k’ is more extensive (in the radial direction) than those for the other meridionals, as is observed. The major differences between the data for the random and rigid models are in the extent of the 2 = 1/7.5 A-’ layer line and the intensity of the diffuse non- meridional layer line at 2 = 1/4.5 A-’. In this respect the random model gives significantly better agreement with the observed data and is considered to be preferable to the rigid conformation in which the aromatic rings are parallel to one another. Similar conclusions were obtained in studies of the 30/70 and 75/25 copolymers.Random torsion angles will lead to chains with a cylindrical cross- section, which will tend to pack hexagonalty. Nevertheless, the existence of three- dimensional order points to axial register for some of the chains, which may require a regular conformation in certain parts of the structure. The calculations so far represent only a starting point for these investigations of the three-dimensional structure and further work is in progress in this area. This work is being supported by NSF Grant no. DMR8 1-07130 from the Polymer Program. ’ J-I. Jin, S. Antoun, C . Ober and R. W. Lenz, Br. Polym. 1, 1980, 12, 132. G. W. Calundann and M. Jaffe, in Roc. Robert A. Welch Found. Symp. XXVI, Synthetic Polymers, 1982, p. 247.+., 0 . 5 p tan (28) 1 .o h 8 0.5 cd c, 0:5 tan (28) 1 :o tan (28) Fig. 7. ( a ) I ( & 2) intensity data for 58/42 copoly(HBA/HNA) in the random conformation, ( b ) cylindrically averaged I ( R, 2) intensity data for 58/42 in the rigid conformation (aromatic groups parallel to each other) and (c) schematic representation of one quadrant of observed data [from ref. (17)].84 X-RAY ANALYSIS OF LIQUID-CRYSTALLINE COPOLYESTERS A. M. Donald, C. Viney and A. H. Windle, Polymer, 1983, 24, 155. M. Y. Cao and B. Wunderlich, Macromolecules, in press. G. A. Gutierrez, R. A. Chivers, J. Blackwell, J. B. Stamatoff and H. Yoon, Polymer, 1983, 24,937. G. W. Calundann, U.S. Patent, 4 161 470,1979. D. J. Blundell, Polymer, 1982, 23, 359. R. A. Chivers, J. Blackwell and G. A. Gutierrez, Polymer, 1984, 25, 435. J. P. Hummell and P. J. Flory, Macromolecules, 1980, 13, 479. J. Blackwell and G. A. Gutierrez, Polymer, 1982, 23, 671. J. Blackwell, G. A. Gutierrez and R. A. Chivers, Macromolecules, 1984, 17, 1219. 1984,23, 1343. l 2 J. Blackwell, G. A. Gutierrez, R. A. Chivers and W. Ruland, J. Polym. Sci., Polym. Phys. Ed., l 3 R. Hosemann, 2. Phys., 1950, 128, 465. l4 S. Hendricks and E. Teller, J. Chem. Phys., 1942, 10, 147. l 5 R. Bonart, Prog. Colloid Polym. Sci., 1975, 58, 36. l 6 J. Blackwell, A. Biswas and R. Bonart, Macromolecules, submitted for publication. l7 R. A. Chivers and J. Blackwell, Polymer, in press. l 8 G. A. Gutierrez, R. A. Chivers and J. Blackwell, Polymer, in press. l9 G. A. Gutierrez and J. Blackwell, Macromolecules, in press.

ISSN:0301-7249    

DOI:10.1039/DC9857900073

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

GENERAL DISCUSSION t Prof. E. Chiellini (University of Pisa, Italy) said: My question is related to the very last part of Prof. Samulski’s presentation and is concerned with the change in the sense of helicity of the cholesteric array in poly( y-benzyl-L-glutamate) which accompanies a change in the nature of the solvent, i.e. in going from dioxane to dichloromethane. Is that change accompanied by a change in the local helicity of the individual macromolecules in dilute solution? In other words, is there any possibility of predicting a correlation between the helicity of single cholesterogenic macromolecules and the helicity of the cholesteric supermolecular array in both lyotropic and thermotropic systems? Prof. E. T. Samulski (University of Connecticut, U.S.A.) replied: Classical 0.r.d.and c.d. experiments with PBLG in dilute solutions using dioxane or dichloromethane (and their mixtures) indicate that the a-helix maintains a right- handed conformation; i.e. insofar as one can extrapolate such findings to concen- trated, lyotropic solutions, there does not appear to be a correlation between the sense of the PBLG backbone helix and the sense of the cholesteric twist. Moreover, it has been demonstrated that a single chiral doping agent will induce a cholesteric pitch sense that depends on the specific achiral thermotropic nematic being doped. Hence, it does not appear that there is a straightforward way of predicting the macroscopic twist sense given the microscopic chirality in either lyotropic or ther- motropic phases. Prof.G. C. Berry (Carnegie-MeZZon University, U.S.A.) said: Liquid crystals are often studied, processed or utilized as thin layers bounded by surfaces. Without special preparations, the surfaces may orient the director in the plane of the surface, but with an orientational coherence length in the plane comparable with the sample thickness, producing a complex three-dimensional director orientation throughout the sample. Can Prof. Samulski comment on whether such effects might be par- ticularly important with long-chain polymeric liquid crystals, and what the effect of such a director field might be in the presence of orientational fields such as a magnetic or electric field or a shear deformation? For example, in the latter case, I suspect such effects could introduce boundary layer effects, giving rise to the observed increase in the apparent viscosity with decreasing rate of deformation for slow flows.Prof. E. T. Samulski (University of Connecticut, U.S.A.) responded: In the early studies of polypeptide liquid crystals it was noted that the cholesteric axis aligns normal to planar surfaces and that the resulting aligned domain could traverse the thickness of the sample cell - a distance of some millimetres [see ref. (9) of my paper]. More recently cell thickness has been exploited to affect textures (cholesteric versus nematic) and alignment mode (homeotropic versus planar).’ Thus boundary surface effects are not negligible in polymeric liquid crystals and may, as Prof. Berry suggests, profoundly influence delicate threshold level perturbations of the director by external fields.As to the relative importance of such effects in monomeric and polymeric liquid crystals, I can only speculate that as a coarse approximation one might anticipate t Plates 1-9 follow p. 123. 8586 GENERAL DISCUSSION the coherence length to scale with molecular dimensions. Hence for a high- molecular-weight polymer the propagation of boundary effects could conceivably exceed by two to three orders of magnitude coherence lengths commonly encoun- tered in monomeric liquid crystals. Y. Uematsu and I. Uematsu, Pofyrn. Prepr., 1979, 20, 66. Dr L. L. Chapoy (Technical University of Denmark) addressed Prof. Samulski: You said that the theory of liquid crystalline polymers is progressing. What is your feeling about the ability to predict the effect of defect structures, e.g.groups containing kinks, flexible links and aromatic substituents, in converting aromatic polymers which are largely intractable into thermotropic, rigid-rod, liquid-crystalline polymers? Prof. E. T. Samulski ( University of Connecticut, U.S.A.) (communicated): Let me begin to answer your question by stating that theoretical modelling of monomeric liquid crystals is not yet at a stage that enables the inclusion of specific chemical idiosyncracies of mesogens; for the most part, researchers are limited to tabulating trends within a given class. of mesogens, rank ordering ‘defect structures’ according to their influence on transition temperatures. However, one should not infer from the status of such modelling that the role of defect structures in polymeric liquid crystals will be even more elusive.On the contrary, subtle changes affected by chemical modification of monomer liquid crystals are frequently exaggerated in polymers. Thus it may be quite possible to categorize defect structures in a semiem- pirical manner via studies of their influence in ‘ideal’ polymeric liquid crystals, and thereby construct prescriptions for the use of defect structures in the modification of potential (and often intractable) polymeric liquid crystals. Moreover, as the modelling of semiflexible polymers progresses, it may be possible to delineate the influence of specific defect structures to a degree of sophistication that is currently not possible in theories of monomeric liquid crystals.It is this kind of interplay between research on monomer and polymer liquid crystals that would constitute the synergism that I anticipate will characterize progress in our understanding of liquid crystals in the next decade. Prof. R. B. Meyer (Brandeis University, U.S.A.) said: In the past we have discussed the question of a convenient material on which physicists interested in polymer liquid crystals might perform experiments. In the case of polymer nematics, this might really be a small family of materials rather than one. For instance, one might be a low-melting-point semiflexible polymer. Another might be a rigid-rod polymer conveniently soluble at room temperature. Each of these should be chemi- cally stable and available in various molecular weights.The general knowledge and availability of such materials as a starting point for physicists would be very useful. Does Prof. Samulski have any suggestions along these lines? Prof. E. T. Samulski (University of Connecticut, U.S.A.) replied: The establish- ment of a class of polymeric liquid crystal standards would indeed expedite the advancement of our understanding of these materials just as the development of stable, room-temperature, monomeric liquid crystals fifteen years ago for electro- optic applications facilitated fundamental experimental investigations. In the case of thermotropic polymers there is, unfortunately, no comparable impetus from the applications sectors ; industry is interested in high-temperature materials whichGENERAL DISCUSSION 87 generally preclude careful characterization. Such materials will have to come from a conscientious synthetic laboratory. I think the synthetic polypeptides are capable of fulfilling the role of a rigid-rod standard for lyotropic polymeric materials.Again there is a real need to make well characterized (narrow molecular-weight distribution) samples available to researchers. However, as the polypeptides are commercially available, perhaps a group of interested experimentalists could approach the U.S. National Bureau of Standards and request that such standard samples be made available. Dr W. J. Jackson (Eastman Koduk, Tennessee, U.S.A.) said: Prof. Lenz’s idea of quantitatively determining the ‘degree of liquid crystallinity’ is a very intriguing concept, and it certainly would be a very useful measure to have. In his preprint he suggested determining the degree of liquid crystallinity by measuring the area of the d.s.c.endotherm which occurs at the transition from the liquid-crystalline to the isotropic state upon heating (and the area of the exotherm which occurs at the transition from the isotropic state back to the liquid-crystalline mesophase upon cooling). Thermotropic, liquid-crystalline polyester plastics and fibres which have the highest tensile strength and stiffness contain only a limited amount of a flexible component or kinking component ;l consequently, the temperature at which the transition to the isotropic state occurs is appreciably higher than 400°C, which is above the thermal stability limit of all polyesters.Even if the d1s.c. thermogram is obtained at a high scan rate to minimize thermal decomposition, to the extent that decomposition does occur, the quantitative aspect of the determination is lost. Broad-line n.m.r. spectra obtained on melts often provide a basis for determining the amount and type of material in each phase of two-phase isotropic/anisotropic melts, and this could be a better approach to the determination of the degree of liquid crystallinity. Prof. Lenz showed an example of this type of data which was obtained by Dr V. A. Nicely of the Eastman Chemicals Division from examination of PET/p-hydroxybenzoic acid copolyesters. When sufficient information is avail- able to show that the narrow line is due to an isotropic melt and that the broad line is due to an anisotropic melt, the broad-line n.m.r.spectrum provides a direct measure of the amount of liquid-crystalline phase or, in effect, of the degree of liquid crystallinity. (Dr Nicely plans to submit further data and interpretation to Macromolecules ; his earlier publication2 describes the determination of the broad- line n.m.r. spectra.) ’ W. J. Jackson Jr, Br. Polyrn. J., 1980, 12, 154. F. E. McFarlane, V. A. Nicely and T. G. Davis, in Contemporary Topics in Polymer Science, ed. E. M. Pierce and J. R. Schaefgen (Plenum, New York, 1977), p. 109. Prof. R. B. Blumstein (University of Lowell, U.S.A.) said: I also turn to Prof. Lenz. Concerning your discussion of copolymers of linear mesogenic with non-linear non-mesogenic repeating units, let me suggest that reference to ‘degree of liquid crystallinity’ might be misleading inasmuch as it implies a conceptual analogy with ‘degree of crystallinity’ as applied to semi-crystalline polymers.Discussion of the nematic-isotropic biphase in copolymers is complicated by simultaneous presence of chain length polydispersity and heterogeneity of composi- tion. In the specific instance of your P/ M copolymers (p. 3 l), irreversible rearrange- ment to block sequences upon thermal treatment during lengthy experiments such as broad-line n.m.r. (which you use to estimate the fraction of isotropic phase88 GENERAL DISCUSSION present) cannot be excluded and may further complicate matters. Thus it might be difficult to distinguish between an N+ I biphase due to selective partitioning of chain lengths between the isotropic and anisotropic components' and microphase separation due to blockiness.In our investigation of nematic homopolymers with flexible spacer groups we have observed a broad N I biphase in polydisperse samples of molecular weight up to ca. 5000-6000. As M, increases, the biphasic range shrinks to a width of ca. 7-8 O C 2 The width of the N + I biphase and fN, the fraction of nematic phase present, can be measured by broad-line n.m.r. experiments.192 Comparison of n.m.r., d.s.c. and microscopy data indicates that the width of the d.s.c. peak gives a reasonable first approximation of the N + I biphase width, which microscopy tends to underestimate, especially in situations where trailing of a relatively minor isotropic component is ~bserved.~ As for copolymers, they usually show very broad phase transitions compared with homopolymers of similar M,.This might explain the problems which you encounter in observing d.s.c. transitions of your P/M copolymers. The magnitude of the d.s.c. peak area should not be used to estimate the 'amount of nematic phase', as the enthalpy at the N/I transition is merely indicative of the degree of nematic order present in the system. This is illustrated by the well documented instances of odd-even oscillations of nematic order, with concomitant odd-even oscillations of AHNI. Copolymers display a degree of nematic order and a value of AHNI that are dependent on composition. My second question concerns your interesting observation of spontaneous homeotropic alignment in polymers I1 (n = 2-4) in table 1 of your paper.You do not report the molecular weight of these samples. Have you investigated the influence of molecular weight on occurrence of homeotropic alignment? In our experience with the nematic series and twin model compounds with central spacers (rigid-flexible-rigid sequence; n up to 18) we have observed spontaneous development of homeotropic regions in samples having M, values up to ca. 5000-6000. For higher values of a,, textures with dense disclination lines develop, and homeotropic alignment is no longer observed. ' F. Volino, J. M. Alloneau, A. M. Giroud-Godquin, R. B. Blumstein, E. M. Stickles and A. Blumstein, Mol. Cryst. Liq. Cryst. (Lett.), 1984, 102, 21. R. B. Blumstein, E. M.Stickles, M. M. Gauthier, A. Blumstein and F. Volino, Macromolecules, 1984, 17, 177. R. B. Blumstein, 0. Thomas, M. M. Gauthier, J. Asrar and A. Blumstein, in Polymeric Liquid Crystals, ed. A. Blumstein (Plenum Press, New York, 1985), p. 249. Prof. A. Blumstein (University of Lowell, U.S.A.) said: It is interesting that in the series of polymers 11, i.e. endowed with an oxyethylene spacer, the increase in n produces a change in the mesophase from smectic to nematic. This is the reverse of the usual trend in alkyl spacers, in which an increase in the number of methyleneGENERAL DISCUSSION 89 groups, n, produces a change from a nematic to a smectic mesophase. Can this unusual observation be explained? Could it be due to the fact that n is an average value and that the polymers are not fractionated? Dr R.Zentel (University of Mainz, West Germany) said: I turn to Prof. Lenz. In your talk you used the term ‘degree of liquid crystallinity’, and I would like to comment on this. As far as I know, there is nothing like this in liquid-crystalline side-chain polymers and in main-chain polymers with small molecular weight distributions and without chemical inhomogeneities due to different copolymer compositions, except for a small temperature region at the nematic-isotropic transi- tion. However, if one is working with liquid-crystalline main-chain copolymers one often finds biphasic regions over a very broad temperature range. In my opinion this is due to the broad molecular-weight distribution and the chemical inhomogeneity in these multicomponent systems, and has nothing to do with partially crystalline polymers, for which motions are frozen in and one cannot reach equili- brium.Therefore it would be better to use the expression ‘volume fraction of nematic phase’ or something like this to explain your results instead of ‘degree of liquid crystallinity’. Prof. E. T. Samulski (University of Connecticut, U.S.A.) said: I wish to make two points about the use of the term ‘degree of liquid crystallinity’ in the characteriz- ation of the apparent two-phase regime reported for many thermotropic polymeric liquid crystals. (1) Nematics only‘differ from isotropic fluids by the subtle, long- range, orientational order [ extending over many hundreds of thousands of molecules (monomers) in a homogeneous ‘domain’] imposed on the fluid mesogenic units.(2) At a fixed pressure and temperature, Gibbs tells us that a single-component material may exhibit only one phase. The first point suggests that defects in the primary structure of the polymer backbone (‘non-mesogenic segments’) could not be excluded from the liquid crystal (as they might be excluded from a single crystal) because of the rather mild orientational constraints of a nematic environment compared with that of a single crystal and because of the dimensional scale of a nematic domain (i.e. a single chain with defects would not span typical macroscopic domain dimensions). Therefore, the second point implies that thermotropic polymers exhibiting multiple phases must be composed of ‘sufficiently different’ species, i.e.high polydispersity and/or primary structures of distinctly different chemical compositions. In short, the so- called degree of liquid crystallinity is merely a coarse indicator of sample purity. One might obviate this problem by centrifuging a sample in the two-phase regime and examining the behaviour of the separate components, exploiting the mesophase’s natural tendency to fractionate chains according to molecular weight. In a sufficiently pure fraction, a first-order phase transition at a well defined temperature should be exhibited on heating and cooling thermotropic polymeric liquid crystals, as is found in their monomeric precursors. Prof. W. R. Krigbaum (Duke University, U.S.A.) said: Prof. Samulski has indi- cated that the Gibbs phase rule does not permit coexistence of the nematic and isotropic phases over a range of temperatures.This statement assumes that a polymer can be treated as a single component, despite its molecular-weight heterogeneity. This would be true if the properties of interest were independent of molecular weight. However, the melting and clearing temperatures of nematogenic polymers are observed to increase rapidly for low molecular weights and eventually to reach90 GENERAL DISCUSSION plateau values at higher molecular weights. Hence a polymer can be treated as a single component only if its molecular weight distribution is quite narrow or its average molecular weight is sufficiently high such that species of low molecular weight are present in negligible amounts.The average molecular weight plays another important role in nematogenic polymers. One frequently finds the homeotropic texture in samples of low molecular weight, while higher-molecular-weight samples of the same polymer exhibit a planar texture. Since formation of the homeotropic texture involves anchoring of the chain ends to the glass surface, this difference probably arises from the higher number density of chain ends in polymers of low molecular weight. Prof. P. J. Flory (Stanford University, U.S.A.) said: The nematic domains in a liquid-crystalline polymer should not be regarded as equivalent to the crystalline regions in a semicrystalline polymer or copolymer. In the first place, nematic domains are liquid rather than solid, and the molecules therein partake of the mobility characteristic of a liquid, apart from their axial orientation. Moreover, the stringent requirements of exact registry between a molecule and its neighbours in the crystalline state are inoperative in a nematic. For these reasons, the sizes of liquid-crystalline domains are typically very large in all directions, unlike submicro- scopic crystallites, whose extensions in space are severely restricteb by kinetic factors that control crystal growth. The distinction is most striking in the case of a copolymer, the co-unit of which is rejected by the crystalline phase comprising the rigid, or crystallizable, units.In the liquid-crystalline state, on the other hand, the co-units are not rejected and hence may be interspersed with the rigid member units of the polymeric molecules.Such intermixing is implicit in the liquid nature of the nematic and cholesteric states. On another matter, the coexistence of nematic and isotropic phases over a finite range of temperature is unambiguously indicative of a plurality of components, as follows from the phase rule. The comparatively broad biphasic ranges for the copolymers reported in this paper may reflect polydispersity in molecular weight or in composition. If the length of the chain is very great, a purely statistical variation in composition should be quite small and, hence, insufficient to promote the frac- tional partitioning of molecules between the coexisting phases that would be required to account for the biphasic gap. Dr H. J. Coles ( University of Manchester) said: In his paper Prof.Lenz mentions the concept of 'degree of liquid crystallinity' which seems to be related to the occurrence of two-phase regions above the melting point, T,, of the polymers. Is this a new concept particular to these main-chain polymer liquid crystals or is this a manifestation of distribution in size of the polymers he is studying? In the latter case, low-molecular-weight components would be expected to undergo a transition to the isotropic phase at much lower temperatures than the high-molecular-weight components, and this could explain the existence of the biphasic regions reported. Dr A. H. Windle (University of Cambridge) said: Prof. Lenz has presented data which he has interpreted as showing that there are two phases in a 60/40 random copolymer of PHB/PET, one of which is isotropic and is present in gradually increasing proportions over the approximate temperature range 125-400 "C.We have looked at copolyesters of this composition (Tennessee Eastman X7G) using hot- stage microscopy and are not able to confirm his proposal.' Fig. 1 shows a summary of the phases observed as a function of temperature. The mesophase and isotropic91 4 20 3 50 340 9 \ h 190 GENERAL DISCUSSION sanple optically isotropic development of optically isotropic phase nucleation of isotropic phase continuous Schlieren texture interrupted Schlieren texture onset of mobility ? 20 T Fig. 1. Diagram of phase stability regions of 60/40 PHB/PET determined by hot-stage microscopy and d.s.c.' phase do indeed coexist, but only within the temperature range 350-400°C.The endotherm corresponding to the transition is also confined within this temperature range. At lower temperatures, in samples which have been held in the liquid state for sufficient time to produce a coarser texture, two phases can be observed, but they are both optically anisotropic. Plate 1 shows such a microstructure at 150 "C. The phase within circular boundaries (the 'ring' structures of Mackley et aL2) is the first to become isotropic, but not until 350°C. ' C. Viney and A. H. Windle, J. Muter. Sci., 1982, 17, 2661. ' M. R. Mackley, F. Pinaud and G. Siekmann, Polymer, 1981, 22, 437. Dr G. R. Mitchell (University of Reading) said: I would like to comment upon the evidence presented by Prof. Lenz, and in particular the n.m.r.results, to support his notion of a two-phase structure in the rigid-chain thermotropic copolyester of hydroxybenzoic acid and poly( ethylene terephthalate). This copolymer exhibits a number of transitions as revealed by thermal analysis.' At ca. 190 "C the optical textures become mobile, while below that temperature the material is solid and immobile on a 1 pm scale, although dielectric measurements292 GENERAL DISCUSSION indicate significant segmental motion. The long-axis orientation parameters have been measured using wide-angle X-ray scattering techniques described el~ewhere.~ Up to temperatures of 230°C there is no significant reduction in the long-axis orientation parameters, the orientation parameter (P2) is ca. 0.6. Above 250 "C the global long-axis orientation is rapidly lost, although of course locally the material is aligned as seen in the birefringent textures.The detailed optical microscopy on the HBA/PET copolyester described by Viney et al.' shows that optically an isotropic component is not observed until temperatures above 350°C. On cooling, this component transforms back to a liquid-crystal phase. In fact Viney et al. were only able to retain an isotropic component at room temperature by extremely rapid cooling of very thin samples, and even then the isotropic component existed only at the edges of the slice. We have also performed broad-line n.m.r. spectroscopy on the melt-extruded pellets and the detailed results of this are recorded el~ewhere.~ We have measured the second moment of the absorption curve both as a function of temperature in the range -150 to 200 "C and as a function of y, the angle between the extrusion direction and the magnetic field vector of the spectrometer. The use of aligned samples is particularly useful for discriminating between the different possible motions in the material.5 Fig.2 shows some of the results of these experiments taken from ref. (4) and some more recent unpublished work.6 As the temperature increases from -150 "C, there is a general drop in the values of the second moments for all three y values shown. There is a more rapid drop for y = 45 and 90 " in the region of 50"C, which we may associate with the glass-like transition observed in the thermal-analysis curves. This 'motional' narrowing of the n.m.r.absorption peak arises from two sources. The first is the interchain proton interactions, which are anisotropic in n a t ~ r e . ~ The second, which is the more substantial contribution, arises from intrachain proton interactions. There are two types of protons in the HBA/PET chain which are, however, sufficiently widely separated in terms of distance to allow them to be considered as non-interacting independent units in relationship to the broad-line n.m.r. spectrum. Thus the protons may be separated into those of the phenyl group, and those of the -CH2-CH2- linkage. The interproton vectors within the phenyl units which make a significant contribution to the absorption spectra are parallel to the chain direction. If rotation of the chain, or of a particular phenyl group, occurs, since its rotation axis is parallel to the proton-proton vector, these rotations will not lead to any substantial motional narrowing.On the other hand, rotation of the chain or the -CH2--CH2- linkage will have a considerable effect upon the proton-proton vector, magnetic field vector angle, which will result in motional narrowing. Thus the reductions in the second moments with temperature are greatest for y = 45 and 90 ", and results from the loss of interchain interactions, and from the mobility of the ethylene linkage. This is not to say the motion of the aromatic rings does not take place, but simply it does not seriously affect the width of the absorption peak. In fact, model calculations4 show that at 200"C, the second moments approach the values which relate to the phenyl contribution alone, indicating considerable mobility of the ethylene com- ponent.That match also confirms that there is no significant motion of the chain about an axis not parallel to the chain direction. This is expected in view of the X-ray orientation measurements described above. In fact, if we cool the n.m.r. sample from 200 "C back down to - 150 "C, we recover the same second moments measured previously. This indicates that there has been no change in the orientation of the molecular chains, and that the reduction in second moments arose purely from rotations of segments or chains about the chain axes.GENERAL DISCUSSION 93 08 O oQoo 0 - 0.20 - 0.15 rn f - 0.10 .. - 0.05 0 B 0 I I I I , GI -100 0 100 200 T / "C Fig.2. Temperature dependence of the observed second moments (AH2) of the broad-line n.m.r. absorption peak for melt-extruded pellets of the random copolyester HBA/ PET (60/40) measured at y = 0" (0), y = 45 O (0) and y = 90 O (0). y is the angle between the extrusion axis and the magnetic field vector of the spectrometer. The right-hand axis corresponds to the measured mobile fraction f (D), which was independent of y. The measured molecular orientation parameter (from wide-angle X-ray scattering measurements3), { P2), is ca. 0.65. There is a further observation from the n.m.r. results which Prof. Lenz drew attention to, and which is displayed in fig. 2. I refer to the resolution-limited narrow component which appears in the absorption spectra above 130 "C. This is plotted as solid squares in fig.2, and these points refer to the right-hand axis of the plot, where f is the fraction of the absorption curve which relates to this narrow com- ponent. Prof. Lenz interpreted this narrow component as arising from an isotropic component, and hence f in his terms is equated to the fraction of the copolyester in the isotropic phase. I would like to propose an alternative interpretation which is more consistent with the optical microscopy, X-ray scattering analysis and dielectric measurements. Let us assume that the narrow component of the n.m.r. signal does relate to an isotropic component. Since, if we reduce the temperature, the narrow component is lost, it is clear within the above assumption that the isotropic phase transforms to a liquid-crystalline form.This re-formed liquid-crystal material would then have a range of preferred molecular orientations, since it will have lost the memory of the original melt-extruded direction. Under those circumstances we would expect a substantial change in the measured molecular orientation since we would have at least 20% of the material (fig. 2) randomly disposed throughout the sample. No such changes are observed either using quantitative X-ray analysis of the molecular orientation at the elevated temperatures or from n.m.r. or X-ray measurements made at room temperature or below. We suggest that the narrow component does not arise from an isotropic com- ponent. Instead, it is suggested that the narrow component corresponds to the increasing liquid-like motion of the ethylene linkage at temperatures above 130 "C, prompted by the relative freedom of motion of the -0-CH2-CH2-O- linkage.Note that for the 60/40 composition of the HBA/PET compolymer some 28% of the relevant protons reside in this linkage. It would seem that the strong interactions94 GENERAL DISCUSSION between the aromatic components maintain the long-axis orientation while the ethylene linkage has almost liquid-like freedom. This mobility facilitates changes in the rotational configurations of the chains (with respect to rotational correlations), and it is these rotation motions which are related to the mobility of the optical textures observed at ca. 190 0C.1,5 The high mobility of the ethylene linkages occurs within the framework of a liquid-crystal phase and simply reflects the configurational freedom of those subchains, despite a measure of constraint from their connection to the relatively static aromatic components.Thus it is not necessary to invoke an isotropic phase to account for the n.m.r. results reported here and el~ewhere,~ and this interpretation has the utility of being consistent with the other wide-ranging experimental data available. However, it is also appropriate to present a caution concerning the consistency of the material. The results reported here are all for material from one batch of the copolyester supplied by Eastman Kodak. There are obvious possibilities that the molar mass distribution, the distribution of monomer units within the chain (which for materials used in this study were shown to be r a n d ~ m ) ~ and the levels of impurities or side-products, may vary from batch to batch, and thus could account for variations in experimental results.' C. Viney and A. H. Windle, J. Muter. Sci., 1982, 17, 2661. Y. Takase, G. R. Mitchell and A. H. Windle, Polym. Commun., in press. G. R. Mitchell and A. H. Windle, Pol-vmer, 1983, 24, 1513. G. R. Mitchell and F. Ishii, Polym. Commun., 1985, 26, 34. V. J. McBrierty, Polymer, 1974, 15, 503. F. Ishii and G. R. Mitchell, unpublished work. ' G. R. Mitchell and A. H. Windle, Polymer, 1982, 23, 1269. Prof. R. W. Lenz ( University of Massachusetts) (communicated): I am grateful to questioners for their interest in my paper and respond to them as follows. ( 1 ) First Dr Jackson: I agree that the quantitative interpretation of the d.s.c.endotherm can only be applied to a copolymer which has its isotropization temperature well below its degradation temperature. Therefore, this approach would not apply to the 60/40 copolymer of p-hydroxybenzoic acid and ethylene terephthalate units which is referred to as X7G by the Eastman Kodak Company and which has a value of T well above 350 "C. However, we have other copolyesters containing both mesogenic and non-mesogenic groups which have Ti transitions well below their thermal decomposition temperatures. (2) Secondly Professors R. B. and A. Blumstein: In your application of n.m.r. spectroscopy and d.s.c. to the determination of the nematic phase within the N + I biphase region, could you not be observing two effects, the decrease in order within the nematic phase and the formation of the separate isotropic phase? The d.s.c.endotherm should contain information on both of these contributions. Similarly, for our copolymers, we believe that the visual observations of a non-birefringent phase with a polarizing microscope correlates with the smaller d.s.c. endotherm of the T, transition, and both indicate that we also have biphasic behaviour over the entire temperature range between T, and T for such copolymers. The anisotropic phase may also be of lower order in our copolymer systems, as you suggest, and that could also contribute to the smaller endotherms for copolymers containing increasing amounts of non-mesogenic units. With regard to your second point, I must say, no, we have only observed this effect on one sample of the polymer, which most likely had a molecular weight wellGENERAL DISCUSSION 95 below 10000.We intend to study the homeotropic orientation of these polymers much more carefully in the future. Finally in this section I confirm to Professor R. B. Blumstein that the polymers in this series in which n had values of 1 , 2 , 3 and 4 were prepared from pure glycols of those compositions, not from oligomeric glycols. You will note that we used the same aromatic ester triad for the mesogenic groups in this series as we did for the series with the polymethylene spacers. In the latter case we did not observe a simple change from nematic to smectic phase formation with increasing number of methyl- ene units.Instead, we observed an odd-even effect in which the polymers with n = 4 and 6 formed nematic phases, while those with n = 3, 5 , 7 and higher formed smectic phases. (3) Thirdly Dr Zentel: The degree of crystallinity of copolymers is determined by their chemical heterogeneity, which is apparently also responsible for the formation of two phases in our copolymers, which contain both mesogenic units and non-mesogenic units. That is, we suggest that the isotropic phase contains copolymer chains which have either larger amounts or longer sequences of non-mesogenic units. On this basis, your term ‘volume fraction of nematic phase’ and our term ‘degree of liquid crystallinity’ are not different in concept. (4) My response to Professor Samulski is the same as those above to Professor Blumstein and Dr Zentel.We are very much interested in doing exactly what you suggested, i.e. separating the isotropic and anisotropic phases for analysis. As Dr Jackson has pointed out, and as I mentioned in my presentation, in a sense that is what Dr Nicely has done by broad-line n.m.r. spectroscopy, which enables him to analyse the two phases independently. (5) Dr Griffin’s informal observation of the range of compositions in X7G is in agreement with Dr Nicely’s results, which I described and refer to above. The compositions of the isotropic and anisotropic phases, which he estimated by fast Fourier transform broad-line n.m.r. spectroscopy, differed considerably in oxyben- zoate content. The isotropic phase contained cu. 35 mol% oxybenzoate units, while the anisotropic phase contained ca.80molOh of such units. Dr Nicely intends to publish these results in the near future. (6) I turn now to Professor Flory’s remarks: I agree that crystallization is kinetically controlled, although by proper methods and data interpretation it is possible to estimate an equilibrium melting point if not an equilibrium degree of crystallinity. I also agree that the isotropization of a liquid-crystalline phase, and its re-formation from the isotropic phase, should be essentially equilibrium processes, and the lack of undercooling in deisotropization indicates that is so. However, as discussed above, we believe that the chemical heterogeneity of mesogen-non-mesogen copolymers can lead to the formation of a two-phase melt in which the anisotropic and isotropic phases can exist in equilibrium.(7) In response to Dr Coles: I would merely comment that I suggested the term ‘degree of liquid crystallinity’ in broad analogy to ‘degree of crystallinity’ to describe, in at least a semi-quantitative manner, a liquid-crystalline melt in which an isotropic phase can exist in equilibrium with a nematic phase, as discussed above for the reasons mentioned. I believe that the isotropic phase differs in composition from the nematic phase and not just in molecular weight, although the latter could contribute also. There is no doubt from96 GENERAL DISCUSSION our studies that such a biphasic behaviour can exist for certain types of copolymers, as indicated by a variety of experimental characterization methods, including polar- ized-light microscopy, d.s.c., broad-line n.m.r. spectroscopy, density, melt viscosity and small-angle light scattering.(8) I am grateful to Dr Windle for showing us his new results. We have also observed the presence of a second, apparently denser texture, region in samples of X7G by polarized-light microscopy. However, we could not see non-birefringent regions in the melt of this copolymer below 350 "C as we can with other copolymers containing non-mesogenic units. (9) Finally I turn to Dr Mitchell: I must emphasize that the broad-line n.m.r. results on X7G were not from our work but were given to me for review by Dr Vincent Nicely of the Research Division of the Eastman Kodak Company in Kingsport, Tennessee. However, in contrast to your conclusions, Dr Nicely was able to measure directly the composition of the narrow-line component, and he found that it contained substantial amounts of oxybenzoate units, as I discussed in my previous responses.Dr K. F. Wissbrun ( Celanese Corprution, N.J., U.S.A.) addressed Prof. Blum- stein: You report the interesting observation of the odd-even effect on both the order parameter and on the X-ray diffraction evidence for the occurrence of 'cybotac- tic domains'. I wonder whether this effect may not also extend to the rheology of your polymers. Looking at your fig. 2 (as well as at similar data that I have seen from your laboratory on higher-molecular-weight samples), it appears that the shape of the flow curve of DDA-9 shows a greater tendency to resemble the three-region behaviour proposed by Onogi and Asada than does that of the AZA-9 polymer. Also, the decrease in viscosity upon going from the isotropic phase to the mesophase is much greater for MA-9, which does not have cybotactic structure, than for DDA-9, which does.The question is then whether you think it likely that the same structural factors that lead to a high order parameter and to cybotactic domain structure may also be responsible for these apparent differences in rheology? I might add that we have observed in our laboratory some systematic differences in shapes of flow curves with variation of chemical structure of aromatic copolyesters, but have no explana- tion for these observations at this time. Prof. A. Blumstein (University of Lowell, U.S.A.) replied: Your question is very interesting.It is difficult to answer it from the data provided in our fig. 2 because the comparison here is made between unfractionated samples of DDA-9 and frac- tionated samples of AZA-9 of similar Mn. Very recent measurements of q*(o) over the range'l0 d O / O S lo2 were performed on fractionated samples of DDA-9.' The results'indicate that the high degree of shear sensitivity of DDA-9 shown in fig. 2 of our paper is due mainly to the polydispersity of DDA-9, albeit that a larger, shear dependence for DDA-9 is still present for fractionated samples. The viscosity difference between the isotropic and the nematic states is larger for DDA-9 than for MA-9 of similar molecular mass, and not vice versa. The shapes of the q * ( o ) curves for DDA-9 and AZA-9 appear very similar for 1 0 lo2.The conclusions from our very limited study of sharp fractions of AZA-9 and DDA-9 is that, taken as a first approximation, both polyesters behave in a similarGENERAL DISCUSSION 97 way. Small differences such as a increased shear sensitivity of DDA-9 and a lower viscosity increase at the N/ I transition are apparent. Whether these differences are significant enough to be due to the structural differences between DDA-9 and MA-9 can only be answered with many more measurements performed on well fractionated and well characterized samples. ' A. Blumstein, 0. Thomas and S. Kumar, J. Polym. Sci., Polym. Phys. Ed., in press. Dr H. J. Coles (University of Munchester) said: In table 1 of Prof.Blumstein's paper the biphasic range is given as a function of the length, n, of the flexible spacer. An interesting feature of these data is the odd-even alternation of both the width of the biphasic region and the order parameter, S,. A wide biphasic region is synonymous with a high order parameter, and this is an extremely important obsermtion for polymer liquid crystals. Does he have any further experimental evidence to link the width of the biphasic region in these materials to their distribution in size with changing n? Prof. A. Blumstein (University of Lowell, U.S.A.) replied: We do have some limited additional experimental evidence (p.m.r.) that even sharp fractions of DDA-9 ( n = 10) when compared with sharp fractions MA-9 ( n = 7 ) display a larger width of the biphasic region.A thorough exploration of this question is important and is under way in our laboratory. Prof. E. L. Thomas (University of Massachusetts, U.S.A.) said: It is interesting to note the appearance of both smectic and nematic order in the various flexible- spacer thermotropic liquid-crystalline polymers materials such as those synthesized by Profs. Blumstein and Lenz. Various researchers have now reported the dependence of liquid-crystal type on such variables as spacer length and temperature. As is indicated in the published work of Dr Noel,' the identification of the type of liquid-crystal phase of a polymer is a difficult task. Moreover, the papers at this Discussion by Windle et al. and by Blackwell et al. underscore the need for carefully addressing the degree of axial shift ciisorder in liquid-crystalline polymers.There- fore, I would like to emphasize a point here that is made in our later contribution, namely a continuous spectrum of liquid-crystal states of increasing axial registration are possible between the axially disordered nematic and axially ordered smectic states. Fig. 3 of our contribution presents idealized scattering from a nematic, a smectic and a hybrid liquid-crystalline structure with partial axial registration over finite lateral regions. One may view a general smectic polymer structure as one in which the direct correlation2 between chains is given by two terms: (i) a liquid-like correlation in the lateral direction and (ii) a finite (non-zero) correlation in the chain axis direction.I.e. the structure could be treated as two-dimensional liquid in the lateral direction and a one-dimensional crystal in the axial direction. If the correlations in the lateral and axial directions are independent, then the direct correlation function C(r, z) may be given by a product of the lateral and axial correlations in which the lateral correlation is, for example, the hard-disc correlation function CHD(r), and the chain axis correlation is a gaussian distribution of finite width A, analogous to the type I1 disorder in crystals.3 Thus98 GENERAL DISCUSSION Fig. 3. Schematic intensity distribution in normal-beam photograph of cybotactic nematic (after de Vries). The magnitude of A determines the nature of the phase, with A = O the ideal smectic and A = a0 the ideal nematic as limits to the continuous distribution. ‘ C.Noel et al., Polymer, 1984, 25, 1281. * As defined by L. S. Ornstein and F. Zernike, Roc. Akad. Sci. (Amsterdam), 1914, 17, 793. Y. Cohen, R. Saraf and E. L. Thomas, Mol. Cryst. Liq. Cryst., in press. Prof. A. Keller ( University of Bristol) commented: Prof. Thomas has just presen- ted a classification of the different states of mesomorphic order to rationalize some of our present definitions. I am puzzled as to where the ‘nematic cybotactic’ fits in. Or rather, why is it necessary to invoke a category under this name? I would have thought that by Prof. Thomas’ criteria the ‘nematic cybotactic’ would fall in the appropriate subgroup of the smectic family. Prof. E. L. Thomas ( University of Massachusetts, U.S.A.) responded: The charac- teristic feature of the so-called cybotactic nematic structure is the set of four low-angle reflections tilted with respect to the molecular axis in the X-ray scattering pattern from a uniaxially aligned sample (see fig.3). deVries’ recorded such patterns from small-molecule liquid crystals and proposed that the molecules are organized into groups which have all the ends of the molecules lying in planes at a large angle to the molecular axis. Azaroff2 criticized de\-’ries’ model and proposed instead a structure consisting of parallel sheets of tilted layers of molecules. I believe Prof. Blumstein’s cybotactic nematic structure is a version of the deVries model adapted for rigid/ flexible thermotropic liquid-crystal polymer molecules.All these proposed models could be classified in the family of smectic liquid-crystal structures. ’ A deVries, Mol. Cryst. Liq. Cryst., 1970, 10, 219. L. V. Azaroff, Roc. Natl. Acad. Sci. USA, 1980, 77, 1252. Prof. A. Blumstein (University of Lowell, U.S.A.) said: I also would like to respond to Prof. Keller because he posed a good question bearing on the discrepancy between the mesophase assignment based on crystallographic models and miscibility experiments accepted as criteria for mesophase assignment. There is definitely a difficulty to overcome in reconciling the model of a nematic cybotactic suggested by the SAX diffraction pattern’ and the miscibility criterion put forward by Arnold and Sackmann.2 Cybotactic nematic has been so named because of its complete miscibility with other nematic systems and only partial miscibility with smectic A or smectic C systems.The polymer DDA-9 is totally miscible with AZA-9 and PAA, which are classical nematics. ’ A. deVries, Mol. Cryst. Liq. Cryst., 1970, 10, 219. * H. Arnold and H. Sackmann, 2. Phys. Chem., 1960, 213, 137.GENERAL DISCUSSION 99 Prof. F. C. Frank (University of Bristol) said: The lateral correlation length in a smectic is in any case in principle non-infinite. A non-infinite length may be long or short, but that affords no sharp distinction. The cybotactic nematic structure, as described, falls within the class smectic C, and is not a sub-class of nematic structure. Perhaps it is useful to have some way of saying it is nearly nematic, but for purposes of phase classification I think ‘cybotactic nematic’ is a term which can be dispensed with.Prof. E. T. Samulski (University of Connecticutt, U.S.A.) said: Dr Yoon’s paper marks a significant step toward an understanding of the conformational constraints imposed on semiflexible polymers in liquid-crystal phases. However, I do have some reservations about the ability of n.m.r. data to confirm unambiguously that the spacer conformers of odd-parity alkyl chains are limited to those that place every second bond exclusively in the trans state. While the authors give thermody- namic arguments consonant with these conclusions, I am uncomfortable with the highly constrained spacer proposed for this fluid state. Could Dr Yoon alleviate some of my discomfort by commenting on the following: The ‘linewidths’ (ca.7 kHz) of the polymer deuterium n.m.r. spectra (fig. 7 of the paper) are significantly broader than those encountered in monomer and dimer liquid crystals and there would appear to be evidence for unresolved quadrupolar splittings on the inner edges of the spectra. Note that spectra of the structurally related dimer and the same polymer mixed with a monomer liquid crystal show resolved splittings which would be masked by the reported linewidth (see fig. 4 of my Introductory Lecture). Such splittings, if present, would imply that the averaging of the C-H bond vectors relative to the nematic director is not the same at each methylene in the spacer, and thereby imply the existence of less severe conformational constraints, i.e. a gradient of spacer conformational freedom along its length. Prof. R. B. Blurnstein (University of Lowell, U.S.A.) said: It is apparent from fig. 7 and 10 in Dr Yoon’s paper that all the n.m.r. spectra were traced in the nematic-isotropic biphasic region. P.m.r. measurements carried out in the N + I biphase of polymer IV’-2 show that the temperature dependence of the mesogen order parameter is more pronounced in the biphasic region than it is in the homogeneous nematic phase. In contrast, Dr Yoon reports a nearly constant orientational order parameter of ca. 0.8 in the biphasic region, and I wonder whether he would comment on the seeming discrepancy between his data and our data on mesogen order in the N + I biphase. Also, both d.m.r. and p.m.r.data on alkyl chain flexibility in the homologous series described in ref. (3) indicate that in the homogeneous nematic phase the spacer disorders more rapidly than the mesogen. R. B. Blumstein, E. M. Stickles, M. M. Gauthier, A. Blumstein and F. Volino, Macromolecules, 1984, 17, 177. * F. Volino, J. M. Alloneau, A. M. Giroud-Godquin, R. B. Blumstein, E. M. Stickles and A. Blumstein, Mol. Cryst. Liq. Cryst., (Lett), 1984, 102, 21. A. Blumstein, M. M. Gauthier, 0. Thomas and R. B. Blumstein, Faraday Discuss. Chem SOC., 1985, 79, 33, and references therein. Dr D. Y. Yoon (IBM, U.S.A.) (communicated): The observation of Prof. Samulski concerning the asymmetric profile of our peaks in fig. 7 is correct. As he suggests, this implies the presence of less extended conformers.However, the extent of deviation to produce this asymmetric profile is small and thus does not affect the main conclusion of our paper. Concerning the seeming discrepancy between our100 GENERAL DISCUSSION results of the order parameter and those of Prof. R. Blumstein’s group, it is likely that both the temperature dependence of the order parameter near the istotropic - nematic transition and the magnitude of the order parameter are highly dependent on the molecular weights of the polymer. In this regard, I note that the average molecular weights of the samples used by Prof. Blumstein’s group (for p.m.r. measurements) are rather low. Dr R. Zentel (University of Mainz, West Germany) said: Dr Windle and his colleagues have found evidence for the existence of a biaxial orientation in the partially crystalline material, which is not surprising, because a lot of crystals are optically biaxial.However, they give no direct evidence for a biaxial orientation in the nematic phase, except for the conservation of the texture and of the orientation of the sample as I can see it. In my opinion this may be a simple paramorphism, whilst a biaxial nematic phase means that the rotations around the long axis of the liquid-crystalline polymers are still strongly restricted. This has nothing to do with a conservation of the orientation of the long axes. I would thus like to ask whether they have more evidence for a biaxial nematic phase. Dr G. R. Mitchell ( University of Reading) (communicated): Dr Zentel has raised doubts concerning the interpretation of the optical microscopy/ X-ray diffraction observations on rigid-chain thermotropic copolyesters presented in our paper and in particular, the relationship of the observations made at room temperature and their extension to the liquid-crystal phase.I would like to draw attention to the following points. The X-ray scattering patterns of these copolyesters recorded at room temperature (fig. 2 of our paper) show no evidence of crystallinity. Some results have been interpreted by others [ e.g. ref. (l)] as indicating a low level of crystallinity (ca. 10% ), although that figure and its derivation may also be open to question. However, even if that were the case, it does not offer a satisfactory and consistent explanation of the results presented in our paper and elsewhere.273 The optical textures observed are the same in their appearance throughout the sample; there are no features which may be interpreted as crystallites.The optical textures observed for the copolyesters upon heating through the softening point remain the same, other than it becomes m ~ b i l e . ~ If the optical textures and their biaxial properties arise from some crystal- linity as supposed by Zentel, then we would expect some particular change in the optical properties when these ‘crystallites’ melt out. No such change is ob~erved.~ Thus, even if there is at room temperature a small fraction of crystallites, they have no significant effect upon the optical observations and their interpretation. In that case, the biaxial optical properties of the copolyester samples at room temperature arise from an optically biaxial non-crystalline structure.The correspon- dence between optical textures and other physical properties above and below the softening point provide considerable evidence that the room-temperature structure is essentially a static version (frozen in) of the mobile liquid-crystal phase. It has been shown that for very thin samples it is even possible to quench in the isotropic phase observed at 420 “C for one of the copolye~ters.~ D. J. Blundell, Polymer, 1982, 23, 359. * C . Viney, G. R. Mitchell and A. H. Windle, Polym. Commun., 1983, 24, 145. C. Viney, G. R. Mitchell and A. H. Windle, Mol. Crysf. Liq. Cryst., in press. C. Viney and A. H. Windle, J. !dater. Sci., 1982, 17, 2661.Dr A. H. Windle ( University ofcambridge) said: What is the evidence for biaxial properties actually in the melt phase? Although many of our photographs have been taken at room temperature (one cannot use the highest-resolution objectivesGENERAL DISCUSSION 101 on the hot stage), there is an underlying observation that there is no change in microstructure at the solid-liquid transition. The lack of correlation between chain orientation and optical orientation, which is an important aspect of the case for biaxiallity, can still be seen above the solid-liquid transition, although in the case of polymer I (B-N) of our paper, the X-ray orientation relaxes over a minute or so. However, polymer 11, (B-ET), when held above its melting point of 190°C but below 250°C, retains X-ray orientation for several hours.In this case the fine Schlieren texture is mobile on a timescale of a few seconds even though the pre- ferred orientation of the chains has decayed little. This is shown in plate 2. We see these observations as yet further evidence for the lack of correlation between the optical extinction directions and the chain axes, and thus for biaxiallity. Another striking piece of evidence relevant to the same issue has recently been obtained by T. J. Lemmon using the microscope hot stage. Plate 3 is a series of micrographs of one of the phases of polymer I1 (B-ET). The included phase shows a fine Schlieren texture which is stable up to ca. 240"C, above which it begins to transform into a much simpler radial texture.The significant aspect is that the transition is reversible and, on cooling, the simple radial texture changes back into the fine Schlieren network. We were not successful in quenching-in the high-temperature form. We have here a reversible transition between two birefringent liquids. We believe the higher-temperature phase to be uniaxial nematic, which transforms to biaxial nematic on cooling, the development of the fine texture being a direct consequence of the reduction of symmetry. It is reminiscent of 'transformation' or 'multiple' twinning seen in crystalline materials at transitions such as cubic to tetragonal. In his oral question Dr Zentel also asked how we can make deductions on the hgstrom scale from observations made in the light microscope. Given that there is a priori evidence for molecules, then aspects of their behaviour can be inferred from macroscopic observation of physical properties.For example, birefringence can tell us about the first harmonic of the orientation probability function about a symmetry axis, ( P2), and discontinuities in optical properties can indicate rapid changes in orientation on a molecular scale. Furthermore, any observation of macroscopic symmetry will bear important information regarding the local correla- tions of molecular orientation. C . Viney, G. R. Mitchell and A. H. Windle, Mol. Cryst. Liq. Cryst., 1985, 129, 75. Dr K . F. Wissbrun (Celanese Corporation, N.J., U.S.A.) (communicated): Dr Windle's observations of the effect of temperature upon the timescales for microstruc- tural mobility and for loss of global orientation for the B-ET polymer are consistent with the effect of temperature upon the melt rheology of this polymer.' Above the ca.240°C d.s.c. endotherm the melt appears to be typically viscoelastic in nature, albeit with an unusually large relaxation time. However, below this endotherm the melt becomes progressively more solid-like, and eventually exhibits a yield stress. In agreement with Dr Windle's interpretation, we also believe that the 240-250 "C transition is the melting of small crystallites. X-ray diffraction data of fibres spun, annealed and measured at various temperatures support this interpretation and suggest that the crystalline phase is that of the hydroxybenzoate species. The presence of residual crystallites in the melt also explains qualitatively the change in rheology and activation energy of melt viscosity with temperature, and the pronounced effect of thermal history upon the shape of the flow curve and the magnitude of the viscosity.As to the B-N polymer, I would merely note that between ca. 280 and 310 "C there is also a pronounced discontinuity in the activation energy of the melt viscosity. Thermal-history dependence of the melt rheology suggests that the cause of this102 GENERAL DISCUSSION transition is, as in the B-ET polymer, melting of residual crystallinity. However, in this case we have no direct structural observations to support the conclusion. K. F. Wissbrun, Br. Polym. J., 1980 12, 163. Dr A. H. Windle (University of Cambridge) (contributed): Our data relating to timescales appear as plate 2 of this Discussion in our response to Dr Zentel's question. Certainly, d.s.c.traces on B-ET polymers of different compositions show that the small 250 "C endotherm appears and intensifies with increasing HBA content, and I agree it is tempting to associate it with minute reinforcing crystallites of this component. Regarding B-N, we find that the melting transition gives a broad endotherm at ca. 280°C for the 70/30 material, so we are not very sure that one should expect the material to be completely molten below 310 "C. There may well be a few residual crystallites of greater than average perfection still present at 3 10 "C. Prof. F. C. Frank (University of Bristol) said: I .am willing to agree that Dr Windle and his colleagues present evidence making it plausible and even persuasive (although not conclusive) that they have here examples of biaxial nematics: the strongest evidence, perhaps, being that if that is the case it helps one to understand the remarkable lack of observable correlation between chain orientation and optical orientation.On the other hand, I see no force in the quantitative interpretation and analysis of the optical properties put forward by these authors. Their analysis is dependent on two assumptions, one explicit and the other implicit. The implicit assumption is that orientation is uniform through the thickness of the specimen: in view of the small scale on which the optical pattern varies laterally, that is not very likely. The explicit assumption is that the chain orientation coincides with a principal axis of the optical indicatrix.I see no good reason for making that assumption, save when the indicatrix is uniaxial: both in B-N and in B-ET the lack of symmetry axes along the chain in any of the chain constituents makes that improbable (although pairing to produce an effective diad axis is not impossible). Consequently I place no reliance at all on the deduction of the angle between optic axes. Dr A. H. Windle (University of Cambridge) said: Sir Charles Frank points to two assumptions in our study of optical microstructures. The first concerns through-thickness variations in structure where the scale of the texture is of the same order as the thickness of the sample. Our optical analysis aims to relate the extinction orientations (crossed polars), observed at any position, to the principal axes of the in-plane section of the indicatrix representing the local optical properties.For this approach to be meaningful we need to assume that the optical orientation of any resolvable portion of the microstructure can be described by a single indicatrix. In terms of the samples which might be examined, there are obviously two extreme cases. In one, the presence of 'macro domains', within which the indicatrix orientation is constant, and having lateral extent much greater than the specimen thickness, would enable the assumption to be made without qfiestion. In the other extreme one can imagine the indicatrix orientation changing on a scale very much smaller than the specimen thickness.Light traversing the specimen will therefore sample a number o'f different indicatrix orientations, and it will be imposs- ible to relate its subsequent polarization state to any particular orientation within the microstructure. The issue seems to boil down to this: are we entitled to relate a fine-scale Schlieren texture such as that of plate 4 (thickness 2 pm) to particular variations in the optical orientation across the sample? We believe that the specimens are sufficiently thin for them to approximate satisfactorily to two-dimensional sections for the purposesGENERAL DISCUSSION 103 of our arguments. The criteria by which we assess that a section is sufficiently thin are as follows. ( a ) Every portion of the sample under examination shows full extinction at some rotation angle between the crossed polars, and this extinction is repeated with an angular period of ~ / 2 .Overlaid layers of different optical orientation will in general preclude full extinction at any rotation angle. (This aspect of the optical behaviour is discussed in greater detail by Dr Viney in a written communication below). ( b ) The dark regions of the microstructure form a resolvable pattern, which in the case of Schlieren textures, consists of dark bands which locally scan the specimen as the sample is rotated. Accord with both these conditions is lost when the sample thickness is increased in relation to the scale of the microstructure. Plate 5 shows a sample, melted on a glass slide, which changes abruptly in thickness from 2 to 8 pm.Similarly, plate 6 is a series of cut sections of increasing thickness prepared by Dr Viney. The loss of both ‘connectivity’ of texture and of contrast as the thickness is increased is apparent between crossed polars. Furthermore, the thicker samples begin to show contrast without the polars being present. This is presumably the result of selective loss of optical information at the back focal plane of the microscope, leading to phase contrast, this being more apparent in the thicker samples since they allow for a greater variation in phase from position to position. The second assumption concerns our estimate of the angle between optic axes. Our calculations of the relative proportions of dark areas visible in circularly polarized light, as a function of section angle [plate 3 ( c ) of our paper] for the biaxial cases ( i e .p # 0) are for the special geometry in which one principal axis of the indicatrix is parallel to the chain axis. We have not yet treated the general case, which for a biaxial indicatrix would require a further term (P,) in the product in the equation for P (page 62 of our paper). The term would be the magnitude of the harmonic function at x, which is the angle between the largest principal axis of the biaxial indicatrix and the local chain axis. We accept Sir Charles’ caveat on this point. Dr C . Viney (University of Cambridge) (communicated): Sir Charles Frank has drawn attention to the problems involved in interpreting textures in specimens whose thickness may be greater than the lateral distance over which molecular orientation changes significantly.In other words: can one be certain that the specimen is thinner than a ‘domain’ size, so that molecular orientation is roughly constant through the specimen thickness at any point? If, during microscopic examination of the specimen, light does follow paths along which the optical orientation changes, individual regions in the specimen will not show extinction between crossed polars. Either one can treat such paths as consisting of a series of layers having discrete optical orientations, or one may have to regard the optical orientation as varying continuously and sufficiently gradually for optical guiding to occur. In the former case, whatever the direction of polarisation of the wave incident on the bottom of the specimen, the light will always have been resolved into two vibration directions by the time that it reaches the top.Thus, the light emerging from the top surface of the specimen cannot be extinguished by an analyser, whether crossed relative to the polariser or not. The overall contrast in the observed texture will therefore be diminished, with individual areas showing four intensity minima and four intensity maxima per 360” rotation of the crossed polars.’ (One could of course obtain extinction in monochromatic light if the path difference of the two emergent waves corresponded to a whole number of wavelengths in the specimen. However, all our microscopy was performed in white light.)104 GENERAL DISCUSSION If, on the other hand, optical guiding is occurring, the vibration directions of the light change continuously as it travels upwards through the specimen.In this case, extinction can always be achieved by uncrossing the polars, provided that the incident light is polarised parallel to a vibration direction at the bottom of the specimen. However, extinction between crossed polars can only be observed in the particular instances when the optical orientation of the top of the specimen differs from that of the bottom by a multiple of 180". If sufficiently thin sections (G2 pm) of our specimens are viewed between crossed polars, it is observed that individual areas show 4 extinctions per 360" rotation of the crossed polars, so one can be confident that there is no significant change in optical orientation through the specimen thickness in this case.A question related to that asked by Sir Charles Frank is whether light, in passing through our thin specimens, encounters optical discontinuities (refractive-index changes) which are severe enough to deviate light from travelling parallel to the thin direction of the specimens. If this were the case, some areas of the specimen might appear dark simply because there is no light emerging there. However, the light which was refracted away from the specimen normal must be expected to emerge somewhere else. After being deviated from its original path, it will have encountered sections of optical indicatrix which are not representative of the specimen plane. Such light has in effect followed a path along which changes in optical orientation have occurred.Therefore, the area where it emerges will not show extiiiction, in accordance with the arguments given earlier. Our thin specimens (e.g. plates 1 and 2 in our paper) do not contain any areas which never show extinction. The textures should therefore be a direct and reliable indication of the optical orientation in these specimens. ' C. Viney, G. R. Mitchell and A. H. Windle, Mol. Cryst. Liq. Cryst., 1985, 129, 75. Prof. F. C. Frank (University of Bristol) (communicated): To Dr Viney: Resolution of a wave into two conjugately polarized waves (e.g. two plane-polarized waves with crossed planes of polarization, or two oppositely cir- cularly polarized waves, or waves in any other two states of polarization represented by diametrically opposite points on the Poincark sphere) is not a physical phenomenon, but a mathematical operation performed by the physicist to help him understand the physics.To Drs Viney and Windle: All possible states of plane polarization for the input light are represented on the Poincari sphere by points on the equator, and if the polarizer is turned at uniform angular velocity the representative point travels round the equator at twice that angular velocity, making an excursion of 360" while the polarizer rotates through 180". The effect on the light of a plane-parallel slab of birefringent material, of thickness h, and strength of effective birefringence Anl, is to rotate all representative points on the Poincark sphere by an angle w1 (proportional to hlAn,) about an axis represented by the unit vector I , .I , lies on the equatorial plane: +Il corresponds to the major and -Il to the minor principal axes of effective birefringence. The resulting states of polarization are represented by a great circle inclined by w1 to the equator, and intersecting the equator at two points, representing plane polariz- ation: one at + I , corresponding to the input polarization and the other at - l , , crossed with respect to the input polarization. For uniform rotation of the polarizer the representative point travels uniformly round the inclined great circle, with twice the angular velocity as before.GENERAL DISCUSSION 105 A second birefringent layer effects a second rotation w2 about a second axis 1,.The orbit of representative points becomes another inclined great circle, around which the representative point travels uniformly for uniform rotation of the polarizer. However, if 1, differs from ll the orbit no longer crosses the equator at either of the points lI or 12. With any further increase in the number of birefringent layers, each effects successive rotations which transform the great circles into great circles and preserve the uniformity of orbital motion around them. The orbit therefore always crosses the equator at two points (unless it totally coincides with the equator). The crossing points are 180" apart on the equator, and also on the orbit. There are thus in any 180" rotation of the polarizer two settings producing plane- polarized light, and therefore capable of being extinguished with an analyser.These polarizer settings are 90" apart, and so are the corresponding analyser settings for extinction; however, the polarizer and analyser settings are in general not 90" apart, in distinction from the case of the single homogeneous birefringent layer. The theory applied here is not exact, making no allowance for back-reflected waves, but the italicized results are topologically robust, and are insensitive to small errors: they disagree with the second sentence of point ( a ) in Dr Windle's response. Let the rotations (1, w ) be represented by their Rodrigues vectors R = 1 tan ( w / 2 ) . Suppose that in a particular case both they and their cumulative resultant rotations are small (note that this does not preclude the angle between different axes li and l j being large).Then since the rule of combination of Rodrigues vectors representing successive rotations is the correction terms for non-commutativity remain of the second order of smallness. Since all the vectors Ri lie in the equatorial plane, then within second-order errors so does their resultant. The orbit becomes moderately inclined to the equator, a first-order effect, but there is negligible displacement of points around the orbit. This behaviour is indistinguishable from that of a single birefringent layer, giving extinction at the appropriate two settings in a 180" rotation between crossed polarizer and analyser. Here we have an important class of examples of inhomogeneous structures which would pass the test which Windle and Viney regard as a test for homogeneity.The theory employed above is a single-beam theory. If beams traversing different paths in the specimen are observed together they will not in general reach extinction conditions simultaneously, and may not have the necessary coherence to produce a joint extinction. When the observation is made microscopically beams in several directions are necessarily employed. If the inhomogeneity is not in the form of parallel layers as assumed above we shall have refractions with ordinary and extraordinary rays following deviant paths; scattering can give rise to more widely deviant independent paths by which light reaches a particular point in the image. Minima of intensity rather than perfect extinctions are therefore always to be expected: generally speaking, the more so the thicker the specimens.Dr C: Viney and Dr A. H. Windle ( University of Cambridge) (contributed). First, is the presence of two waves having crossed planes of polarisation a mathematical convenience or a physical reality, when light travels through a linearly birefringent medium? The only simple plane-polarised harmonic waves which can describe an optical disturbance within an anisotropic medium are those for which the wavevector106 GENERAL DISCUSSION k, the electric field vector E, and the electric displacement vector D are eoplanar. ( D is normal to k.) These conditions are met for two orthogonal orientations of D which are parallel to the semi-axes of the appropriate section of the optical indicatrix.We are persuaded that these two waves are physically distinct by the phenomenon of double refraction seen so clearly in calcite cleavage rhombs. Although the k vectors of the two waves are identical, their E vectors are in general not. In one case (the ‘ordinary’ ray), E is collinear with D ; in the other (the ‘extraordinary’ ray), it is not. Because the ray propagation direction is normal to E (it is in fact along E X & the direction of Poynting’s vector), the two simple harmonic waves are each identified with an experimentally distinguishable ray. They can each be extinguished separately with an analyser. Through the elegant application of the Poincark sphere representation, Sir Charles Frank has demonstrated that, for specimens consisting of a succession of birefringent layers, there are two distinguishable settings of the polariser, 90” apart, for which light after transmission is capable of being extinguished by the analyser; the analyser settings necessary for extinction are in general not crossed relative to the polariser.It follows that extinction will, in general, not be obtained in multi-layer specimens observed between crossed polars at any setting of the crossed polars relative to the specimen. This is exactly what was said in our previous comment (A. H. Windle replying to F. C . Frank) in setting down the first criterion ( a ) for testing through-thickness uniformity of our specimens. There is another point we should make here. Our observations have been made in white light, and our extinction criterion is for that condition.Sir Charles’ result is for monochromatic light. Since the retardation introduced by individual birefrin- gent layers depends on the wavelength of light passing through it, the final orbit of representative points will cross the equator of the Poincark sphere at opposite points of a diameter whose azimuth depends on the wavelength of light used. In white light, for a multi-layer specimen, the colour components will extinguish at different settings of polariser and analyser. If a specimen were to have the particular characteristics of a series of infinitesimally thin birefringent layers, which both separately and cumulatively are represented as small rotations of the Poincari sphere, the polariser and analyser settings required to produce extinction will be practically the same for each wavelength.The specimen will then indeed behave as a single birefringent layer,’ as Sir Charles points out. However, we have observed birefringence of up to 0.1 in the B-ET and B-N copolyesters; such a value is especially high for a polymer. We would not expect a model based on small rotations to be appropriate in this case. Finally, we appreciate the concern that ordinary and extraordinary rays (or, in the general case of an optically biaxial region, the separate extraordinary rays) may follow significantly deviating paths. For light incident at a given angle on a birefrin- gent layer, the angle between the two transmitted rays can be calculated from the geometrical properties of the indicatrix, if the principal refractive indices are known.Given a maximum birefringence of 0.1, one can make a rough estimate of the relevant principal refractive indices in a B-ET domain by interpolation from data2 for PET fibres. We estimate that the maximum angular deviation between the two transmitted rays is of the order of 3”. Thus we do not expect them to experience significantly different refractions. Also, after travelling through 2pm of sample, their spatial separation resolution is only of the order of 0.1 pm, which is below the resolution limit of the microscope. F. Pockels, Lehrbuch der Krisralloptik (Teubner, Leipzig, 1906). D. W. van Krevelen, Properties of Polymers (Elsevier, Amsterdam, 1976), p. 320.GENERAL DISCUSSION 107 Prof. A. J. Leadbetter ( Rutherford - Appleton Laboratory, Chilton) said: A biaxial nematic must have an intrinsically anisotropic diffraction pattern in the equatorial plane. Has Dr Windle observed this? The observation of a diffuse scattering peak at slightly higher scattering vectors than the first (hkO) Bragg peak from the average hexagonal packing of long molecules is commonly observed in diffraction patterns of smectic B phases of thermotropic liquid crystals.In this case it arises from a local herringbone type of packing with lower symmetry than hexagonal. Could this be the explanation of similar observa- tions in the experiments reported by Dr Windle and Prof. Blackwell? Dr A. H. Windle (University of Cambridge) said: Prof. Leadbetter suggests that a diffraction pattern obtained with the beam parallel to the chain axis would be useful in confirming biaxiallity. We agree that it is an important experiment, but the difficulty lies in the making of a large enough monodomain sample of suitable orientation.Also, if such an experiment showed no clear evidence of non-axial packing, one would still have to be very careful in ruling out biaxiallity. Long-range rotational correlation about the chain axes may well contribute to biaxial optical properties, without necessarily disturbing the chain packing significantly from a two-dimensional liquid-like correlation. The pattern would be intrinsically anisotropic, but the anisotropy may not be very apparent. Dr G. R. Mitchell (University of Reading) said: I would first like to address my comments to the remarks of Prof.Leadbetter, relating to the desirability of generating a monodomain in the biaxial nematic phase of the copolyesters described in our paper. Naturally the availability of a monodomain sample, or indeed a sample with domains larger than ca. 1 p m observed at present, would be particularly useful for conoscopic imaging. The viscosity of these rigid-chain systems would appear to preclude the effective use of electric and magnetic fields, although attempts have been and will continue to be made. The use of a mechanical field was also used to attempt some realignment of the ‘domains’. The material used was the Eastman Kodak X7G copolymer of hydroxybenzoic acid and poly( ethylene terephthalate), and was in the form of a melt-extruded rod. X-ray scattering analysis showed the sample had a high global orientation (P2) value of ca.0.7,’ while X-ray scattering patterns taken with the X-ray beam along the extrusion axis contained a uniform distribution of intensity along a ring, indicating no preferential alignment transverse to the chain direction.’ A sample of that extrudate was compressed normal to the extrusion direction. Plate 7 shows the X-ray scattering pattern obtained when the X-ray beam is parallel to the original extrusion axis. The compression axis is horizontal. The scattering observed arises from spatial correlations between chain segments (ca. 5 A). The scattering pattern clearly exhibits some anisotropy, which might as a minimum be interpreted as indicating some structural unit which is anisotropic transverse to the chain direction, and thus responds to the mechanical field.However, the interpretation of this result is not so straightforward on two accounts. The first relates to the effect of some misorientation of the long axis of the chains in the starting sample. The effect of the lateral compression will also affect those chains, which will also result in an anisotropic scattering pattern similar to plate 7. However, even if it were possible to obtain a large monodomain, it is not clear that the scattering pattern obtained would exhibit an orthorhombic or other ‘biaxial’ packing arrangement. The rotational correlations in a biaxial nematic phase result from the presence of some transverse anisotropy, which may be repulsive or shape-related, or due to attractive interactions. These attractive correlations108 GENERAL DISCUSSION might, for example, arise from dipole-dipole interactions of the ester units.The X-ray scattering observed at s == 1.5 A-1 arises principally from correlations between aromatic rings (the greater proportion of the sample). The correla- tions between esters may be maintained in the absence of significant rotational correlations between aromatic rings. Thus rotational correlations could exist without the need for some ‘biaxial’ packing arrangement of the chains as a whole. ’ G. R. Mitchell and A. H. Windle, Polymer, 1983, 24, 1513. Prof. J. Blackwell (Case Western Reserve University, U.S.A.) said: I would like to comment on the paper by Dr Windle on a topic that is not discussed in our paper, but in which we have considerable interest.The X-ray patterns of the melt-spun fibres show evidence for ordering of the copolymer chains. Bragg maxima on the equator point to hexagonal packing, and the presence of an off-equatorial at d = 3.1 A shows that there is three-dimensional order, i.e. some of the chain sequences are in register rather than randomly staggered as in a nematic structure. Dr Windle has suggested that this three-dimensional order is due to registration of short, identical sequences. In the example that he gives, he discusses the probability of the formation of crystallites ca. 3 0 8, in width by association of pentamer sequen- ces, and concludes that this is a conceivable mechanism for the development of order. The problem with this explanation of the ordering is that it underestimates the size of the ordered regions.Our measurements of ‘crystallite size’ by application of the Scherrer equation to the equatorial peak widths lead to lateral dimensions of ca. 90 A for the as-spun copoly(HBA/HNA) fibres, and this dimension increases on thermal annealing. These observations point to the existence of domains consist- ing of ordered arrays of at least 300 chains. Blundell’ has reported a ‘degree of crystallinity’ of ca. 21 ‘/o for copoly(HBA/HNA; 40/60). Whatever the exact mean- ing of this measurement for this type of copolymer, it is clear that a significant fraction of the chains is involved in formation of ordered regions. Given the large number of identical sequences that would need to be brought into register, it is unlikely that this can explain the development of order.Our view is that the order is developed by lateral aggregation of random sequences. The homopolymers poly( HBA) and poly( HNA) probably pack in a similar manner, and these structures may be able to tolerate a significant number of the other monomer as defects while still retaining the basic homopolymer packing. The distortion necessary to accommodate different monomers will be appreciable, but not all that great in view of the similarity of the HBA and HNA structures. The fact that the residue lengths are in the approximate ratio of 3 : 4 may in itself lead to significant register between totally random sequences. It is possible that there is some degree of segregation such that the ordered regions consist of HBA-rich or HNA-rich sequences, but at present there is no physical evidence for this.D. L. Blundell, Polymer, 1982, 23, 359. Dr D. J. Blundell (ICI, Wilton) said: I would like to make some comments relevant to papers by Dr Windle and Prof. Blackwell concerning the nature of the three dimensional ordered regions in thermotropic copolyesters of the form:GENERAL DISCUSSION 109 35 30 25 20 15 10 2 e p Fig. 4. WAXS diffractograms for polymer with x = 0.4 (see text). ( a ) 20, ( b ) 230 and (c) 300 "C. These comments are based on data that were used as a basis for a previous publication.' Fibres which are melt-drawn from this family of polymers show broadly similar X-ray patterns. As shown in Prof. Blackwell's paper the main variation with composition x is in the meridional reflections. The insert in fig.4 shows a tracing of an X-ray photo of a high orientation fibre from a polymer with x=O.73. There is also diffuse background diffraction on the equator which is not shown on the tracing. For samples with lower orientation the sharp diffraction spots spread into arcs. The WAXS diffractograms shown in the main part of fig. 4 were obtained from a near randomly oriented sample of polymer with x=O.4. The sample was prepared by loading small pieces of polymer into a 2 mm diameter thin-walled glass tube which was then mounted in a transmission hot stage. The WAXS scan at room temperature shows two sharp diffraction peaks superimposed on an amorphous background. The peak at 28 = 20" corresponds to the main equatorial reflection in the fibre pattern, while the shallow reflection at 28 = 26" corresponds to the off-axis spots.The presence of this reflection shows that three-dimensional ordered regions are present in this sample. The weak outer equatorial spots in the fibre pattern are out of the range of the diffractometer hot cell. When the temperature of the cell was increased the intensity of the two sharp reflections reduced in a way that correlated with the broad d.s.c. melting endotherm. The peaks completely disappeared at 300 "C, which is above the final d.s.c. melting process, leaving only the amorphous background. The peaks reformed on sub- sequent cooling. The area under the sharp peaks relative to the area under the amorphous background, indicates that ca.20% of the polymer is associated with the three- dimensional ordered regions that form on cooling. The d.s.c. melting endotherm indicates a heat of fusion of 20 kJ kg-' of 'crystalline' material. The half-width of the diffraction peaks imply that the lateral size of the ordered regions are CQ. 100 8, across. This is significantly larger than the 30A suggested by Dr Windle and is closer to the estimate made by Prof. Blackwell in his paper.110 GENERAL DISCUSSION - Fig. 5. Tracings of X-ray photographs of polymer with x = 0.73. The first point I would therefore like to make to Dr Windle is that if he is to succeed with explaining the three-dimensional ordered regions in terms of his new and interesting concept of a non-periodic layer crystal, then his model must be able to accommodate 20% of the material and involve lateral sizes of ca.lOOA in individual crystals. The second point I would like to make involves the degree of order of the 'crystalline' regions. Our observations indicate a significantly higher order than suggested by Dr Windle and are closer to those shown in Prof. Blackwell's paper. In our experience, most forms of extrusions, mouldings and fibres show evidence of the 26" off-axis reflection ( i e . indicative of three-dimensional order) as well as the stronger 20" equatorial reflection. On annealing, significantly higher degrees of order can be attained. This is exemplified in fig. 5 , which shows tracings of X-ray photographs of polymer with x = 0.73 which ( a ) has been freshly melt-drawn into a fibre and (6) has been subsequently annealed at progressively higher temperatures near the melting point.The annealed pattern shows a greater number of diffraction spots arranged in a pattern of higher symmetry. After projection onto the (hkO) plane, the spots in pattern ( a ) can be shown to be consistent with a hexagonal packing of chains. In view of the biaxial nematic state deduced in Dr Windle's paper, is it not surprising that the most ordered regions in the system appear to choose to pack with a hexagonal symmetry rather than a 'biaxial' one? It appears that it is only after prolonged annealing that the more ordered regions move from a hexagonal packing to one possessing a degree of biaxiality as in pattern (b). ' D. J. Blundell, Polymer, 1982, 23, 359.Dr A. H. Windle ( University of Cambridge) (partly communicated): Dr Blundell disagrees with our statement about the lateral size of the ordered material as estimated from the half-width of the equatorial reflection.GENERAL DISCUSSION 111 h U .- 2 U .- ......... .... .... * . . . . . . . . a . 10 20 30 2 e p Fig. 6. Reflection diffractometer scan of a sample of polymer B-N (75/25 HBA: HNA) without macroscopic orientation. The radiation used was filtered CuKa, and the data points were obtained by step scanning at 0.05” (28) intervals. The background is an estimate by eye (see text). In our paper we describe our estimate as a lower limit and add the rider that the ‘actual dimension will almost certainly be larger’. Dr Blundell obtains a dimension of ‘ca. 100A’ and makes an issue of this value.His first figure (room- temperature plot) makes assumptions about a two-phase structure and he draws in a background in that light. We do not disagree with his choice of background, except that it leaves the peak tails rather lopsided, nor with the fact that it leads to a smaller value of peak half-width. However, choosing to draw a background at all raises an additional issue which would detract from the generality of our argument. Incidentally our measurement of Dr Blundell’s half-width on his figure using his background gives an apparent crystallite size of 76 A. We have now carried out a room-temperature reflection diff ractometer scan on a nominally unoriented sample on a composition which compares more closely with Dr Blundell’s fibre diagrams and also the previous data in our paper.The result is shown below, together with our own estimate of an amorphous-type background (fig. 6 ) . The half-width of the main equatorial peak gives an apparent size of 63 A. Bearing in mind the different sample compositions, the fact that the thermal histories were not necessarily the same (our sample was cooled from the mesophase at ca. 1 K s-’), and the inevitable differences in diffractometer geometries, it is difficult to see the contention. If one draws in a halo-shaped background, the minimum size is in the region of 60-80A, if one does not then the absolute minimum will be around 30A. Also, I must re-emphasize that the actual size will be considerably greater than these values if there is a substantial type-I1 disorder contribution to the peak half-width.Before leaving the matter of size we should also like to draw attention to a feature of Dr Blundell’s scan at 230°C. The ‘crystal’ peak is noticeably sharper than at room temperature. This indicates that some of the smaller ordered entities112 GENERAL DISCUSSION have 'melted out' at 230 "C even though the solid-liquid transition does not occur until ca. 290 "C. We see this effect as supporting evidence for the d.s.c.-based argument in our paper that the entities melt out over a very wide temperature range. The concept of non-periodic layer (n.p.1.) crystallites accounts for the parallel observations that the positions of the meridional peaks is consistent with the non-periodicity of the random copolymer molecules,132 and yet on annealing the intensity of the lowest-order peak concentrates onto the meridian without any other sampling of that 'layer line'.Thus we have comparatively extensive lateral order associated with an intrinsically aperiodic molecule. A crystallinity of 20% and a lateral size of 100 A, as suggested by Dr Blundell, could be satisfactorily accommo- dated by the model. Finally, Dr Blundell discusses the particular arrangement of equatorid maxima seen in his photographs. His statement that the 26" off-axis reflection is 'indicative of three-dimensional order' needs qualification. This maximum, which is also a feature of our quantitative contour plots (see for example our response to Prof. Blackwell's question), is particularly significant in that its displacement in the meridional direction is not sufficient to place it on even the first 'layer line' at ca.1 kl. The full implications of the off-equatorial component are uncertain, but we see it as representing an association between the units of adjacent chains which gives long-range near-lateral order despite the aperiodicity along the chain axis. Dr Blundell's contention that the arrangement of equatorial and near-equatorial maxima, when projected onto (hkO), indicates hexagonal packing, is difficult to substantiate. Hexagonal packing implies that the first two equatorial maxima have scattering vectors with amplitudes in the ratio of 1 : 1.732. The observed ratio, as projected and stated elsewhere in his question, is of the order of 1 : 1.3 (20": 26").The extra order shown by the appearance of additional peaks in the equatorial cluster in samples heated for substantial periods near to their melting points has also been observed by ourselves. It must represent a change in local molecular organization, but before attempting to draw any conclusion one has to be very careful to ensure that the molecules have not themselves become more periodic as a consequence of a process such as transesterification. G. A. Guitierrez, R. A. Chivers, J. Blackwell, J. B. Stamatoff and H. Yoon, Polymer, 1983'24,937. G. R. Mitchell and A. H. Windle, Colloid Polyrn. Sci., 1985, 263, 230. Dr G. R. Mitchell (University of Reading) (communicated): I also would like to address some of the issues raised by Dr Blundell. It is clear from the thermograms presented by Dr Blundell,' from those of our paper at this Discussion and elsewhere, that some first-order transition occurs at ca.300 "C. The first question to answer is whether this transition corresponds to a weak transition of the complete sample volume, or if it arises from the transition of a small component of that sample volume. If it is the latter, then the interaction of this small component must be such that it locks the remaining bulk of the sample, since the optical structure exhibits no mobility below the softening point.* If we accept the evidence presented in our paper that the hydroxybenzoic acid/ hydroxynapthoic acid copolymers (HBA/ HNA) exist in a biaxial nematic phase above the softening point, in other words if we exclude the possibility of the softening point being a biaxial/uniaxial nematic transition, the softening point must correspond to the melting out of crystallites or some other enhanced order.Dr Blundell uses particular interpretations of the experimental X-ray scattering patterns to support the notion of a significant level of crystallinity. Those interpreta-GENERAL DISCUSSION 113 tions, while highly plausible, are not unique, and I would like to draw attention to the following points. Fig. 12 of our paper shows the equivalent X-ray scattering patterns of HBA/HNA at room temperature and at a temperature above the softening point, as displayed by Dr Blundell. There is clearly a marked reduction in the positional order upon passing through the softening point.The two peaks in the 20-30”26 region are related to interactions between chains and both these peaks, albeit considerably broadened, are still present in the ‘melt’. The peak at ca. 45” 26 arises from correla- tions within single-chain molecules, and its breadth is related in part to the correlation length of the copolyester molecule^.^ It is clear that the correlation length of the molecules is also reduced above the softening point. If we compare the two curves of fig. 12 in our paper, there appears to be no reason to assume that the room- temperature curve consists of two components (crystalline and ‘amorphous’) as argued by Blundell. It has the appearance of simply a ‘sharper’ version of the high-temperature curve. This reasoning would support the idea that the softening transition is a homogeneous event.This has additional support from the results of optical microscopy and from electron microscopy, which confirm the absence of a two-phase structure unless the samples are annealed and particularly if the samples are first highly oriented. Small-angle X-ray scattering curves show no evidence for any significant density variations2 The interpretation followed by Dr Blundell of the X-ray scattering data also utilizes the presence of off-equatorial diffraction maxima to support the concept of crystallinity. The first point relating to this line of reasoning is that those reflections do not lie on a layer line corresponding to the first meridional peak, nor do they lie on a layer line appropriate to the napthoic repeat unit (however unlikely that might be).A model which satisfactorily accounts for the presence of these off- equatorial maxima and also accounts for their position off a layer line is displayed in fig. 7. It is suggested that those off-equatorial maxima result from the loose correlation of aromatic units which are inclined to the general chain direction. The experimentally recorded azimuthal angle and the scattering vector of the off - equatorial peaks are in accord with such a model. In addition, the off-equatorial maxima are sharper (in terms of scattering vector) than the main equatorial maxima at s = 1.5 A-’. The association of these off-equatorial maxima with a specific correlation within the structure is again in agreement with such an observation. There is strong evidence to suggest that, even in non-crystalline polymers, specific interactions will occur between phenyl groups when present.4i5 The longitudinal correlation shown schematically in fig.7 is observed in cylindrical distribution functions derived from the X-ray scattering patterns of aligned copolyester ~arnples.~.’ These functions show that the axial register is limited to a scale of the order of aromatic units rather than a point-to-point atom correlation. It is also reasonable to expect that this limited axial register will develop after and additional to the biaxial ordering of the molecules. If we relate these off-equatorial peaks to the correlations outlined above, then the assignment of a ‘hexagonal’ lateral packing scheme is void. The fact that a weak peak is seen at the same scattering vector (s = 1.9 A-’) in the scattering curve for the globally isotropic sample at a temperature above the softening point [fig. 12(b) of our paper] demonstrates the retention of some level of rotational correlation between chains as would be expected in a biaxial nematic structure. It would appear that this rotational correlation is consolidated and enhanced by some limited axial register in one direction (normal to the aromatic ring plane) upon cooling the sample to the solid state.These correlations clearly cannot be equated with three-dimensional crystallinity.114 I I I I 1 I I I I I GENERAL DISCUSSION Fig. 7. Schematic representation of the solid-state molecular organization in the HBA/ HNA random copolymers, to account for the occurrence of the off-equatorial maxima at s = 1.9 A-’.The ‘molecules’ are viewed edge-on to the aromatic rings. The development of a further additional level of order upon annealing, as demonstrated in our paper by thermal analysis and electron microscopy, requires careful thought. We can partition the possible crystalline models into those which ( a ) involve significant run lengths of one of the homopolymers, (6) involve some distorted structure, predominantly of one homopolymer type, but which allows the introduction of some defects in the form of the other monomer type, and ( c ) involve in any one crystal only one particular random sequence of monomers. The latter are the non-periodic layer crystals described in our paper. The first of these choices will certaintly not match the 20% suggested by Dr Blundell, since the molecular chains have been shown to have a random distribution of monomers [ref.(3) and papers by Windle et al. and Blackwell et al. at this Discussion]. ’ D. J. Blundell, Polymer, 1982, 23, 359. * C. Viney, G. R. Mitchell and A. H. Windle, Mol. Crysf. Liq. Cryst., in press. G. R. Mitchell and A. H. Windle, Colloid Polym. Sci., 1985, 263, 230. T. P. H. Jones, G. R. Mitchell and A. H. Windle, Colloid Polym. Sci., 1983, 261, 110. G. R. Mitchell and A. H. Windle, Colloid Polym. Sci., 1985, 263, 280. G. R. Mitchell and A. H. Windle, Polymer, 1982, 23, 1269. ’ G. R. Mitchell contribution to this Discussion (as a comment to Prof. Blackwell’s paper.)GENERAL DISCUSSION 115 Dr G.R. Mitchell (University of Reading) said: I would like now to direct attention to the paper of Blackwell et al. We also have performed analyses of the scattering patterns from aligned samples of the rigid-chain thermotropic copolyesters of hydroxybenzoic and hydroxynapthoic acids. These structural investigations have proceeded in two stages. The first objective was to obtain chemical and structural parameters relating to individual molecules, and in particular the nature of the distribution of the ‘monomers’ within each molecule. We utilized an approach for calculating the meridional intensities which was a development of that proposed initially by Zernicke and Prins’ for simple disordered systems. We extended the method to a two-component system which allows the meridional scattering for a random copolymer to be written in analytical form.2 The comparison of such calculated scattering patterns with those obtained experimentally confirmed the initial expectations of randomness.Scattering patterns calculated for ‘blocky’ sys- tems were at variance with the experimental data. The meridional scattering for the HBA/HNA copolymer exhibits an intense peak at s = 3 A-’, which is almost invariant in position with changing HBA and HNA proportions. It may be considered to be a combination of the fourth-order maxima from the HNA repeat (ca. 6.3 A) and the third-order maxima from the HBA repeat (ca. 6.3 A). The fact that there is not an exact match of those repeats leads to a broadening component, which is additional to any broadening due to restricted correlation along the chain resulting from bending or other defects.We have calculated from the model scattering2 the breadth of the peak at s = 3 A-’ as a function of the correlation length of the molecule, and the results are shown in fig. 8. The straight line represents the relationship between breadth and correlation length for a perfectly invariant peak. The curve away from that straight line results from the slight mismatch of the repeat units and must be taken into account when determining the Correlation length. In a particular melt-extruded pellet of HBA/HNA (70/30) examined the breadth of the peak at s 3 A-’ was measured at As = 0.1 A-’, which corresponds to a correlation length involving ca. 10 repeat units or ca. 69 A. It would be reasonable to expect that this correlation length would be dependent upon both the particular chemical composition and the mechanical and thermal treatment of the sample.The second stage of the analysis is concerned with the packing of these random copolymer molecules. We have obtained quantitative X-ray scattering data (fig. 2 of our paper) which have been transformed to a real-space cylindrical distribution function W( r, a) using procedures described in detail el~ewhere.~ Fig. 9( a ) and (6) show these cylindrical distribution functions for the Eastman Kodak X7G copolymer of hydroxybenzoic acid and poly( ethylene terephthalate) and for the HBA/HNA (70/30) copolymer. There are three distinctive features of these func- tions for both copolymers. The first is the peaks which are centred along the meridian or extrusion axis. Those peaks represent the correlations with each molecule.The second are the columns which extend up from the equator. These are the correlations between neighbouring chain segments. The lack of arcing of these columns indicates a high level of local segmental correlations, although the global-orientation ( P2) value is only ca. 0.6. Both the features outlined above are expected for liquid-crystal systems of highly oriented almost rigid rods. However, a third feature indicates an additional level of correlation. Both cylindrical distribution functions exhibit a maximum in those columns which is out of the equatorial plane. This indicates a preference for some level of longitudinal register between adjacent molecules.However, this register is limited and appears to be restricted to register between aromatic groups. In other words, it is not a point-for-point atom-position register.116 GENERAL DISCUSSION 0 10 20 30 LO 50 60 number of units, n Fig. 8. Plot of the reciprocal of the calculated half-width (As) of the peak in the region of s = 3 A-' against the number of units in the correlation length of the chain. The solid line is for an invariant peak ( i e . with exactly matching repeat units of 6 and 8 A). The points are for the equivalent peak but with the real repeat lengths of HBA of 6.3 8, and HNA of 8.3 A. In the case of the HNA/HNA cylindrical distribution function, the correlation appears at a distance commensurate with the napthoic unit repeat. We need to interpret these results with caution.However, they do indicate a preference for an axial shift in addition to lateral spatial correlations. This enhancement of the local order may only occur in a limited volume of the material, and perhaps represents the nearest to crystallization that a random copolymer can attain. ' F. Zernicke and J. A. Prins 2. Phys., 1927,41, 184. G. R. Mitchell and A. H. Windle, Colloid Polym. Sci., 1985, 263, 230. G. R. Mitchell and R. Lovell, Acta Crystallogr., Sect. A, 1981, 37, 189. Br R. A. Chivers (ICI, Wilton) said: Another aromatic copolyester which forms a liquid-crystalline melt is that known commonly as HNATH. This material is made from 2-hydroxy-6-naphthoic acid (HNA) and equimolar quantities of terephthalic acid (TPA) and hydroquinone (HQ).I have prepared fibres from the molten polymer, and the X-ray diffraction pattern of fibres of HNA/TPA/HQ 50/25/25 is shown in plate 8. This pattern is very similar to those of the HBA/HNA system' in that aperiodic meridional maxima (in this case five out to d = 2 .$) are seen, and also intense equatorials and off -equatorials on broad, more diffuse equatorial scatter. Calculations have been performed on the meridional diffraction of the HNATH system and will be reported fully elsewhere. In essence, the meridional diffraction can be explained as being derived from a random copolyester chain as has been shown by us for other systems.',2 However, there is one clear difference between the diffraction from HNATH and that from these other systems.This is the presence of further off-axial maxima, most clearly those continuing the ca. 4.2 8, 'layer line'. Maxima can also be resolved on the upper and lower extremes of the broad equatorialGENERAL DISCUSSION 117 r / A 1 ° 15 15 Fig. 9. Cylindrical distribution functions W( r, a) derived from the scattering data of fig. 2 of our paper using the procedures described in ref. (3). The extrusion axis is vertical. ( a ) For a melt-extruded pellet of a random copolymer of hydroxybenzoic and hydroxynaphthoic acids in the ratio 70/30. ( b ) For a melt-extruded pellet of a random copolymer of hydroxyben- zoic acid and poly(ethy1ene terephthalate) in the ratio 60/40. The dashed lines represent contours for negative values. scatter that are quite distinct from, and at lower d-spacings than, the usual off- equatorials.My calculations of the full three-dimensional transform of the atomic chain: similar to those reported here for HBA/HNA,' show considerable intensity out along the ca. 4.2. A layer line which may help to explain some of these maxima. It may be also that the lateral intermolecular packing is better in this system. HNATH is therefore a similar material to the others discussed, but it shows interesting differences in its diffraction which make it worthy of further study. The work described here was performed at Case Western Reserve University, Cleveland, Ohio, and I am grateful to Celanese Research Company, Summit, N.J. for provision of the polymer. J. Blackwell, A. Biswas, G. A. Gutierrez and R. A. Chivers, Faraday Discuss.Chem. SOC., 1985, 79, 73. * G. A. Gutierrez, J. Blackwell and R. A. Chivers, Polymer, 1985, 26, 348. Prof. J. Blackwell (Case Western Reserve University, U.S.A.) said: I agree with Dr Chivers in that these off-equatorials for HNATH [copolyesters of HNA, hydro- quinone and terephthalic acid (TPA)] occur further from the equator than those seen for copoly( HBA/HNA), which suggests that there is more extensive lateral register of the chains in the former copolymer. This observation contrasts with what we have previously reported for copolymers prepared from HBA, 2,6-dihydroxy- naphthylene (DHN) and terephthalic acid.' The latter copolymer has a similar chemical structure to that of HNATH except for a reversal in sense of some of the ester groups.The X-ray patterns of as-spun fibres of three HBA/DHN/TPA molar ratios (60/20/20, 50/25/25, 40/30/30) show an intense off-equatorial on a 'layer118 GENERAL DISCUSSION line' at ca. 12 A, as is seen for both copoly(HBA/HNA) and HNATH, but there are no strong off-meridional maxima as far from the equator as those seen for HNATH. This is a further example of the differences that can result from an apparently minor change in polarity of the ester groups, as was commented on earlier by Dr Yoon. It suggests that this polarity affects the lateral registration of the chains, although it should be noted that the HNATH preparations contain higher proportions of naphthylene units. * J. Blackwell and G. A. Gutierrez, Polymer, 1982, 23, 671. Dr A. M. Donald (University of Cambridge) said: Dr Chivers has shown that the ternary copolyester based on hydroxynaphthoic acid, hydroquinone and tereph- thalic acid (N-QT) may show a greater richness of structure, as revealed by X-ray diffraction, than the simple random copolyester discussed in detail by Prof.Black- well. My own electron-diffraction studies of members of the N-QT series, in which the ratio of the components was varied, supports this conclusion. When thin samples of the polymer are annealed for 10 min at 300 "C on, for convenience, a rocksalt substrate, dramatic changes in both the meridional and equatorial reflections may occur. Faint hints of off-meridional reflections, perhaps related to those observed by Dr Chivers, can occasionally be seen, but the most striking feature is the appearance of a set of aperiodic meridional lines which are much less arced than the main 3s line (which is still present), as shown in plate 9.Only a small percentage (estimated to be ca. 5%) of the specimen exhibits these additional reflections along the meridional, and hence it is probable that X-ray techniques will be hard-pressed to detect their presence, demonstrating the usefulness of electron microscopic tech- niques. Most intriguing of all, different regions may show a different set of the aperiodic lines, as can be seen by comparing plates 9 ( a ) and ( b ) , both taken for specimens with a ratio of 70: 15 : 15 for the naphthoic: hydroquinone: terephthalic residues, respectively. Interestingly, a comparison of the meridional lines shows that plate 9( a ) contains more lines, and they are sharper than in plate 9( b ) ; however, the converse statement is true for the equatorial reflections.Additional reflections are likewise seen for the other compositions examined, namely samples containing 60 and 50% of the naphthoic residue. As yet, the origin of these lines is not clear, but what is demonstrated by the diffraction patterns is that quite marked changes in packing are occurring locally during annealing, on a size scale which is hard for standard X-ray techniques to probe. Dr A. H. Windle (University of Cambridge) said: I should first like to comment on Prof. Blackwell's paper. He alludes to the less-than-satisfactory agreement between the best model predictions for the intensity of the first meridional peak and the experimental data.We should like to support his suggestion that this is due to 'preferred axial stagger'. We showed in our paper that this first meridional maximum, observed in electron diffraction patterns of thin (1000 A) samples, con- centrated on the meridian after the polymer had been annealed in the solid state. We have now performed a similar experiment, employing X-ray diffraction and using fibres of some Celanese 73/27 copoly (HBA/HNA) material (B-N). Fig. 10 shows recent data collected by Ruth Golombok. It compares the patterns of as-spun fibres with those annealed for 1 day at ca. 10 "C below the melting point. The anneal4 3 s/A-' 1 0 4 3 2 1 0 1 2 3 A Fig. 10. Wide-angle scattering intensity for two fibre samples of B-N. ( a ) Unannealed.( b ) Annealed at 280°C. The fibre axis is vertical.120 GENERAL DISCUSSION 0 1 . 0 s/A-' 0 s/A-' Fig. 11. Detail of fig. 10 showing the intensification of the s = 1 k' reflection onto the meridian in ( a ) the unannealed sample and (b) the annealed sample. has had comparatively little effect on the third (s == 3 &*) strong meridional maximum, some influence on the second peak and a strong effect on the first, which concentrated onto the meridian as shown in detail in fig. 11. In the annealed material the apparent lateral width of the 1s peak is 0.14 A-*, whereas that of the 3s peak is ca. 1.6 kl. Hence the longitudinal register between units 6-8 A long in the chain directions (the copolymer units) appears to extend laterally for at least 50 A.However, the register is not especially perfect and matching on the finer scale of 2 8, is difficult to recognize. As with the electron diffraction, the only sampling of the first 'layer line' is on the meridian, and this again requires interpretation in terms of non-periodic layer (n.p.1.) crystallites. In response to Prof. Blackwell's informally posed question it is possible, indeed likely, that there are specific mistakes in the matched sequences and that the lateral extent of the register significantly exceeds 50 A. We believe, however, that the n.p.1. crystallites represent the most useful starting point in considering the order in these systems. The annealing treatment decreases the arcing of the equatorial peaks and shar- pens equatorial maxima. The asymmetric nature of the equatorial maxima prompts my second comment, which is a rather more general one. Non-crystalline polymers which contain aromatic groups within their backbones have a special signature on their diffraction scans. The main interchain peak is asymmetric, with a distinct shoulder or even subsidiary maximum at s = 1.9 A-'. Data collected by Dr T. P. H. Jones for several such polymer glasses are reproduced in fig. 12. Furthermore, the scattering from liquid benzene also shows a shoulder in the same position (fig. 13) which has been interpreted by Narten in terms of special correlations between the rings. Short-range rotational correlation (or biaxial- lity!) thus seems to be a feature of aromatic backbones. The fact that the diffraction patterns of all the aromatic ester liquid-crystal polymers examined show a high-angle shoulder which resolves to a width corresponding to at least 100 A in the solid and less in the liquid implies that there is rotational correlation of the aromatic rings. Although such correlations may be associated with biaxial order, they are notGENERAL DISCUSSION 121 d 0 1 2 3 4 5 1 0 1 2 3 4 5 6 0 1 2 3 4 5 6 s/A-' Fig. 12. Interference functions from a number of glassy polymers containing aromatic groups in the backbone.' In each case a shoulder or small peak at s = 1.9 A-1 is apparent. ( a ) Poly( p-phenylene sulphide), ( b ) poly( dimethylphenylene oxide), (c) polycarbonate, ( d ) poly(ether sulphone), ( e ) poly(ether ether ketone) and (f) polysulphone (UDEL).122 GENERAL DISCUSSION u 0 4 slA-1 6 Fig. 13. Interference function of liquid benzene showing the shoulder at s = 1.9 A-’ on the ‘high-angle’ side of the main intermolecular interference peak.* PHBA PHNA Fig. 14. Sketch showing repeat units of PHBA and PHNA in an ‘extended’ conformation.GENERAL DISCUSSION 123 necessarily the cause of it, as the dipoles on the ester groups are likely to have a greater influence. The s = 1.9 A-' shoulder in copolyesters makes an angle of 13-14" with the equator. If the molecule is in an extended-chain conformation it is noteworthy that the projection of the vertical bond (through the aromatic ring) makes an angle of this order with the local chain axis (fig. 14). T. P. H. Jones, Ph.D. Thesis (University of Cambridge, 1983). ' A. H. Narten, J. Chem. Phys., 1977, 70, 299.Plate 1. The microstructure of 60/40 PHB/PET at 150 "C, showing the two phase structure. (T. J. Lemmon). [ f o o n g page 124Plate 2. A sequence of micrographs of the same area of a thin section of B-ET at 220°C between crossed polars: ( a ) 0, ( b ) 15 s, (c) 30 s and ( d ) 10 min. The texture is seen to be mobile on a timescale of seconds, but the X-ray orientation (determined at room temperature from the same thin section) has decayed little after 10 min.'Plate 3. A series of three micrographs of the same area of B-ET sample 8 pm thick. The microstructure, showing two phases, has been established by annealing at 290 "C for 1 h. On cooling below 250 "C the discontinuous phase has a fine Schlieren texture (a). This texture disappears at 245 "C on heating (b), to be replaced by a course radial texture ( c ) . The transition is reversible on cooling. The specimen was photographed at the temperatures indicated.Plate 4. Fine Schlieren texture in a 2 pm thick sample of B-ET between crossed polars (T. J. Lemmon).Plate 5. Sample of B-ET of thickness 1-2 pm in the centre of the field and 8 pm to the right. The uneven boundary on the left is the specimen edge (T. J. Lemmon).( a ) - 10 ptn (6)- 10 pm (d- 10 pm Plate 6. A series of micrographs of B-ET showing sections of increasing thickness: ( a ) 2, ( b ) 5 and ( c ) 10p.m. The top row shows textures between crossed polars, while the bottom row shows the corresponding contrast obtained when both polars are removed.Plate 7. X-Ray scattering pattern of a melt-extruded pellet of Eastman Kodak X7G cop- olyester which has been compressed at room temperature in a direction normal to the extrusion axis. The incident X-ray beam is parallel to the extrusion axis, and the compression direction is horizontal in the figure. The scattering maxima corresponds to interchain correlations, real space distance of ca. 5 A. Plate 8. X-Ray fibre diffraction from melt-spun HNA/TPA/HQ, 50/25/25.Plate 9. Selected area electron diffraction patterns of N-QT of composition 70: 15 : 15 follow- ing a 10min anneal at 300°C.

ISSN:0301-7249    

DOI:10.1039/DC9857900085

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1985, 79,125-132 Measurements of the Anisotropic Viscous and Elastic Properties of Lyotropic Polymer Nematics BY ROBERT B. MEYER," FRANKLIN LONBERG, VICTOR TARATUTA, SETH FRADEN, SIN-DOO LEE AND ALAN J. HURD The Martin Fisher School of Physics, Brandeis University, Waltham, Massachusetts 02254, U S A . Received 9th May, 1985 We summarize the current status of our studies of the macroscopic linear mechanical properties of nematic liquid crystals formed from solutions of rigid or semirigid rod-like polymers. The polymer system we have studied is a racemic mixture of poly(benzy1 glutamate) dissolved in a mixture of dioxane and methylene chloride. We have also studied nematics formed from colloidal suspensions of tobacco mosaic virus which may be viewed as an ideal model system representing a rigid-rod polymer solution. We review briefly the current theoretical understanding of the elastic and viscous parameters characterizing a nematic.Then we discuss our experiments, both Frederiks-transition studies and quasielastic Rayleigh scattering. The Frederiks transition in its well known simple forms is not easily utilized in these systems, mainly because of the intervention of a number of phenomena not normally encountered in low-molecular-weight liquid crystals. However, these phenomena have been analysed theoretically and can be used to extract information on elastic and viscous properties. Quasielastic Rayleigh scattering on well oriented single crystals has proved to be an excellent technique for measuring ratios of elastic and viscous parameters.We describe the scattering geometries we have used and our results. An important goal in the study of polymer nematic liquid crystals is the complete understanding of their mechanical properties. We have undertaken a program of research concerning the linear mechanical properties of systems of rod-like or semiflexible polymers that form nematic phases in solution. These form a fairly simple limiting case for two reasons. First, fairly simple theoretical ideas based on a dilute-solution limit might be almost correct. Secondly, the preparation of and experimentation on such samples is easier than the study of melts in a number of ways. In particular, we have studied racemic mixtures of poly(benzy1 glutamate) (PBG) in a solvent which is a mixture of dioxane and methylene chloride, to suppress any remaining cholesteric helicity.' We have also studied the nematic phase of colloidal suspensions of tobacco mosaic virus (TMV), which in this context may be thought of as an ideal model of a rigid-rod polymer solution.The experimental methods we have used include the Frederiks transition and quasielastic Rayleigh scattering. The first of these has been used extensively in the study of low-molecular-weight nematics to determine their elastic constants and viscosities.* The second has been used in the study of n e m a t i ~ s , ~ but it presents some serious technical problems for the routine determination of mechanical proper- ties owing to the large birefringence of the nematic phase.4 However, in our materials, which have very low birefringence, this technique works very well.Rather, it is the Frederiks transition that presents difficulties of various kinds. Several new phenomena are encountered when trying to study the Frederiks transition which prevent the straightforward extraction of information from this experiment. 125126 MECHANICAL PROPERTIES OF POLYMER NEMATICS However, these new phenomena are very interesting in their own right, and in the light of theoretical analysis they provide new ways of studying the mechanical properties of polymer nematics. In the following sections we first review our understanding of the theory of the elastic and viscous properties of nematics composed of rigid rods or semirigid chains, and then we discuss our experimental findings. THEORY The starting point for theory of lyotropic nematics is the hard-rod model first investigated by On~ager.~ This views the interaction between particles as arising entirely out of the excluded-volume effect.The appearance of the nematic phase is the result of maximizing the entropy of the system; in the nematic phase, rotational entropy is sacrificed to increase the configurational entropy of the molecular positions. In the approximation used by Onsager all that matters are two-body interactions, which are treated in a mean-field approximation. At the level of this approximation one can solve the equations describing the nematic-isotropic phase equilibrium, and compute the orientational distribution function for the molecules in the nematic phase.From this distribution function, and still at the level of approximation using two-body interactions in a mean-field calculation, one can compute the elastic constand and viscosities’ of the nematic phase. We have carried out these computations as a starting point for comparison of experimental results with theory.* The results are summarized in table 1. For describing TMV, the theory at the Onsager level might be at least a good starting point. However, for longer and more flexible particles, such as PBG, and surely for even longer and much more flexible particles, such as many other currently studied polymers, or for more concentrated solutions and melts, other theoretical ideas are needed. Here we consider only the possibility of semiflexible particles, which are non-rigid but are still fully extended in the nematic phase.In the Onsager picture of the elastic constants, all that is taken into account is the change in the two-particle excluded volume produced by spatial distortions of the director field. All the elastic constants scale with the square of the volume fraction of polymer in solution and the square of the length-to-diameter ratio. For highly oriented states of the nematic, this theory predicts that the bend elastic constant will be large, while those for splay and twist will be small, since bend always increases interference effects, while splay and twist do so only as second-order effects. Moreover, the bend elastic constant increases roughly linearly with order parameter in the range of experimentally reasonable order parameters, diverging to infinity at perfect order.This is a consequence of the singular form of the two-body excluded-volume function, which varies as the absolute value of the sine of the angle between two rods. These are interesting predictions to check. Beyond the Onsager picture of the elastic constants there are two ideas for longer, more flexible chain^.^ First, the splay elastic constant has a component arising from a single particle contribution to the entropy. Splay requires concentrat- ing like ends of the chains (‘top’ or ‘bottom’ ends), which reduces their entropy. This contribution exists for hard rods and for more flexible chains, and increases linearly with chain length and concentration. It is larger than the two-particle interference contribution to the splay elastic constant.For flexible chains the bend elastic constant is limited by the flexibility of the chains. It no longer increases with chain length, and varies linearly with concentration rather than quadratically. It simply reflects the energy per unit volume needed to bend the chains. Thus forR. B. MEYER et a1 127 Table 1. Elastic constants and viscosities of the nematic phasesn hard-rod theory S = 0.7 S = 0.8 S = 0.9 TMV PBG 1.8 0.6 8.0 3 13 0-054 0.032 1.13 0.15 34.8 elastic moduli 2.1 3.3 0.69 1.1 14.0 43.0 3 3 21 42 viscosities 0.034 0.017 0.016 0.004 1.11 1.06 0.11 0.06 69.6 240 2.5 42.5 4.1 0.36 4.7 11.4 13.0 34.7 0.16 35.1 S0.16 34.8 0.044 0.015 0.0046 0.95 1 .o 0.21 63.3 220 The notation follows P.G. deGennes, The Physics of Liquid Crystals (Clarendon Press, Oxford, 1974). S is the nematic order parameter. The units for Ki are lo-’ dyn, assuming a diameter for PBG of 15 A. The units for the viscosities are poise. The accuracy of the PBG measurements is *7%. fully extended but non-rigid chains, the ratio of splay to bend elastic constants should be relatively large, independent of concentration and proportional to chain length, rather different from the hard-rod limit, in which this ratio would be small and decreasing with increasing chain length. The hard-rod picture qualitatively predicts that certain viscosities grow very large with molecular length and increasing order parameter, while others remain small.7 All the viscosities are proportional to a fundamental rotational viscosity for a single particle which depends on its shape and the solvent viscosity.For semiflexible chains, the same qualitative conclusions apply. The ability of semiflexible chains to wrap rather gently around one another might modify the picture, but there is not yet any quantitative description of those effects. THE FREDERIKS TRANSITION The Frederiks transition provides a simple means for determining material parameters for nematic liquid crystals, provided that one can prepare well aligned single-crystal samples with strong anchoring of the director at the parallel plates defining the sample boundaries. We have been able to prepare such samples of PBG nematics. Either homeotropic or parallel boundary conditions can be utilized to fix the director in a unique orientation at the parallel-plate sample surfaces. Homeotropic alignment occurs spontaneously for glass, fused silica or indium-tin oxide surfaces, presumably owing to hydrogen bonding of the PBG at the oxide surface.We have prepared parallel128 MECHANICAL PROPERTIES OF POLYMER NEMATICS boundary conditions by coating these oxide surfaces with a thin highly cross-linked polyethylene layer deposited by plasma polymerization of ethylene gas. lo For TMV, parallel orientation occurs spontaneously, and we have not thought of any way of preparing homeotropic samples." The magnetic anisotropy of PBG is positive, so that it aligns parallel to a magnetic field.12 For high-frequency electric fields (above ca. 100 kHz) the dielectric anisotropy is negative and electrohydrodynamic effects have disappeared, so the director aligns perpendicular to an electric field.13 The magnetic anisotropy of TMV is p~sitive.'~ In the following sections we describe the three main geometries for the Frederiks transition, together with our findings.BEND GEOMETRY With homeotropic boundary conditions and a magnetic field in the plane of the sample, or an electric field parallel to the director, the critical field for the Frederiks transition determines the bend elastic constant. The effective viscosity for this mode of distortion is very large, so the critical field is hard to find. This is the first kind of problem encountered, although it is one of inconvenience rather than fundamental difficulty. After exceeding the critical field by a small amount and creating a small distortion, one can change the field and see whether this distortion grows or shrinks with time, and thus bracket the critical-field value.By making a quantitative measurement of the amplitude of the distortion, for instance by monitoring the optical anisotropy of the sample, one sees that the response at a fixed field is an exponential relaxation in time toward the equilibrium state. By plotting the inverse of the time constant of this exponential against field, one can measure the critical field from the zero crossing and the effective viscosity from the slope of this line. This is slow work. However, above twice the critical field a new and very fast distortion appears which is the second harmonic (mode two) of the fundamental distortion (mode one) occuring at the lower critical field.' The effective viscosity for this mode of distortion is very small.Repeating the previous experiment near this critical field does not seem to be too difficult, since one can study the rapid response of mode two before mode one does anything (the ratio of speeds is of the order of a hundred or more). However, one discovers that the system does not respond exponentially even at rather small distortion amplitudes!" This is the first real problem encoun- tered. The non-linear behaviour of this fast mode of distortion we have attributed to the influence of elongational flow viscosity, which appears as a second-order effect. The equation of motion for the angular distortion 8 induced by the field can be written in the long-chain limit as for small amplitudes.Because the first-order viscosity, Tb, for this mode is very small while the coefficient of the non-linear term, al, is of the order of a hundred times larger, the non-linear effects become important at very small amplitude, making the first-order viscosity hard to measure accurately. This is a new phenomenon that is not encountered in low-molecular-weight liquid crystals. It gives us a direct means of measuring the relative magnitude of the elongational flow viscosity.R. B. MEYER et al. 129 If one knows the appropriate diamagnetic or dielectric anisotropy and one can measure the time dependence of the bend distortion accurately, one can determine K3 and several viscosities using this geometry.TWIST GEOMETRY With parallel boundary conditions and the magnetic field parallel to the plane of the sample, we examined the twist Frederiks transition. Upon application of the field, a pattern of stripes appeared, oriented perpendicular to the urlisturbed director, once again a new phen~menon.'~ Exploring the field dependence of the stripe spacing and the time dependence of the pattern, we realized that this was a dynamic effect due to the large anisotropy of the viscosity of the material. For a suddenly applied field, above the critical field for the Frederiks transition, the periodic structure has a faster response time than a uniform twist response. There is a particular wavelength for which the stripes grow the fastest. That wavelength dominates the response, producing the stripes that are observed.This is a transient response, in the sense that eventually the stripes disappear, leaving the sample in a uniformly twisted state. The uniform twist transition has viscosity yl, which is very large. The stripe pattern involves coupled rotation and velocity fields; the magnetic field drives the director field of counter-rotating stripes, which in turn induces the velocity field. This combined set of fields has a much lower viscosity than y l . Of course the stripe pattern has more internal elastic energy than the uniform twist distortion. It involves bend distortion as well as twist, and has in-plane components of curvature that are not involved in the uniform twist mode. This has two consequences. First, for a range of fields above the critical field for the uniform twist transition, the uniform mode may be the fastest.At some higher field the stripe pattern becomes faster. Secondly, the wavelength of the stripe pattern that maximizes the speed of response is kept long by the elastic energy, while the viscous factor favours a short wavelength. Therefore the observed wavelength is a compromise. By measuring the wavelength of the observed stripe pattern as a function of applied field, one determines several things. The critical field for the ordinary twist Frederiks transition can still be determined, yielding KZ. The behaviour above critical field gives an accurate value of K 2 / K 3 , and two ratios of viscosities. Data for one preparation of TMV and details of the analysis are published elsewhere." SPLAY GEOMETRY With parallel boundary conditions and the field perpendicular to the plane of the sample, we attempted to study the splay Frederiks transition.Once again, on suddenly applying a large magnetic field, we observed the appearance of arrays of stripes, this time oriented obliquely to the original director orientation. The orienta- tion and wavelength of the stripes depended on the field strength. Again, this was found to be a transient effect due to the anisotropy of the viscosity of the material. A theoretical analysis indicates how this effect can be used to determine ratios of material parameters. In this case, because of the complexity of the director and velocity fields, the analysis is rather complicated, and we have not found any simple way to extract data from the experiments.Nonetheless, if one knows some parameters, these measurements can be used to determine some others. Data on TMV and a detailed presentation of the theory for this instability and data analysis are presented e1~ewhere.l~130 MECHANICAL PROPERTIES OF POLYMER NEMATICS (a) ( b ) Fig. 1. Schematic representation of the replacement of a uniform splay structure in a planar sample ( a ) by a periodic twist pattern ( b ) . This might also be referred to as a splay- compensated structure, since apparent splay in the plane of the sample is cancelled by another component of apparent splay perpendicular to that plane. Attempting to avoid the stripes and determine the critical field for the splay Frederiks transition, we discovered another phenomenon.We could not achieve a uniform splay Frederiks transition in PBG. At the lowest field giving a response, we still observed stripes; parallel to the undisturbed director. This is a static effect. The explanation is that a new form of Frederiks transition replaces the uniform splay response. It is a periodic structure composed mostly of twist distortion (see fig. l), which has a lower threshold field than the simple splay transition. It occurs if the ratio of splay to twist elastic constants is above 3.3. What is remarkable is that one can construct a stripe pattern that satisfies the boundary conditions that the distortion disappear at the surfaces, that couples to the external field, and that is composed of pure twist! The theory of this effect, predicting the critical field and the wavelength of the stripes, has been developed and has been published else- where.I8 Neither the wavelength nor the critical field depends very strongly on the splay elastic constant, and both of those quantities are difficult to measure accurately.This leaves us in the position of having no simple means of measuring the splay elastic constant via the Frederiks transition. LIGHT SCA'ITERING Quasielastic depolarized Rayleigh scattering has been shown to be a useful way of determining elastic constants and viscosities of nematic liquid crystal^."^ The PBG nematics are especially well suited to this technique because they have very low birefringence. This eliminates multiple scattering and makes the optical propa- gation equivalent to that in an isotropic medium.There is still enough birefringence to make the depolarized light scattering cross-section large enough for an adequate signal, using an argon laser at ca. 100mW incident power. With this technique one can study both the intensity of the scattered light and the time dependence of the fluctuations that scatter the light. The intensity is determined by the amplitude of the thermally excited director fluctuations which produce depolarized scattering. That amplitude depends only on the energy of the fluctuation, which is proportional to the appropriate elastic constant. One does not measure absolute intensities of scattered light, but in a situation in which one can measure a ratio of intensities or the dependence of intensity on some external variable, one can deduce information about the relative value of an elastic constant.R.B. MEYER et a1 131 The time dependence is most efficiently studied by computing the autocorrelation function of the scattered light intensity in real time with a digital autocorrelator. The fluctuation modes in nematics are all overdamped, since their elastic energy is small and their viscosity is large. The autocorrelation function for a single mode is an exponential decay, the time constant of which depends on the ratio of a viscosity to an elastic constant. We have studied two scattering geometries in samples with parallel boundary conditions.’’ In the first the director is perpendicular to the scattering plane, with the incident polarization parallel to the director, and the scattered polarization parallel to the scattering plane.The plane of the sample is oriented so that the scattering wavevector is parallel to the glass plates. In the second geometry, every- thing is the same as in the first, except that the sample is rotated about an axis normal to its surface so that the director is parallel to the scattering wavevector. The first geometry allows one to see scattering from a superposition of pure splay and pure twist modes. The ratio of the amplitudes of these modes depends on the ratio of twist and splay elastic constants, and their relaxation times depend on their effective viscosities. In the second geometry one sees scattering from a pure bend mode. Keeping the entire scattering geometry fixed and switching from the first sample orientation to the second, one can compare the amplitudes of the pure bend mode and the splay and twist modes.In this way one measures the ratio of the bend elastic constant to the others. The relaxation time of the pure bend mode determines another effective viscosity. These measurements are repeated at a number of scattering angles to confirm the correct angular dependence of the scattering intensities and relaxation times. The results of these measurements are several ratios of elastic constants and viscosities. Combining these measurements with one of the critical fields for the Frederiks transition and a knowledge of the diamagnetic anisotropy leads to an absolute determination of a number of viscosities and the elastic constants of the sample.It is important to note that all the field and light scattering experiments can be performed on a single sealed sample of PBG or TMV, so that the data can be combined unambiguously. The birefringence and ultraviolet absorption spectrum2’ can also be measured on the same sample to determine its concentration and degree of orientational order. DISCUSSION We now have some quantitative data on the elastic and viscous properties of TMV’’7’7 and PBG” nematics. We have deduced ratios of elastic constants and viscosities from measurements on the dynamic-stripe phenomena in one preparation of TMV. For PBG we have performed light-scattering experiments and some Frederiks-transition measurements on the same samples, which give compatible results.The TMV is a carefully extracted preparation of monodisperse virus. From X-ray studies of the same preparation we roughly estimate its order parameter to be 0.9. Table 1 compares our measurements with hard-rod theory. For an order parameter of 0.8 the agreement would be remarkably good. The way these data are obtained is not via the most direct experiments, and the complex fitting procedures used for analysing the oblique stripes do not necessarily determine the parameters uniquely. Moreover, we only have ratios of various parameters, rather than absolute values. Nonetheless, the results are suggestive that the hard-rod model in the Onsager limit is not far from correct.132 MECHANICAL PROPERTIES OF POLYMER NEMATICS We have extensive data on one sample of PBG at a concentration just above the coexistence region, reported in table 1.The absolute values reported for various parameters are based on the published value for the diamagnetic anisotropy of PBG. The saxple is obtained from the Sigma Chemical Co., which gives its degree of polymerization as 700, implying a length-to-diameter ratio of 70. The data could be consistent with either the hard-rod model or the semiflexible-chain limit. The concentration dependence and the molecular-weight dependence of the elastic constants and the viscosities will be crucial for distinguishing between these possibilities. These experimental methods should also be useful for several other lyotropic polymer nematics, providing the opportunity to see if these systems are at all universal in their behaviour. We thank Gerry Swislow for extensive assistance and instruction on computation. This research was supported in part by the U.S.National Science Foundation through grant no. DMR-8210477, and by the Martin Fisher School of Physics, Brandeis University. C. Robinson, Tetrahedron, 1961, 13, 219. (Academic Press, New York, 1978), vol 14, pp. 77-107. Orsay Group on Liquid Crystals, J. Chem. Phys., 1969, 51, 816. Orsay Group on Liquid Crystals, Phys. Rev. Lett., 1969, 22, 1361. L. Onsager, Ann. N. Y. Acad. Sci., 1949, 51, 627. J. P. Straley, Phys. .Rev. A, 1973, 8, 2181. N. Kuzuu and M. Doi, J. Phys. SOC. Jpn, 1983, 52, 3486; 1984, 53, 1031. Sin-Doo Lee and Robert B. Meyer, in preparation. R. B. Meyer, in Polymer Liquid Crystals, ed. A. Ciferri, W. R. Krigbaum and R. B. Meyer (Academic Press, New York, 1982), chap. 6. S. Fraden, A. J. Hurd, R. B. Meyer,< M. Cahoon and D. L. D. Caspar, Roc. of the les Houches Workshop on Colloidal Crystals, J. Phys. (Paris), 1985, 46, C3-85. ' H. J. Deuling, in Solid State Physics, ed. H. Ehrenreich, F. Seitz, D. Turnbull and L. Liebert l o V. G. Taratuta, G. M. Srajer and R. B. Meyer, Mol. Cryst. Liq. Cryst., 1985, 116, 245. I t l 2 N. S. Murthy, J. R. Knox and E. T. Samulski, J. Chem. Phys., 1976, 65, 4835. l 3 F. Lonberg and R. B. Meyer, personal communication. l4 G. Maret, S. Fraden, M. Cahoon and D. L. D. Caspar, work in preparation. ' 5 F. Lonberg and R. B. Meyer, personal communication. l6 F. Lonberg, S. Fraden, A. J. Hurd and R. B. Meyer, Phys. Rev. Lett, 1984, 52, 1903. l7 A. J. Hurd, S. Fraden, F. Lonberg and R. B. Meyer, J. Phys. (Paris), 1985, 46, 905. l8 F. Lonberg and R. B. Meyer, Phys. Rev. Lett., 1985, 55, 718. l9 V. G. Taratuta, A. J. Hurd and R. B. Meyer, Phys. Rev. Lett., 1985, 55, 246. 2o V. G. Taratuta, personal communication.

ISSN:0301-7249    

DOI:10.1039/DC9857900125

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

Faraday Discuss. Chem. Soc., 1985, 79, 133-140 Phase Studies of Binary Mesogenic Systems BY WILLIAM R. KRIGBAUM Department of Chemistry, Duke University, Durham, North Carolina 27706, U.S.A. Received 26th November, 1984 Although several theories exist for rod-like nematogens, our understanding of thermotropic mesomorphism in semiflexible chain polymers is less well developed. Flory observed that the nematic behaviour of Kuhn chain polymers is governed by the axial ratio of the Kuhn link and is nearly independent of the contour length of the chain. Following this lead, we attempted to fit binary phase diagrams of two polymer + diluent systems using the Warner- Flory treatment of rod-like nematogens. The only modification involved replacement of the axial ratio of the rod by that of the Kuhn link.This theory assumes the nematic phase i s stabilized by orientation-dependent interactions and that the axial ratio of the rod is indepen- dent of temperature. Poor agreement with the experimental phase diagrams suggests some other factor is important. We then took into account the temperature dependence of the unperturbed dimensions through appropriate modification of Flory’s treatment of Kuhn chain polymers. This predicts a thermotropic nematic-isotropic transition, even in the absence of orientation-dependent interactions: at the temperature which reduces the axial ratio of the Kuhn link to the critical value. Persistence lengths measured for two polymers at different temperatures, when extrapolated to their isotropization temperatures, produce axial ratios very near the critical value.This treatment predicts that all semiflexible polymers that behave as Kuhn chains should be in corresponding states at their isotropization temperatures. The application of this concept is illustrated by examining the thermal behaviour of a number of substituted poly(p-phenylene terephthalates). According to current theory of rod-like mesogens, thermotropic behaviour requires orientation-dependent interactions and an axial ratio less than the critical value. Orientation-dependent forces are required to provide an enthalpic contribu- tion to the stabilization of the nematic phase. If the axial ratio exceeds the critical value, the entropic contribution provides ample stabilization, and no thermotropic transition to the isotropic phase is possible.Flory and Ronca,’ in seeking an experimental system for comparison with theoretical predictions, emphasized the requirement that the nematogen be a rigid rod with nearly cylindrical symmetry. They proposed the p-phenylene oligomers as ideal candidates. This system has been extensively studied,*-’ but it presents serious experimental difficulties because the transition temperatures increase rapidly, accompanied by a corresponding decrease in solubility, as the size of the oligomer increases. Similar problems can be anticipated6 for other rod-like mesogens owing to the small entropy change accompanying their crystal-nematic transitions. An extension of the Flory-Ronca theory by Warner and Flory7 accounts for the effect of diluent in diminishing the orientation-dependent interactions between nematogenic molecules.This affords predictions for binary systems which are much more amenable to experimental study. Our theoretical understanding of semiflexible polymers which exhibit a ther- motropic nematic phase is currently in a more rudimentary state. Flory,* in his treatment of Kuhn chain polymers, observed that their nematic behaviour is governed by the axial ratio of the Kuhn link and is almost independent of the contour length 133134 PHASE STUDIES OF BINARY MESOGENIC SYSTEMS N I I I I I I I I 1 I 0 2 Fig. 1. ( a ) Variation of the nematic-isotropic transition temperature of PHIC in toluene (open circles and dotted curve) compared with the temperature dependence of the biphasic region predicted according to the Warner-Flory treatment with axial ratio 40.(b) A similar comparison of the data of Conio et af." for hydroxypropylcellulose in dimethylacetamide (dotted curves) with the Warner-Flory predictions for axial ratios 6 (full curves) and 2 (dashed curves). of the chain. This suggests that the Warner-Flory treatment' might be applicable to binary semiflexible polymer + diluent systems simply by replacing the axial ratio of the rod by that of the Kuhn link. RESULTS AND DISCUSSION Krigbaum et aL9 collected data for the binary system poly(n-hexyl isocyanate), PHIC, in toluene to provide a test of this procedure. They also utilized the data of Conio et al. l o for hydroxypropylcellulose, HPC, in dimethylacetamide. The experi- mental observations are represented by the open circles and dotted curves in fig.1.W. R. KRIGBAUM 135 PHIC undergoes a thermotropic nematic-isotropic transition, and the transition temperature is depressed by the addition of toluene. We have assigned the axial ratio of the Kuhn link as l=40, which is approximately the value deduced from experimental data at 25 "C. Because this is larger than the critical value, 6.42, theory predicts no thermotropic transition for the bulk polymer. The binary system is predicted to exhibit a narrow biphasic region, as indicated by the two vertical lines in fig. l ( a ) . H ydroxypropylcellulose exhibits a thermotropic cholesteric-isotropic transition, and the transition temperature is again depressed by the addition of diluent, dimethylacetamide.For this system we have assigned the value of the axial ratio, l= 6, obtained by extrapolation to the isotropization temperature, Tf. Since this is less than the critical value, a thermotropic transition is predicted for the bulk polymer. The parameter T", a measure of the strength of the orientation-dependent interactions, can be evaluated from a knowledge of Tf. The cholesteric phase is designated as Ch in fig. 1 (6). The predicted biphasic region is indicated by the two full lines labelled l= 6 in the figure. The predicted depressions of the biphasic temperatures are much too steep, and remain so even when the calculations are repeated for a much smaller axial ratio, 5 = 2 [as shown by the pair of dashed curves in fig. l(b)]. It is evident that the observed transition temperatures for both systems span a much broader range of compositions than can be accommodated by the Warner- Flory treatment.In fact, the discrepancies between the theoretical predictions and observation are large enough to suggest that some other factor must play a significant role in the isotropization transition. In the foregoing treatment the axial ratio was assumed to be independent of temperature. It is well recognized that the unperturbed dimensions of coiling macromolecules (and the axial ratio of the Kuhn link) vary with temperature. We suggest that this temperature dependence is the missing factor which can provide an alternative mechanism for the nematic-isotropic transition. Even if the axial ratio exceeds the critical value at some particular temperature, this does not imply that the isotropization temperature of the polymer, Tf, is infinite.On the contrary, Tf will coincide with the temperature at which the axial ratio has decreased to the critical value. As a test of this proposal, Krigbaum et aL9 have measured the persistence length of PHIC in toluene and tetrahydrofuran over a range of temperatures. These data are represented by the open and filled circles in fig. 2, where In 5 is plotted against the reduced temperature, T / Tf. Fig. 2 also includes data collected by Aden et al." for HPC in dimethylacetamide. The critical value of the axial ratio of the Kuhn link given by Flory's treatment, Ccrit = 6.70, is represented by the small cross on the right-hand side of the figure. Extrapolation of the data for both polymers to Tf produces axial ratios very near the critical value, as expected from the foregoing argument.We would anticipate that the orientation-dependent interactions are minimal for PHIC owing to the shielding effect of the long sidechains. The lower extrapolated value for HPC may indicate that these interactions play a small role in stabilizing the nematic phase for this polymer. In order to test further the importance of the temperature dependence of the axial ratio we9 have modified the treatment of Kuhn chain polymers given by Flory* through introduction of a temperature-dependent axial ratio. Values of 6 = -d In l / d T for these two polymers can be obtained from the slopes of the lines in fig. 2. The predictions of this treatment are compared with the experimental data in fig.3. The full curves were calculated using the experimentally determined values,136 PHASE STUDIES OF BINARY MESOGENIC SYSTEMS 0.6 0.7 0.8 0.9 1.0 TR= T/TP Fig. 2. Temperature dependence of the axial ratio of PHIC in toluene (open circles) and tetrahydrofuran (filled circles) plotted as In 6 against the reduced temperature, T / r. The triangles and lower line represent the data of Aden et al" for hydroxypropylcellulose in dimethylacetamide. 6 = 0.01 1 for PHIC and 0.005 for HPC. The predicted depressions are too steep for both systems. For HPC + dimethylacetamide we can achieve nearly quantitative agreement by increasing 5 to 0.0073. For HPIC+toluene an increase of 6 to 0.013 gives a better fit, but the experimental and predicted curves have a different shape.Despite the failure to achieve quantitative agreement, comparison with the results in fig. 1 clearly indicates that the predictions appearing in fig. 3 are significantly better than those based upon the Warner-Flory treatment. An important outcome of this treatment is the concept that all semiflexible polymers which can be represented by the Kuhn chain model should be in corre- sponding states at their respective isotropization temperatures. The application of this concept will be illustrated by considering data for substituted poly(p-phenylene terephthalates). Poly(ppheny1ene terephthalate) is a rigid macromolecule with a high crystal melting temperature. The persistence length of this polymer has not been determined experimentally, but Erman et aL12 have calculated a value of 784 A.This is substan- tially larger that the experimental value, cu. 200 for the corresponding aromatic polyamide. Jackson14 reported the crystal melting temperature of the aromatic polyester to be 600°C. As disclosed by the early patent of Goodman et u1.,l5 the melting temperature of the parent polyester can be lowered by placing a substituent on one or both of the aromatic rings. We16 have investigated the thermal behaviour of a number of substituents. The repeating unit will be represented by X Y so monosubstituted polymers will be symbolized by X/H or H/Y and disubstituted polymers by X/Y. The results of this study are summarized in table 1.W. R. KRIGBAUM 137 450 k4 2400 350 v2 Fig.3. Comparison of the phase diagram of ( a ) PHIC in toluene and ( b ) hydroxypropyl- cellulose in dimethylacetamide with predictions based upon a temperature-dependent axial ratio. The full and dashed curves for PHIC were calculated using ( = O . O l l and 0.013, respectively. The full and dashed curves for HPC represent predictions for (=0.005 and 0.0073, respectively. As shown in column three, a substituent such as a methyl group or a halogen atom lowers tKN into the range 370-405 "C owing to positional isomerism of the substituent. Larger depressions are obtained with phenyl or hexyl substituents which provide additional rotational or conformational degrees of freedom per repeating unit. As claimed by Harris,I7 the H/C6H5 polyester has a lower melting temperature than the C6H5/H polymer, the values being ca.290 and 345 "C, respectively. Substitu- tion on both aromatic rings affords still lower melting temperatures. With one exception, the range of tKN values for the disubstituted polymers is ca. 205-235 "C. The one exception is the C6H5/C6HS polymer, which is amorphous because the melting temperature has been lowered into the vicinity of the glass transition temperature.138 PHASE STUDIES OF BINARY MESOGENIC SYSTEMS Table 1. Transition temperatures (in "C) of substituted poly(p-pheny- lene terephthalates) substituent t G ~ K N tNI ( tNI - tKN) unsubstituted H/H 26718 66014 - - monosubstituted X/ H C1/ H 220 372 5 10 138 C6H5/ 170 346 475 129 C6H13/H - 330 462 132 Br/ H - 353 475 122 monosubstituted H/Y H/ C1 220 402 490 88 H/ Br - 405 490 85 H/C6H5 130 287 369 82 disubstituted X/Y C1/ Br 120 213 362 149 ClIC6H5 113 233 3 60 127 C6H5/Br 120 222 376 154 C6H5/C1 108 206 368 162 C6H 13/ Br 121 208 365 157 C6H5/C6H5 122 - 23 1 - We initially anticipated that the substituted poly(p-phenylene terephthalates) would retain the high chain extension of the parent polyester.It was therefore surprising to find that the substituted polymers exhibit a thermotropic transition to the isotropic phase. If all these polymers are in corresponding states at their isotropization temperatures, this implies that substitution on one or both aromatic rings reduces the unperturbed molecular dimensions (and the axial ratio of the Kuhn link). Column four lists the isotropization temperatures, tNI, while column five gives the temperature range of the nematic phase, ( tNI - tKN).Except for H/C6H5, the tNI values for the monosubstituted polymers fall in the range 460-5 10 "C, and (except for C ~ H ~ / C ~ H S ) those of the disubstituted derivatives range from 360 to 376 "C. We can infer from these data that the unperturbed molecular dimensions are reduced by substitution, and the reduction is larger for the disubstituted polymers. The unusually low tKN values for H/C6H5 and C ~ H ~ / C ~ H S suggest that a phenyl sub- stituent on the terephthalate ring probably forces the nearest carbonyl group out of the plane of the ring, creating a flexible link. The relatively smaller nematic temperature range, ( tNI- fKN), for the H/Y polymers may indicate that any substituent on terephthalic acid tends to destabilize the nematic phase more than when it is located on hydroquinone. Further support for these conclusions is provided by the glass-transition tem- peratures listed in column two.The glass-transition temperature, tG, of the parent polymer, 267 "C,'* is reduced to ca. 220 "C by a single substituent, to 170 "C for C6H5/H and to 130 "C for H/C&,. The tc values for the disubstituted polyestersW. R. KRIGBAUM 139 fall in the range 100-122 "C. Since the glass-transition temperature decreases as the chain becomes more flexible, these observations lead to the same conclusions as those reached from the tNI values. CONCLUSIONS We believe the results presented above strongly support our contention that the decrease in unperturbed dimensions with increasing temperature plays a significant role in bringing about the nematic-isotropic transition in semiflexible polymers.The observation that the axial ratios for two different polymers, when extrapolated to their isotropization temperatures, are very close to the critical value is particularly persuasive. The predicted binary phase diagrams, while not providing a quantitative fit to the experimental observations, nevertheless represent a significant improvement over the results obtained using the Warner-Flory treatment. Our predictions are based upon Flory's theory' for Kuhn chain polymers. Unfortunately that treatment does not predict the orientation distribution of the Kuhn links in the nematic phase, so that orientation-dependent interactions cannot be taken into account.Both ten Bosh et U L ' ~ and Ronca and Yoon20 have recently developed theories for the nematic behaviour of a thread-like model chain. The partition function used by the former workers does not include a factor for the number of degrees of conformational freedom accessible to a system of chains in the nematic phase, while the latter workers have not yet incorporated orientation-dependent interactions into their treatment. We anticipate that further improvements in the treatment of the mesomor- phic behaviour of semiflexible chain polymers will appear shortly. Consideration of the temperature dependence of the unperturbed dimensions and its influence upon the transition to the isotropic phase leads to the conclusion that all polymers meeting the requirements of a Kuhn chain will be in corresponding states at their isotropization temperatures.This conclusion has far reaching implica- tions. One possible practical application of thermotropic polymers is to produce ultra-high-modulus fibres by melt spinning. We, in collaboration with Acierno,2' 322 as well as Simoff and have spun semiflexible polymers from the nematic phase and obtained poor mechanical properties. In view of these results, we turned our attention to the class of rigid-chain polymers. As exemplified by poly(p- phenylene terephthalate) these have melting temperatures which are too high to permit melt spinning. By introducing randomness along the chain through substitu- tion one might hope to reduce the melting temperature while retaining the high chain rigidity.This proved not to be possible for the poly(p-phenylene terephtha- lates), as indicated above. Instead, both tKN and tNI are reduced simultaneously, which implies that those polymers having lower melting temperatures also have a more semiflexible chain character. Based upon our spinning experience cited above, we would expect the fibre modulus to be reduced in parallel with the isotropization temperature. One piece of confirmatory evidence is obtained upon comparing the literature values of the modulus for heat-treated fibres of H/C& and C&/H. These are 352 l7 and 910 l4 g denier-', respectively. As expected, the polymer having the lower isotropization temperature exhibits the lower fibre modulus. While the influence of temperature on the unperturbed dimensions has been investigated experimentally for several polymers, the effect of temperature upon the average conformation of small molecules is less well explored.It is entirely possible that the population of conformers for many low-molecular-weight mesogens may be temperature-dependent. This variation could have a significant effect upon the nematic-isotropic transition temperature of this class of molecule as well.140 PHASE STUDIES OF BINARY MESOGENIC SYSTEMS We acknowledge the support of the U.S. Army Research Office through contract DAAG-84-K-0033, and the National Science Foundation Industry/ University Cooperative Research Activity through grant DMR-8 106 160. ' P. J. Flory and G. Ronca, Mol. Cryst. Liq.Cryst., 1979, 54, 3 11. G. W. Smith, Mol. Cryst. Liq. Cryst., 1979, 49, 207. I. C. Lewis and C. A. Kovac, Mol. Cryst. Liq. Cryst., 1979, 51, 173. 1. C. Lewis and J. B. Barr, Mol. Cryst. Liq. Cryst., 1981, 72, 65. W. R. Krigbaum and A. Ciferri, J. Polym. Sci., Polym. Lett. Ed., 1980, 18, 253. M. Warner and P. J. Flory, J. Chem. Phys., 1980, 73, 6327. P. J. Flory, Macromolecules, 1978, 11, 1141. W. R. Krigbaum, H. Hakemi, A. Ciferri and G. Conio, Macromolecules, 1983, 16, 1264. M. A. Aden, E. Bianchi, A. Ciferri, A. Conio and A. Tealdi, Macromolecules, in press. ' P. A. Irvine, Da Chung Wu and P. J. Flory, J. Chem. Soc., Faraday Trans. I , 1984, 80, 1795. lo G. Conio, E. Bianchi, A. Ciferri, A. Tealdi and M. A. Aden, Macromolecules, 1983, 16, 1264. '' B. Erman, P. J. Flory and J. P. Hummel, Macromolecules, 1980, 13, 484. l 3 M. Arpin and G. Strazielle, Polymer, 1977, 18, 591. l4 W. J. Jackson Jr, Br. Polym. J., 1980, 12, 154. l6 W. R. Krigbaum, H. Hakemi and R. Kotek, Macromolecules, in press. l7 J. F. Hams Jr, US. Patent 4 294 955, 1981. '* V. Frosini, G. Levita, J. Landis and A. E. Woodward, J. Polym. Sci., Polym. Phys. Ed., 1977,15,239. l9 A. ten Bosh, P. Maissa and P. Sixou, J. Phys. (Paris) Lett., 1983, 44, L105; J. Chem. Phys., 1983, 2o G. Ronca and D. Y. Yoon, J. Chem. Phys., 1982,76, 2395; 1984,80,925,930. 'I D. Acierno, F. P. La Manta, G. Polizzotti, A. Ciferri, W. R. Krigbaum and R. Kotek, J. Polym. 22 W. R. Krigbaum, A. Cifem and D. A. Acierno,, J. Appl. Polym. Sci., Appl. Polym. Symp., in press. 23 D. A. Simoff and R. S. Porter, Mol. Cryst. Liq. Cryst., 1984, 110, 1. I. Goodman, J. E. McIntyre and D. H. Aldred, British Patent 993 272, 1975. 79, 3462. Sci., Polym. Phys. Ed., 1983, 21, 2027.

ISSN:0301-7249    

DOI:10.1039/DC9857900133

出版商:RSC    

年代:1985

数据来源: RSC