摘要:

Faraday Discuss. Chem. Soc., 1985, 79, 251-264 Chemical Characteristics of Cellulosic Liquid Crystals BY DEREK G. GRAY Pulp and Paper Research Institute of Canada and Department of Chemistry, McGill University, Montreal, Canada H3A 2A7 Received 17 th December, 1984 Cellulosic derivatives form an interesting range of polymer liquid crystals. They belong to a class of mesophases in which the polymer chain is stiff or semiflexible and is chemically homogeneous. The cellulose backbone, a single-stranded homopolymer of @linked 1,4- anhydroglucose units, contains three hydroxy groups per anhydroglucose unit which provide convenient sites for substitution reactions, leading to a wide variety of cellulose ethers and esters. Many of these derivatives form cholesteric liquid-crystalline phases in suitable solvents, and some derivatives with relatively large (but non-mesogenic) side chains form both lytropic and thermotropic liquid crystals.Published observations on the phase separation of cellulose-based polymers indicate that the mesophases form at critical volume fractions of polymer ranging from 0.3 to 0.5 for high molar mass samples at room temperature. The critical volume fractions for a given polymer and solvent decrease with increasing molar mass to approach these asymptotic values. The critical volume fraction increases with temperature. The nature of the solvent is also important; highly polar or acidic solvents generally favour mesophase separation at lower critical concentrations than simple organic solvents. Furthermore, whereas heavily substituted deriva- tives with long flexible side-chains form mesophases easily in a wide range of solvents, the more familiar derivatives with smaller substituents seem to form mesophases with specific solvents, but not with others. We have measured the critical concentrations of fractions of (acetoxypropy1)cellulose in dialkyl phthalate solvents over a large temperature range and compared the results with predictions of theories for the phase separation of freely jointed and worm-like chains.The results indicate that chain geometry and stiff ness are the major factors controlling liquid- crystalline phase formation and that anisotropic inter-chain interactions play a minor role. Thus the chemical structure of polymer and solvent govern the phase separation indirectly in these systems, primarily through effects on the chain conformation. In considering the chemical characteristics which give rise to polymer liquid crystals, some classification is helpful.Polymeric liquid crystals are usually classified according to the mesophase structure (nematic, cholesteric etc.) associated with conventional low molar mass liquid crystals. However, this classification is not always sufficient. Polypeptides in certain solvents form cholesteric mesophases,* as do the thermotropic melts of some polysiloxanes with cholesteryl ester side-chains.2 Although the mesophase structure in both cases is cholesteric, the properties of the materials and even the driving forces for their formation are quite different in these two examples.Some further classification according to chain structure is required. There are two major classes of polymeric liquid crystals. In one structural class of mesophases the polymers contain mesogenic groups, either in the main chain or in side chains, linked by flexible spacers. The mesogenic groups may resemble conven- tional low molar mass liquid crystals, or they may be rigid chain segments. These polymer chains are essentially heterogeneous in structure, with rigid anisotropic segments linked to flexible spacers, and may thus be called ‘heterocamptic’ (hetero + 257253 CELLULOSIC LIQUID CRYSTALS ? Fig. 1. Idealized structure of an acetoxypropylcellulose chain. On average, each anhydro- glucose unit in this sketch contains 2.5 ether substituents and 1.5 ester substituents.K C Z ~ T T U =to bend). Examples of heterocamptic polymer mesophases include the side-chain cholesteryl derivatives of polysiloxanes2 and met ha cry late^^ and the main-chain aromatic polyesters containing flexible aliphatic chain segments.495 The second class of mesophase is derived from polymers with uniformly stiff or semiflexible backbones which are chemically homogeneous on a scale very much shorter than their contour or persistence lengths.6 This class may be called 'homocamptic' mesophases. Examples include the synthetic polypeptides,' aromatic polyamides7 and cellulose derivatives.8 CELLULOSIC MESOPHASES The cellulose backbone is a single-stranded homopolymer of P-linked 1,4- anhydroglucose units. Each anhydroglucose unit contains three hydroxy groups which provide convenient sites for substitution reactions, leading to a wide variety of cellulose ethers and esters.An example of a heavily substituted cellulose chain is shown in fig. 1. Many of these derivatives form cholesteric liquid-crystalline phases in suitable solvents, and some derivatives with relatively large but non- mesogenic side-chains form both lyotropic and thermotropic liquid crystals. Reviews covering the field to early 1982 have appeared.'-'' In work which has been published since these reviews, some new substituted cellulosic derivatives which form ther- motropic cholesteric phases have been prepared,] I and much effort has been devoted to investigating the previously reported systems. Anisotropic solutions of cellulose acetate and triacetate in trifluoroacetic acid have attracted the attention of several groups.Chiroptical properties, l2*I3 the refractive index,14 phase b~undaries,'~ nuclear magnetic resonance spectra ' 6 ~ 1 7 and differential scanning calorimetry 18, '' have been reported for this system. However, trifluoroacetic acid causes degradation of cellulosic polymers; this calls into question some of the physical measurements on these mesophases because time is required for the mesophase solutions to achieve their equilibrium order. Mixtures of trifluoroacetic acid with chlorinated solvents have been employed to minimize this problem,20 and anisotropic solutions of cellulose acetate and triacetate in other solvents have been examined.21922 The mesophase formed by hydroxypropylcellulose (HPC) in water' is stable and easy to handle, and has thus attracted further attention, '7~18923-27 as has the thermotropicD.G. GRAY 259 mesophase of HPC.28 Detailed studies of mesophase formation and chain rigidity for HPC in dimethyl a ~ e t a m i d e ~ ~ and in dichloroacetic acid3' and for the benzoic ester of HPC in acetone and benzene31 have been published. Anisotropic solutions of methylol cellulose in dimethyl s ~ l p h o x i d e ~ ~ and of cellulose in dimethyl acetamide +LiCf3 have been reported. but in the latter case the mesophase forms close to the solubility limits for the system.34 The optical properties of cellulose carbanilate mesophases in methyl ethyl ketone35 and the dielectric properties of ethyl cellulose mesophases in d i ~ x a n e ~ ~ have been investigated, and a route to a cross-linked cholesteric film from a cellulose-based mesophase has been rep~rted.~' The relative abundance of liquid-crystalline systems observed for derivatives of P-linked anhydroglucose polymers has spurred efforts to find liquid-crystalline mesophases from other polysaccharides.Amylose is composed of a-linked 1,4- anhydroglucose units; its triethyl and trimethyl ethers are reported to form liquid- crystalline phases in chlor~form.~~ Liquid-crystalline phases also form in aqueous solutions of the triple helical P-linked 1,3-glucans s~hizophyllan~~ and sclero- g~ucan.~' PHASE SEPARATION AND CHAIN STIFFNESS The primary factor governing liquid-crystalline phase formation for cellulose derivatives does appear to be chain stiffness;l but critical concentration for mesophase separation does also depend to some extent on factors such as the nature and degree of substitution, the solvent, the temperature and the molar mass of the polymer.Quantitative studies of the phase-separation process have been reported for HPC in dimethyl a ~ e t a m i d e ~ ~ and in dichloroacetic acid,30 and for the benzoic acid ester of HPC (BzPC) in acetone and ben~ene.~' The appearance of a liquid- crystalline phase required concentrations of HPC (150 000 g mol-') >40 wt0% in dimethyla~etarnide.~~ The biphasic region extended to a concentration of 50 wt% ; at higher polymer concentrations the solution was assumed to be completely anisotropic because no isotropic phase separated on extensive ultracentrifugation.In the two-phase region the concentrations of the conjugate phases were not constant, but tended to follow the overall concentration. The critical concentrations in dimethyl acetamide were slightly lower than in water, and in both solvents the critical concentrations increased slightly with decreasing molar mass.29 However, in dichloroacetic acid the critical concentration for phase separation was only 13 wt% for high molar mass samples, and a much larger increase in critical concentration with decreasing molar mass was ~bserved.~' The critical concentrations for the BzPC are also constant for high molar mass fractions and increase sharply at low molar ma~ses.~' For each of the above systems the phase-separation data were rationalized .in terms of the chain stiffness.The chain stiffness was estimated from dilute solution viscosities of polymer fractions in the same solvents as the phase-separation measure- ments; the results were expressed as the Kuhn segment length for a random flight chain or the equivalent Kuhn segment length, L = 2 q , for a worm-like chain of persistence length q. The chain diameter, d, was estimated from X-ray or hydrody- namic measurements, and the resultant chain-axis ratios, k / d , were used to test the various phase-separation theories for semiflexible chain molecules. The agreement between theory and experiment was less than perfect.3033' The persistence length for HPC in dichloroacetic acid (10 nm) was significantly larger than in dimethyl acetamide (7 nm). This is qualitatively in line with the lower critical concentration in dichloracetic acid, but the critical concentrations are significantly lower than260 CELLULOSIC LIQUID CRYSTALS those predicted by the Flory lattice for freely jointed chains.The BzPC results were very similar. In both systems the increase in critical concentrations with decreasing molar mass was observed for chains whose contour lengths were still many times longer than their persistence lengths. This increase was of course not predicted by the freely jointed model for phase separation, but it was predicted by a virial treatment for the phase separation of worm-like chains.43 However, quantitative agreement with the latter theory was p ~ o r ; ~ ' , ~ ' the measured critical concentrations were much lower than predicted, and the upswing in critical con- centrations occurred for chains which are longer than predicted by the theory.The agreement for long chains of HPC and BzPC was better with a virial theory for freely jointed but the validity of the virial approximation at high concentra- tions is questionable. The incorporation of anisotropic inter-chain interactions into the lattice theories by Flory and does bring the predictions of these theories into line with the experimental results on HPC.29,30 A temperature depen- dence of the critical concentration for semiflexible chains may be attributed to anisotropic interactions or to changes in the persistence length with temperature. For HPC in water, the lower critical solution temperature of ca.40 "C restricted the observable temperature range for the anisotropic phase However, with dimethyl acetamide as solvent, a strong increase in critical concentration with increasing temperature was This was attributed primarily to a decrease in chain persistence length with temperature, with anisotropic interactions playing a minor ACETOXYPROPYLCELLULOSE + PHTHALATE ESTER DILUENT Acetoxypropylcellulose (APC) was observed to form a thermotropic cholesteric m e ~ o p h a s e . ~ ~ This unexpected finding, coupled with the ease with which APC formed lyotropic solutions with a variety of solvents, suggested that a study of the phase behaviour of APC with a suitable non-volatile diluent might provide useful experimental data at concentrations ranging from the critical concentration up to the pure polymer, and at temperatures up to the clearing temperature of the undiluted anisotropic phase.The phthalate esters gave cholesteric lyotropic mesophases with APC, and the dilute solution viscosities in these solvents were measurable over a large temperature range. Full details are available el~ewhere.~~.~' Briefly, APC (fig. 1) was prepared from HPC and acetic anhydride. The chemical composition of the product was analysed by i.r. and proton n.m.r. spectroscopy and by quantitative but tedious chemical procedures. Each anhydroglucose unit was substituted with approximately three hydroxyether units, and two out of the three hydroxy groups (on the anhydroglucose unit or side chain) were esterified. Fractions ranging in molar mass from 2.2 x lo4 to 6.5 x lo5 g mol-' were obtained by fractional precipitation with n-heptane.The weight-average molar mass of each fraction was measured with static low-angle laser light scattering, and the molar mass distribution was measured by size-exclusion chromatogrpahy with the low-angle laser-light- scattering photometer as detector. In order to characterize the stiffness of the APC chain, the limiting viscosity numbers for dilute solutions of APC fractions in dimethyl phthalate were measured at 25, 105 and 150 "C. The Mark-Houwink-Sakurada exponents for these three temperatures were 0.88, 0.75 and 0.57, respectively, in accord with the expected decrease in chain stiffness with increasing temperature. The decrease in chain stiffness was quantified according to Bohdanecky's treatment" of the Yamakawa-Fujii theory for the viscocity of worm-like chains.The results are shown in table 1. The theory gave ML, the mass per unit length along the chain,D. G. GRAY 26 1 Table 1. Chain dimensions for acetoxypropylcellulose in dimethyl phthalate from viscosity data according to Bohdanecky's formula- tion" of the Yamakawa-Fujii worm-like chain model T / "C 25 105 150 MJdalton nm-' 900 860 630 d /nm 1.26 1.22 1.46 k/nm 14.4 9.46 5.30 kwlnmU 13.5 9.3 7.1 d/nm" 1.14 1.16 1.49 ~~~~~~ ~ ~ ~ ' For ML = 821 dalton nm-' (see text). and d and k, which are the hydrodynamic diameter and equivalent Kuhn length, respectively. The values for ML at 25 and 105 "C were close to the value of 821 dalton nm-' which may be estimated from the molar mass (423 dalton) and projection of the length (0.5 15 nm) of the substituted anhydroglucose units in the chain.The values for the hydrodynamic diameter, d, at 25 and 105 "C are close to the value of the observed X-ray d spacing (1.22 nm) for the bulk mesophase. The results at 150 "C show an increase in d and a decrease in ML. If real, these changes may indicate an extension of the side chains and of the main chain with increasing temperature. A large change in ML seems unlikely, so table 1 includes values for d and k, calculated for a temperature-independent value for ML of 821 dalton nm-'. The decrease in k, with temperature is very marked, with segment axial ratios, k / d , approaching ca. 5 at 150 "C. (The value increases to ca. 6 if the X-ray d-spacing is used for the chain radius.) The values at 150 "C are subject to some uncertainty owing to chain degradation; the addition of an antioxidant, a nitrogen blanket and an acid acceptor reduced but did not entirely eliminate the rapid chain degradation observed without these precautions. The phase-separation data for APC in dibutyl phthalate at 25 "C seemed straight- forward; all fractions and the bulk polymer showed anisotropic behaviour at polymer volume fractions of 0.52 or greater.In contrast to several other cellulosic mesophases, no increase in critical volume fraction, +:, was observed with decreasing molar mass (the lowest molar mass sample had a contour length of 94nm, which is significantly longer than & for APC of 14 nm at 25 "C). The unfractionated solution was biphasic up to a polymer volume fraction of 0.69. The critical concentra- tions were slightly higher than observed for HPC in dimethyl a ~ e t a m i d e ~ ~ and the width of the biphasic region was typical for cellulosics.The compositions in the two-phase regions were analysed for composition by i.r. spectroscopy and for molar mass by size-exclusion chromatography with a low-an le laser-light-scattering photo- meter. Again in accord with the work of Conio et nl?'the concentrations of polymer in the conjugate phases were not constant, but increased with increasing overall concentration, and the molar masses in the anisotropic phases were consistently larger than in the conjugate isotropic phases. This is puzzling in the light of the constant 4: observed for different APC fractions.Conio et aZ. ascribed these results to non-equilibrium effects in the ultracentrifuge which was used to separate the phases.29 We avoided this problem by simply allowing the phases to separate by standing for a couple of years. Unfortunately this introduced another unexpected difficulty; slow degradation of the APC chains occurred in dibutyl phthalate even262 CELLULOSIC LIQUID CRYSTALS Table 2. Experimental critical volume fractions, +:, and axial ratios, kJ d, for acetoxypropylcellulose as a function of temperature 25 0.52 12 105 0.7 1 8 150 0.86 6 182 1 .o (ca. 4.6)b "Bodanecky theorys' for k, in dimethylphthalate ; d taken to be con- stant at 1.2 nm. Value from linear extrapolation of experimental data. at room temperature, so the molar-mass measurements were not of quantitative significance. (The degradation of APC was noted only after long storage in dibutyl phthalate solutions.) As expected, the critical concentration for anisotropic phase formation increased with increasing temperature.The data at the temperatures of the viscosity measure- ments are shown in table 2. The anisotropic phase was still observed at temperatures where the segment axial ratio has dropped below 6.4. This is the critical value for the formation of ordered phases by non-interacting rigid and may indicate that 'soft' anisotropic interactions, characterized by a temperature T*, are involved.46 However, the values of T" required to explain the phase separation increased with temperature from 7'" = 80 at 25 "C to T* == 250 at 182 "C.It must be remembered that the theory of Warner and Fl01-y~~ is for individual rods, whereas the APC chain is modelled as a set of freely jointed rods. Connecting the rods together in a freely jointed chain may make little difference to the critical concentration in the absence of anisotropic segment-segment interaction^,^^ but the difference may be greater in the presence of such interactions. Furthermore, the freely jointed rod or worm-like chain models are at best approximations to the cellulose backbone conformation, and are limiting cases of flexibility distribution along the chain. Any changes in the flexibility distribution with temperature would alter the apparent value of T*. The anisotropic-isotropic transition for APC was also investigated by differential scanning calorimetry and by hot-stage microscopy.The transitions were very broad and the enthalpies were not very reproducible. For unfractionated and undiluted APC, the transition occurred at 182 "C with an enthalpy change of ca. 0.65 cal g-'t polymer at a heating rate of 20 "C min-'. The transition enthalpy dropped quickly with added diluent; at 80 wt% APC in dibutyl phthalate the transition temperature was 103 "C and the enthalpy ca. 0.16 cal g-'. The enthalpy of the phase transition became virtually undetectable at lower polymer concentrations. The decrease in enthalpy with decreasing concentration of cellulose acetate in trifluoroacetic acid has also been noted by Navard et aZ.52 For the fractionated APC, the optical clearing temperature increased linearly with molar mass from 180 "C for 1.3 x l O5 g mol-' to 190 "C for 7.7 x lo5 g mol-', but the enthalpies of transition of ca.0.7 f 0.1 cal g-' showed no detectable trend with molar mass. The results for APC reinforce the conclusion that the phase separation for cellulose derivatives is primarily driven by the chain stiffness, but that anisotropic t 1 cab4.184 J.D. G. GRAY 263 interactions do become important, especially at elevated temperatures where chain flexibility is high.49350 The temperature and solvent effects on the critical concentra- tion primarily reflect changes in the chain conformation and stiffness. OTHER FACTORS IN MESOPHASE FORMATION Liquid-crystalline phases are not observed for all cellulose derivatives. In some cases crystallization, gel formation or aggregation is favoured over mesophase formation.Aggregation and gel formation occur most readily with hydroxy-rich and lightly substituted derivatives in aqueous solvents. The relationship between crystallization and mesophase formation for rod-like polymers has been Cellulose has of course a very strong tendency to crystallize, and the mesophases observed with certain solvents may well be m e t a ~ t a b l e . ~ ~ ' ~ ~ The triesters and triethers of cellulose with small substituent groups also tend to crystallize readily, in some cases as crystalline s ~ l v a t e s . ~ ~ This group of derivatives forms mesophases in some solvents but not in others? Presumably thermodynamically good solvents or solvents which complex strongly with the chain inhibit crystallization.Cellulose derivatives such as the esterified hydroxyethers, which are heavily substituted with large, flexible side-chains, do not crystallize easily, and they readily form liquid-crystalline phases, even in the absence of solvent. l 1 Finally, the chemical characteristics of the polymer and solvent may influence the rate at which the liquid-crystalline phase forms. High molar mass cellulosic samples in viscous solvents take longer to form ordered phases than shorter chains, and of course thermotropic systems only form above the glass-transition temperature. Lyotropic mesophases which order slowly may be overlooked if the solvent is volatile or reactive, and thermal degradation may interfere with detection of slowly ordering thermotropic cellulose derivatives.The presence of extensive side-chains appears to enhance the main-chain mobility and to allow the thermotropic derivatives to achieve their equilibrium ordered arrangement. CONCLUSIONS In 1956 Flory predicted that because of their extended chain conformations cellulose derivatives should form orientationally ordered phases on being cooled from a sufficiently high temperat~re.~~ More than twenty-five years passed before the unequivocal observation of this phenomenon. The experimental results on cellulosics are still best explained in terms of the chain conformation and stiffness, but anisotropic interchain interactions also play a role. The chemical characteristics of the substituents and solvents seem to influence mesophase formation mainly but not exclusively through their effects on chain conformation.I thank the Natural Sciences and Engineering Council of Canada for support of this work. ' C. Robinson, Trans. Furuday SOC., 1956, 52, 571. * H. Finkelmann and G. Rehage, Mukrornol. Chem. Commun., 1980, 1,733. N. A. Plate and V. P. Shibaev, J. Polym. Sci., Polym. Symp., 1980,67, 1. A. Roviello and A. Sirigu, J. Polym. Sci., Polym. Lett. Ed., 1975, 13, 455. W. J. Jackson and H. F. Kuhfuss, J. Polym. Sci., Polym. Chem. Ed., 1976, 14, 2043. A. Cifferi, in Polymer Liquid Crystals, ed. A. Cifferi, W. R. Krigbaum and R. B. Meyer (Academic Press, New York, 1982). S. L. Kwolek, P. W. Morgan, J. R. Schaefgen and L. W. Gulrich, Macromolecules, 1977,10, 1390. D. G. Gray, J. Appl. Polym.Sci., Appl. Polym. Symp., 1983, 37, 179. * R. S. Werbowyj and D. G. Gray, Mol. Cryst. Liq. Cryst. 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ISSN:0301-7249    

DOI:10.1039/DC9857900257

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

Faraday Discuss. Chem. Soc., 1985, 79, 265-272 The Mesophase in Carbonaceous Pitches BY LEONARD S. SINGER Union Carbide Corporation, Electrode Systems Division, Parma Technical Center, P.O. Box 61 16, Cleveland, Ohio 44101, U.S.A. Received 3 rd December. 1984 An intermediate nematic mesophase state has been found to occur in carbonaceous pitches during their thermal transformation to coke and carbon. The molecules responsible for mesophase formation are, by and large, disc-like and mostly aromatic, and their distribution of molecular weights is quite critical. The liquid crystal exhibits uniaxial-negative optical behaviour and can be easily oriented by mechanical and magnetic forces. Mesophase pitches are usually two-phase systems in the liquid state and the mesophase becomes a nematic glass when cooled to the solid state.The two phases can be separated and their constitutions and physical behaviour determined. In certain aspects, the coexisting isotropic and anisotropic phases are remarkably similar. Because of the unique structural and rheological characteristics of the carbonaceous mesophase and its ordering and hardening behaviour, it has been possible to prepare highly oriented, high-performance products such as carbon fibres from mesophase pitch. The present state and future possibilities of the technology based on these phenomena is discussed. The discovery in 1965 by Brooks and Taylor'-3 of an intermediate mesophase state in the carbonization of organic materials has had an enormous impact on all carbon-related technologies. This embryonic liquid-crystal stage of carbon essen- tially determines the ultimate structural destiny and, of course, the resultant mechanical properties of the derived carbon products.Cokes for graphite electrodes for steel manufacture, ultra-high-modulus carbon fibres for everything from aircraft to golf clubs, pitch binders for all specialty carbon products and the new high- performance carbon-carbon composites for aerospace are but a few of the materials which depend critically upon the constitution, polymerization, ordering and ther- mosetting of the carbonaceous mesophase. It is of interest that the technologies of conventional liquid-crystal polymers and the carbonaceous mesophase have developed in a parallel fashion over approxi- mately the same time period with very little interaction between researchers.This is perhaps understandable, but it does not seem desirable. It is therefore hoped that this and other discussions will motivate closer and more frequent interactions between workers who may deal with very different materials but, in many cases, identical physical and chemical principles. This paper has been divided into three main sections, which cover (a) the formation, constitution and structure of mesophase pitch, (b) the effects of mechanical and magnetic forces on orientation of the mesophase and (c) applications of carbonaceous mesophase technology to carbon and graphite products. FORMATION, CONSTITUTION AND STRUCTURE OF MESOPHASE PITCH Since there are now many excellent review^^-^ on the carbonaceous mesophase and hundreds of relevant articles, this paper can present only a very brief review and updating of the subject.265266 MESOPHASE IN CARBONACEOUS PITCHES 50 I00 I50 200 250 300 350 4 molecular weight Fig. 1. Mass spectrogram of a petroleum-derived pitch.6 Perhaps the best place to start is with a definition of the term 'pitch', which is defined by Webster7 as 'a black or dark viscous substance obtained as a residue in distilling tar, wood, petroleum etc.' Interestingly, such materials were recognized, mostly by their undesirable characteristics, in biblical times' as well as more recently by William Shakespeare.' A more suitable technical description of pitch is a complex mixture of hundreds or thousands or predominantly aromatic organic compounds with an average molecular weight of several hundred.These compounds are formed by an array of thermal decomposition, hydrogen transfer and oligomerization reactions. An example of the distribution of masses in a petroleum-derived pitch is shown in the mass spectrogram in fig. 1, in which essentially all masses within the range shown seem to be present? Even pitches prepared from the thermal decomposition of a single organic compound exhibit essentially the same degree of diversity in molecular weight and structure." Although one must be careful in the use of an 'average' chemical structure to describe a complex mixture, the use of a generic molecule for a pitch is not only useful but is reasonably accurate, since all of the molecules in a pitch are of the same general type.There are many examples in the literature in which combinations of molecular- weight determinations and proton and 13C n.m.r. have been used to deduce the structure of an average molecule in a pitch." One such example for the petroleum pitch in fig. 1 is shown in fig. 2.12 Although the separate compounds in a pitch may melt individually at a fairly high temperature (several hundred "C), the pitch mixture melts much lower, e.g. at 50- 100 "C, because of its eutectic behaviour. Both liquid and solid pitch are isotropic materials, the solid being simply a non-crystalline glass. The discovery of the carbonaceous mesophase was a result of the investigations of Brooks and Taylor, 1-3 who pondered the pitch-to-coke transformation. The main question was how a disordered material, such as pitch, polymerizes or oligomerizes only slightly further to produce a highly ordered polymer (coke) which consists of large domains havirlg long-range order and preferred orientation.Using hot-stage polarized-light microscopy techniques, Brooks and Taylor found that as pitch was heated above 400"C, either at constant temperature or withPlate 1. Photomicrographs of a polished section of an acenaphthylene-derived mesophase pitch under polarized light. Views ( a ) and ( 6 ) are for two different angles of microscope stage rotation differing by 45".Plate 2. Polarized-light photomicrographs of schematic diagram of the deformation of mesophase by bubble percolation [fig. 14 of ref. (l4), reproduced by permission].Plate 3.Photomicrographs of polished sections of mesophase spheres derived from acenaph- thylene pitch: ( a ) sample rotated in a magnetic field of 10 kG at 400 "C for 1 h and ( b ) sample rotated in a magnetic field of 10 kG at 430 "C for 2 h. Adjacent pictures for each sample differ by a 45" microscope stage rotation.L. S . SINGER 267 CH, CH, Fig. 2. Structure of the 'average' molecule in a petroleum-derived pitch: C36H25, molecular weight 445. gradually increasing temperature, small spheres appeared and gradually increased in size with time and increasing temperature. The spheres showed interesting and varied extinction contours under polarized light and there was no apparent change in sphere structure when the pitch was cooled from the temperature of formation ( CQ.400 "C) to room temperature. This phenomenon makes it possible to infer the structure of high-temperature materials from room-temperature observations. Plate 1 shows photomicrographs of polished sections of the solidified pitches observed under crossed polars. These spheres, which can grow to hundreds of pm in size, obviously do not possess spherical symmetry. This can be seen by the variations of pattern with 45" stage rotation in plate 1. At the temperature of formation, the spheres are liquid droplets, immiscible with the surrounding isotropic pitch. This condition is clearly seen by high-temperature electron2 and p~larized-light~.'~ microscopy. The spheres are slightly more dense than the isotropic phase and, if left undisturbed in the preparation vessel at high temperatures, slowly settle out.They are also more viscous than the surrounding pitch. When they grow to large sizes, they begin colliding with each other and coalesce to form even larger spheres and ultimately large anisotropic regions. Eventually the entire pitch is converted into anisotropic material, which then becomes a very viscous semi-solid and finally a solid coke. The arrangement of the disc-like aromatic molecules within the liquid crystal or mesophase spheres was determined by Brooks' and Taylor's electron-diff raction work,2 which showed conclusively that the molecules do stack up within the spheres but that the stacks of molecules tend to approach the surface of the sphere at an angle of ca. 90". When the spheres collide, they coalesce, as shown in fig.3, and, depending on their viscosity and how fast they are quenched, either look like one of the intermediate stages or quickly reorganize to a larger sphere with the original structure. As coalescence develops, the dehydrogenative polymerization reactions continue and the molecules get larger and the mesophase more viscous. As hydrogen and other gaseous reaction products escape, bubbles form and produce interesting, but sometimes troublesome, flow patterns and stacking defects. The photomicrograph of White14 in plate 2 illustrates how the deformation of the viscous mesophase occurs by bubble percolation, giving rise to interesting 'kinky' extinction patterns. The short lines represent edges of the disc-like molecules and indicate their orienta- tional arrangement.The thermal dehydrogenative condensation reactions leading to the larger aro- matic molecules capable of forming the mesophase appear to be first order with an activation energy of ca. 50 kcal m01-l.l~ E.p.r. and other studies indicate that both reactive u radicals and more stable T radicals are involved in these When the molecules reach a molecular weight of ca. 1000 they are, apparently, sufficiently large and flat to favour the formation of a liquid crystal or mesophase.268 MESOPHASE IN CARBONACEOUS PITCHES BEFORE CONTACT JUST AFTER CONTACT SHORT TIME AFTER CONTACT REARRANGEMENT Fig. 3. Schematic representation of the collision and coalescence of mesophase spheres. Fig. 4. Possible 'average' structures for molecules of a petroleum-derived mesophase pitch: l 2 C71 H39, molecular weight 89 1 .Two possible average structures for the mesophase formed from the petroleum pitch described in fig. 1 and 2 are shown in fig. 4.'* The n.m.r. techniques cannot distinguish between structure (a) or ( b ) , but diamagnetic susceptibility studies16 have shown that the relatively small diamagnetic susceptibility and anisotropy of an oriented mesophase pitch are more consistent with molecules which are not large, highly condensed aromatic systems. Most mesophase pitches are two-phase emulsions in which the overall molecular- weight distribution of the two phases is not too different. In a recent study of the molecular-weight distributions in the isotropic and anisotropic phases of a petroleum mesophase pitch separated by high-temperature ~entrifugation,'~ the molecular- weight distributions were determined by gel-permeation chromatography (g.p.c.) toL.S . SINGER 269 molecular weight Fig. 5. Molecular-wsight-distribution curves for the isotropic (upper, an = 846) and anisotropic (lower, M , = 937) phases of a petroleum-derived mesophase pitch separated by high-temperature centrifugation. molecular weight Fig. 6. Curve showing the partitioning of molecules between the anisotropic and isotropic phases described in fig. 5, as a function of molecular weight. be as shown in fig. 5. A quantitative analysis of these curves indicates that the partitioning of the molecules between the two phases has the dependence upon molecular weight shown in fig. 6. From the curve in fig.6 it is easy to see why the extraction of a small amount of the lower-molecular-weight species from an isotropic pitch could result in a substantial transformation to a mesophase. These phenomena are, of course, similar to those observed for many liquid-crystal polymer systems.18 Recently, Chen and Diefendorf’’ and Whitehouse and Rand2’ have applied solubility-parameter and phase-diagram concepts to quantify these isotropic- anisotropic phase relationships in pitches. A theory for flat pitch-like molecules270 MESOPHASE IN CARBONACEOUS PITCHES similar to that derived by Flory and Ronca for rod-like molecules2’ would certainly help to clarify many of these observed phenomena. The recent synthesis and studies of disc-like mesogens22 will also undoubtedly contribute significantly to our under- standing of the carbonaceous mesophase.Recent studies of the ‘premesophase’ and ‘dormant’ mesophase by Japanese worker^^^.^^ have added another dimension to carbonaceous mesophase pitches. They have found that hydrotreatment of pitches can change the structure and presumably the planarity of the molecules sufficiently to affect significantly the mesophase-forming characteristics of the pitches. EFFECTS OF MECHANICAL AND MAGNETIC FORCES ON ORIENTATION OF THE MESOPHASE The stacks of flat aromatic molecules in the liquid carbonaceous mesophase can be depicted as a collection of slippery playing cards. The liquid-crystal phase is thus easily oriented by shear or elongational forces and can be extruded and drawn into highly oriented filaments or fibres.25 Such phenomena are the basis of Union Carbide’s process for making high-strength, high-modulus carbon fibres from mesophase pitch.26 Such mechanical forces at the mesophase stage, although not as carefully controlled, are, of course, responsible for the ultimate degree of order and preferred orientation in cokes and practically all other graphitizable car- bonaceous materials.The action of magnetic fields on the carbonaceous mesophase is unique for several reasons. The fact that the carbonaceous mesophase is a uniaxial negative nematic liquid crystal implies that there are two degrees of freedom for the orientation of the liquid crystal in a magnetic field. For example, mesophase spheres suspended in the isotropic pitch can be oriented as shown schematically in fig.7(a) in a stationary sample and as shown in fig. 7 ( b ) for a sample rotated in the magnetic field.2792* Polarized-light photomicrographs of small and large mesophase spheres oriented by rotation in a magnetic field are shown in plate 3. The small and large spheres differ in the amount of internal orientation, which is governed by the competition between surface and magnetic forces.29 A 100% anisotropic pitch has also been magnetically oriented into a disclination- free mesophase for which the diamagnetic anisotropy, e.p.r. and specific heat have been measured.I6 It is apparent from the results that the aromatic molecules are reasonably small and flat, that the stable free radicals are oriented odd-alternate hydrocarbons of similar structure3’ and that long-range lamellar organization is already present.I6 This study has suggested that the ultimate characteristics of graphite, viz.its electronic, thermal and elastic behaviour, are already determined at the mesophase stage of carbonization.16 APPLICATIONS OF CARBONACEOUS MESOPHASE TECHNOLOGY TO CARBON AND GRAPHITE PRODUCTS Since it now seems clear that the ultimate order and properties of subsequent carbons and graphites are determined at the carbonaceous mesophase stage, a number of applications of this mesophase technology have been applied to the modification and improvement of carbon products. Carbon fibre based on mesophase pitch is one of the more recent development^.^^ By controlling the preferred orientation during melt spinning, oxidatively cross-linking to render the ordered structure infusible and then heating to graphitizing temperatures, fibresL. S.SINGER 271 I t z axis S S Fig. 7. Schematic representations of mesophase spheres in a uniform magnetic field H : (a) static sample and ( b ) sample rotated about an axis (2 axis) perpendicular to the field. having high strengths and ultra-high Young’s moduli (ca. 1000 GPa) are now in produ~tion.~~ Experiments have also been reported involving the preparation of oriented cokes by mesophase extrusion techniques3’ 932 ‘Meso-carbon microbeads’, which consist of partially extracted mesophase spheres, have been suggested and utilized for a number of interesting application^,^^ including a column-packing material for liquid chromatography.These and other possibilities have also been described in a recent review of carbon technology in Japan.34 CONCLUSIONS The mesophase in carbonaceous pitch occurs at an early stage of the carboniz- ation process at which the ultimate order and properties of subsequent carbons and graphites are determined. Although there is now a realization of the importance of the mesophase in carbon technology, we do not yet have a detailed understanding of the formation, structure, properties and carbonization behaviour. The future of carbon and graphite technology depends upon this understanding. J. D. Brooks and G. H. Taylor, Nature (London), 1965, 206, 697. J. D. Brooks and G. H. Taylor, Carbon, 1965, 3, 185. J. D. Brooks and G. H. Taylor, in Chemistry and Physics of Carbon, ed.P. L. Walker Jr (Marcel Dekker, New York, 1968), vol. 4, pp. 243-286. H. Marsh and P. L. Walker Jr, in Chemistry and Physics of Carbon, ed. P. L. Walker Jr and P. Thrower (Marcel Dekker, New York, 1979), vol. 15, pp. 229-286.272 MESOPHASE IN CARBONACEOUS PITCHES J. E. Zimmer and J. L. White, Adv. Liq. Cryst., 1982, 5, 157. I. C. Lewis, J. Chim Phys., Phys. Chim. Biol., in press. Webster's New Collegiate Dictionary (G. C. Merriam, Springfield Mass., 1956). The Apocrypha of the Old Testament, ed. B. M. Metzger (Oxford University Press, New York, 1973), Ecclesiasticus, chap. X I I I , verse 1. William Shakespeare, Much Ado About Nothing, act 111, scene 111, statement by Dogberry. E. M. Dickinson, Fuel, 1980, 59, 290. E. M. Dickinson, personal communication.1975, pp. 215 and 216. lo I. C. Lewis, Carbon, 1980, 18, 191. 12 l 3 R. T. Lewis, Extended Abstracts, 12th Biennial Conference on Carben, Pittsburgh, Pennsylvania, l4 J. L. White, Bog. Solid State Chem., 1975, 9, 59. l 5 L. S. Singer and I. C. Lewis, Carbon, 1978, 16, 417. P. Delhaes, J. C. Rouillon, G. Fug and L. S. Singer, Carbon, 1979, 17, 435. L. S. Singer, I. C. Lewis and R. A. Greinke, Extended Abstracts, Carbone 84, International Conference on Carbon, Bordeaux, 1984, pp. 352 and 353. l 8 Polymer Liquid Crystals, ed. A. Ciferri and W. R. Krigbaum (Academic Press, New York, 1982). S. H. Chen and R. J. Diefendorf, Extended Abstracts, 16th Biennial Conference on Carbon, San Diego, California, 1983, pp. 22, 23, 26 and 27. S. Whitehouse and B. Rand, Extended Abstracts, 16th Biennial Conference on Carbon, San Diego, California, 1983, pp. 30 and 31. 16 17 19 20 21 P. J. Flory and G. Ronca, Mol. Cryst. Liq. Cryst., 1979, 54, 31 1. 22 H. Gasparoux, Mol. Cryst. Liq. Cryst., 1981, 63, 231. 23 Y. Yamada, S. Matsumoto, K. Fukuda and H. Honda, Tanso, 1981, 107, 144. 24 S. Otani, US. Patent 4 472 265. 25 L. S. Singer, Fuel, 1981, 60, 839. 26 L. S. Singer, U.S. Patent 4 005 183. 27 L. S. Singer and R. T. Lewis, Extended Abstracts, 11 th Biennial Conference on Carbon, Gatlinburg, Tennessee, 1973, pp. 207 and 208. 28 L. S. Singer, U.S. Patent 3 991 170. L. S. Singer, R. T. Lewis, A. T. Lauria and S. L. Strong, Abstracts of the 5th International Liquid Crystal Conference, Stockholm, 1974, pp. 52 and 53. 1. C. Lewis and L. S. Singer, in Chemistry and Physics of Carbon, ed. P. L. Walker Jr and P. A. Thrower (Marcel Dekker, New York, 1981), vol. 17, pp. 1-88. I. Romey, Extended Abstracts, Carbone 84, International Carbon Conference, Bordeaux, 1984, pp. 340 and 341. 29 30 31 J. C. Jenkins and G. M. Jenkins, Carbon, 1983, 25, 473. 32 33 H. Honda, Mol. Cryst. Liq. Cryst., 1983, 94, 97. 34 Recent Carbon Technology, ed. T. Ishikawa and T. Magaoki (JEC Press, Cleveland Ohio, 1983).

ISSN:0301-7249    

DOI:10.1039/DC9857900265

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

GENERAL DISCUSSION t Dr R. A. Chivers (ICI, Wilton) said: Prof. Thomas and Miss Wood have described the effect of tilt, lateral order and relative axial shift on the diffraction patterns of chain molecules. They have, however, only considered the molecules as lines of points, although the real molecular structure can have a major effect on the pattern. A real chain molecule, C(r, 4, z), can be described by the convolution of a point chain, A( z), and the electron density of one or more monomer ‘motifs’, p( r, 4 , ~ ) : ’ C(r, 4.4 = A ( z ) “A‘, 494. Ic(R, Q 7 2) = I-%c(r, 4, Z)l12 $[C(r, 4, z ) l = mwl * . R P ( r , 4, dl. I C W , @, z) = I.mwll’ - I%dr, 4, z)l12 (1) (2) (3) The diffracted intensity, Ic( R, @, Z), is given by where Therefore = I A Ip. (4) Cylindrical averaging may be performed2 to give a two-dimensional diffraction pattern.The diffraction from a nematic structure of periodic point chains is a series of continuous layer lines, except for the equator on which lie discrete maxima. In the case of aperiodic point chains, the layer lines are aperiodic and may be broadened in the chain dire~tion.~ This is only the I A term of eqn (4). When the atomic structure is also considered, I A is multiplied by the scattered intensity from this structure, Ip. The effect of this is that where Ip is very small or zero, then Ic will be likewise and the layer lines may no longer extend to infinity, possibly being just a short streak. The effect is of especial importance in nematic systems, as described above, in which I A has the form of continuous layer lines.The observed scattering, Ic, is then largely determined by Ip. In the case of a smectic system, however, I A will only consist of discrete maxima on the layer lines, and hence may be a more dominant factor on I , than Ip, which then serves to modulate the intensity of these maxima. Fig. 7 ( a ) and ( 6 ) of the paper by Blackwell et ai3 show the results of calculations of Ic for a random copolyester of p - hydroxybenzoic acid and 2-hydroxy-6-naphthoic acid for both the random and a rigid distribution of inter-residue torsion angles. In both cases, effects of the Ip modulation of I A (aperiodic layer-lines) can be seen, and in fig. 7 ( a ) this gives rise to clear differences in the lateral spread of the meridional maxima, as have been observed experimentally. This effect is of considerable importance if we wish to obtain information on intermolecular registration or ‘crystallite’ size from the lateral spread of meridional maxima.Stamatoff4 has observed discrepancies between equatorial peak widths and those of meridionals which he attributed to disorder caused by chain registration. He did not, however, consider Ip, which would indeed be extremely difficult to do with sufficient accuracy but which may serve to change the basis of some of his 1- Plates 1-8 follow p. 287. 273274 GENERAL DISCUSSION conclusions. Unless Ip is known, all crystallite size and registration information from meridional scattering in nematic systems must be treated with extreme caution. B. K. Vainshtein, Diffraction of X-Rays by Chain Molecules (Elsevier, Amsterdam, 1966), chap.IV. J. Blackwell, A. Biswas, G. A. Gutierrez and R. A. Chivers, Furaduy Discuss. Chem. Soc., 1985, J. B. Stamatoff, Mol. Cryst. Liq. Cryst., 1984, 110, 75. * R. A. Chivers and J. Blackwell, Polymer, in press. 79, 73. Prof. E. L. Thomas and Miss B. A. Wood (University of Massachusetts, U.S.A.) replied: The schematic diagram shown in our fig. 3 was to illustrate the specific effect of axial registry on the scattering patterns, permitting a distinction between nematic and smectic ordering and omitting the effect of the actual electron-density profile ,of the particular molecule on the scattering pattern. We certainly agree with Dr Chivers that this additional important factor should be taken into account for the detailed analysis of any actual polymer, since it is the product of the lattice factor (i.e. our fig.3) and the specific molecular transform which yields the observed scattered intensity distribution. Our point was that partial axial registration (a tendency toward fully smectic order) will be most apparent in the modulation of the scattered intensity on the lowerst-order layer lines. Prof. F. C. Frank (University of Bristol) said: The very interesting and instructive electron micrographs shown by Prof. Thomas and Miss Wood give a clear impression of domains, delimited by characteristic boundaries, which I called ‘caterpillars’ when they were first shown to me, but for which a more professional-sounding and informative name is ‘confluence lines’.I do not like the authors’ name ‘axial lines’ for them, because I do not see a precise significance for the word ‘axial’ in that name. In fact, these domains are an illusion and the confluence lines a mathematical artefact, which we do not immediately recognize as such because we are unaccus- tomed to seeing director fields portrayed by their orthogonal trajectories, as Thomas and Wood demonstrate is the case here. The confluence lines indicate no physical singularity in the underlying director field: in particular, they do not indicate disclinations of strength S = f 1, as suggested, or any other disclinations: but they do frequently terminate on disclinations of S = f $. Example 1 : Consider a sinusoidally wrinkled director field F, represented by Y F = a cos k x + p where p is a parameter distinguishing different members of the family of director trajectories.Differentiating, we obtain dy,/dx = -ak sin kx. Then, for the orthogonal field G: dy,/dx = -l/(dyF/dx) = (l/ak) cosec (kx). This integratesIa to give the orthogonal trajectories as: The integration generates logarithmic singularities wherever dyF/ dx is zero, and these produce the confluence lines. See fig. 1.275 GENERAL DISCUSSION Fig. 1. Example 1: (a) a sinusoidal director field and ( b ) an orthogonal field to that of ( a ) .276 GENERAL DISCUSSION Example 2: the confluence lines:lb Adding a linear term produces alternatively wide and narrow bands between Y ~ = a c o s ( k x ) + b x + p -- dyF - -ak sin (kx) + b dx - l/[ak sin (kx) - b] -- dyG dx tan (x/2) - ak/ b + ( a2k2/b2 - 1)1/2 yG = ( a2k2 - b2)-1/2 In tan (x/2) - ak/ b - ( a2k2/ b2 - 1) v2) + 4.This has confluence lines at kx = arc sin (b/ ak). See fig. 2. Example 3 : A single smoothly curved kink-band may be represented by" yF=barctan(x/a)-ex+p a2+x2 a6 - c( a 2 + x') _ - -- dyG dx X a6 2 + 4. a b l e - a ) YG =-- If c < b/a, this has two confluence lines at x = *(ab/c - a2)lj2. Example 4 : A field of hyperbolae, with asymptotic slopes 6 for x > 0, - b for x < 0 is given byId yF= b(a2/b2+x2)1/2+p (positive root) -- dyF- b(a2/b2+x2)-1/2/x dx -- dyG - -( a2/ b2 + ~ ~ ) ' / ~ / e x dx yG= -(a2+x2/b2)'I2+ (;) 7 In ( a / l ~ + ( a ~ / b ~ + x ~ ) ' / ~ X ) +4. This has a single confluence line on the y axis. See fig. 3. These are all examples of translationally parallel fields, the trajectories of which are families of congruent curves such that the vector p joining points of the same slope on any two members of the family is a constant.Confluence lines in theGENERAL DISCUSSION 277 I i i / 8 i /' c .. Fig. 2. Example 2: ( a ) a sinusoidal field modified by a linear term and ( b ) an orthogonal field to that of ( a ) .278 GENERAL DISCUSSION Fig. 3. Example 4: (a) a hyperbolic director field and ( b ) an orthogonal field to that of fig. 1( a ) (alternatively, a confluent director field).GENERAL DISCUSSION 279 orthogonal trajectories of such a field are produced where p is orthogonal to the tangent vectors t of the field, i.e. where p t = 0. There are no confluence lines for the orthogonal trajectories of either equidistantly parallel fields (in which the distance between any two curves is a constant) or in harmonic fields. The latter correspond to curves of constant value of either the real or the imaginary part of an analytic function of (x + iy ) and describe two-dimensional equilibrium configurations for isotropic curvature elasticity, in which K , = K 3 .Neither of these cases yields con- fluence lines because the condition p - t = 0 is satisfied everywhere. Although translational parallelism is not obeyed over large distances, the ideal- ized examples (1) to (4) so closely simulate textures observed locally by Thomas and Wood that one must infer that translational parallelism is a rather strong approximate rule governing the textures in these specimens.The reason for this is not obvious. A strong splay constant, K1, will go some of the way towards explaining it, but since the textures have been formed in flow it is likely that anisotropic viscosities as well as curvature-elasticity constants exert significant control over the texture. The fields do not appear to be splay-free. In plate 4 of their paper Thomas and Wood have drawn an (unduly straightened) director trajectory across two kink bands. The confluence lines do not bisect the angles between director trajectories inside and outside the kinks: in terms of example 3 we have c > 6 / 2 a : neighbouring director trajectories are therefore closer together outside than inside the kinks: they are actually furthest apart on the confluence lines bounding a kink.Supposing that the director trajectories correspond to highly aligned polymer chains one infers that the material is more highly drawn at the kink boundaries: with corresponding splay in the neighbourhood of these boun- daries, which should be accommodated by an accumulation of chain ends or ‘hairpins’. In this class of material, with flexible links of n methylenes between the rigid segments, the energy cost of a hairpin should be fairly low if n exceeds 5 or 6 (and it is 10 in the material of Thomas and Wood) so that, except for the lowest molecular weight, hairpins should predominate over chain ends for splay accommo- dation in highly aligned material (a condition which has to be distinguished from full extension). These inferences as to splay and non-uniformity of drawing could be made firmly if the specimens were thick: actually they are very thin, and therefore a small variation in thickness can produce compensating splay in the third dimension, so that they may be invalid: nevertheless ‘serpentine’ bending has been reported in relatively thick specimens. The orthogonality relationship between fields F and G is recursive.If G is taken as a director field then F is its field of orthogonal trajectories. This applies particularly to example 4 above. Regarding the field G (with 6 replaced by 6’ = - 1/ 6) as well as the field F as a director field they represent, at least qualitatively, two possible configurations for a symmetrical tilt boundary between two uniformly aligned regions of different orientation: one in which directors bend hyperbolically across the boundary and the other in which they are confluent to it.Plate 7 of Thomas and Wood shows a boundary containing alternating stretches of these two alternative structures: they make disclinations of strength +; or -; at each transition from one structure to the other. (Of course, a stretch appearing hyperbolic in the orthogonal trajectories is confluent for the directors, and conversely). Thomas and Wood have other electron-micrographs showing longer boundaries of this alternating kind. A probable interpretation is that for a given change of orientation one or other of the two alternative configurations has the lower energy: if the flow process has generated the boundary in its higher-energy configuration it can, on annealing, be converted into the other by nucleating pairs of S = *$ disclinations which migrate apart to280 GENERAL DISCUSSION annihilate with their opposites arising from another pair-nucleation point: boun- daries of alternating structure have been caught in the process of conversion.It is intriguing to speculate on the details of molecular mechanism, presumably involving rearrangement of hairpins and chain-ends, and perhaps also involving truns- esterification. Finally, it is interesting to observe that the trajectories shown in the ‘schematic diagram’ of plate 2 in Dr Singer’s paper at this Discussion are qualitatively very similar to director trajectories in the specimens of Thomas and Wood. However, Singer’s trajectories, being those of major polarizability in an optically negative nematic, are orthogonal to the director field.In this case, then, it is the director-field which is relatively rich in confluences. One may associate this relationship between the two cases with the fact that in Singer’s material simple shearing flow is easiest on planes orthogonal to the director, whereas in the material of Thomas and Wood it is easiest on planes parallel to the director. I thank Prof. Thomas and Miss Wood for letting me see some of their electron- micrographs in advance, and Dr John Hannay for constructing fig. 1-3 for me. M. B. Dwight, Tables of Integrals (MacMillan, New York, 1947), ( a ) items 432.10 and 603.6; (6) item 436.00; (c) items 512.3, 140.02 and 160.21; ( d ) item 241.01. Prof. E. L.Thomas and Miss B. A. Wood (University of Massachusetts, U.S.A.) said: We thank Sir Charles Frank for the mathematical development of the relation- ship between the molecular director field and the lamellar trajectories in our images. Two main points are brought out by Frank: (i) the (false) impression of ‘orientation domains’ is due to the singularities in the array of lamellae which in fact arise from a continuous underlying molecular-director field ( i. e., there are no molecular-director domain boundaries as defined by surfaces of discontinuous rotation of the molecular director from one domain to the next) although the eye sees apparent domain boundaries in the lamellar trajectories and (ii) the observed textures imply that translational parallelism of the molecular director field is a significant feature in the behaviour of TLCP materials.We accept Frank’s point that the ‘caterpillar’ type arrangement of lamellae (our axial disclinations: see plates 6 and 7 of our paper) are not disclinations of unit strength as we suggested but are as he aptly terms them, ‘lines of confluence’ in the field of lamellar trajectories. The correspondence between Frank’s sinusoidally wrinkled molecular director field (his example 1) and our plate 9 ( a ) is striking. We also accept that the ‘sawtoothed’ molecular trajectories indicated on our plate 4 are drawn unduly straight; Frank’s example 3 can well produce this type of kink band in the field of lamellae without any discontinuities in the molecular-director field [our plate 9(b)].By appropriate choice of the parameters a, b and c the loci of strong curvature in the molecular director field can be restricted to a scale small relative to the width of a kink band, so any degree of sharpness can be produced with mathematically smooth molecular director curves. Note that the experimental determination of the boundary sharpness is limited to a resolution of ca. 100 8, by the inherent size of the lamellae. Evidence for confluences in the molecular-director field is presented in plate 1 of this comment. This micrograph shows sinusoidal lamellar trajectories in a film of the annealed methyl-substituted polymer. Thick dark lines approximately normal to the lamellae occur at inflection points, producing periodic bands. Construction of the orthogonal set of molecular-director trajectories produces the logarithmic singularities of Frank’s example 1 in the centre of each band; the roles of the lamellarGENERAL DISCUSSION 28 1 and the molecular-director field are now exchanged.It remains to understand the origins and inter-relations between the various translationally parallel molecular- director fields. Their nature may reflect the detailed order of the liquid-crystal medium before cessation of flow and subsequent compressive buckling. We stress that the bands ought not to be construed as an intrinsic orientation domain structure of the liquid-crystal medium. Moreover, since banded structures are apparently ubiquitous in flow-oriented liquid-crystal polymers, we suggest that understanding and ultimately controlling the band texture and associated disclinations may assume an importance once reserved for the domain concept in structure-property relations in liquid-crystal polymers.Dr A. H. Windle (University of Cambridge) (communicated): Prof. Thomas has referred to smectic-type organization in thermotropic polymers containing regular lengths of flexible spacer. I should like to ask him if he considers longitudinal positional correlation (or ‘preferred axial stagger’) between the polymer molecules as sufficient justification for the smectic classification. Perhaps it is reasonable in the case of substantial flexible spacers which segregate so as to be alongside like spacers on neighbouring chains and lead to a layer structure within the mesophase. But what if both the rigid and flexible portions of the chains are much shorter and longitudinal correlation still persists? In the extreme, consider a nematic polymer in which there is longitudinal register between monomer units perhaps only 8 A long, but without any long-range positional order in the plane normal to the chains (otherwise it would be a crystal, or at least a plastic crystal). Is that a smectic or a nematic? Prof.E. L. Thomas ( University of Massachusetts, U.S.A.) replied: Chains aligned in the axial direction with no long-range correlation in the lateral direction may be referred to as ‘smectic’ when axial registration is perfect and ‘nematic’ when no axial registration exists. In between these two limits the structure may be referred to as ‘distorted’ smectic.The extent of registration may be quantified by comparing the intermolecular interference function for various layer lines for a uniaxially oriented system. This may be obtained in the following way: if the cylindrically averaged molecular structure factor of the chain is known, then the interchain interference function, Z,, may be obtained by analysis of the scattered intensity distribution of the zeroth-order layer line. Similarly the corresponding inter-chain interference func- tion, Z1, for the first layer line may be determined. Z1 would be a constant for nematic ordering and Z1 = 2, for perfect registration, i.e. smectic ordering.’ In some situations the degree of registration may be defined by a parameter A (see my contribution to the discussion of Prof. Blumstein’s paper).R. Saraf and Y. Cohen, personal communication. Dr A. M. Donald (University of Cambridge) said: Prof. Thomas has presented electron micrographs purporting to show domain structures in a rigid-flexible-spacer spacer main-chain thermotropic polymer. Sir Charles Frank claims these are not ‘domains’ since there need be no abrupt discontinuity in the director field associated with the observed structures. Both these contributions refer to two-dimensional structures. In the thermotropic random copolyester I have been studying in the TEM, the structure cannot always be treated as two-dimensional, which introduces an additional complexity into the problem. The polymer, designated B-N, is based282 GENERAL DISCUSSION shear direction Fig. 4 on hydroxybenzoic anc hydroxynaphthoic acid residues (in the ratio of 7 : 3), and it is upon annealing thin specimens on a rocksalt substrate that a three-dimensional structure develops.When B-N is sheared at 300°C to produce a thin film suitable for the TEM, a banded structure develops. These banded structures, as we have heard in several other presentations, seem to be something of a ubiquitous phenomenon when liquid-crystalline polymers are sheared. The electron microscope is ideally suited to unravelling the underlying molecular trajectory using dark-field techniques, as shown in plate 2. The two dark-field images [plates 2 ( a ) and ( b ) ] are formed from the two wings (ends) of the equatorial arc, and show complementary contrast. The bright-field image [plate 2( c ) ] shows only thickness variations, parallel to the direction of shear, owing to the method of sample preparation.If the objective aperture is moved from the end of the equatorial arc towards the centre, the width of a bright band in the dark-field image increases, showing that there is a smooth variation in the molecular orientation. Fig. 4 shows the molecular trajectory as determined from dark-field micrographs. Three things should be noted in connection with this diagram. First, the TEM provides unambiguous information on the direction of the molecule because direct correlation with the diffraction pattern can be made; the problems associated with optical microscopy because of the possibility of optical biaxiality are sidestepped. Secondly, the distortions involved are mainly bend distortions.The structure does not involve twist distortions at all, which is commonly expected (although I believe without firm evidence for a main-chain thermotropic polymer) to be the lowest- energy distortion. Thirdly, this structure differs from the geometry proposed by Prof. Meyer in his talk. The bands seen here, and in most polymers upon shear, lie perpendicular to the prior shear direction and not parallel. The mechanisms by which they form is still not clear. This banded structure is still essentially a planar structure, but the act of annealing it on a rocksalt substrate leads to a rapid transformation to a very different structure, which at first glance appears to be a ‘domain’ structure. Bright-field contrast is now apparent, with dark ‘veins’ running parallel to the prior shear direction [plate 3( b ) ] .These veins correspond to regions of homeotropy, or near-homeotropy, and look dark because they have a greater cross-section for scattering. In the equatorial dark-field image [plate 3 ( a ) ] , it is apparent that the veins separate regions possessing alternating senses of in-plane misorientation, and the discontinuity appears to be sharp. However, this appearance may be misleading, since the full three-dimensional structure must be considered. Further information on this point can be obtained by tilting the specimen about an axis perpendicular to the prior shear direction. When the specimen is flat the veins are dark, but the contrast elsewhere is fairly uniform [plate 4( a)].Upon tiltingGENERAL DISCUSSION 283 Fig. 5 in one sense alternate ‘domains’ appear dark and light [plate 4(b)]; tilting in the other sense the contrast is reversed [plate 4(c)]. Clearly the veins separate regions which possess opposite senses of molecular tilt out of the specimen plane. Close examination of the appearance of the veins themselves upon such a tilting sequence is also revealing. In plate 5 some rocksalt debris has remained on the specimen surface to act as a marker. The apparent position of the vein, relative to a piece of rocksalt, moves as the specimen is tilted. Remembering that the darkest contrast will be seen where the molecules lie most nearly parallel to the electron beam direction, plate 5 shows that the molecules change tilt smoothly across the veins.Here again we do not have an abrupt discontinuity in the director field, but in this case a variation confined to a few tens of nanometres. Away from this region, the molecular orientation is sensibly constant over several pm. Is this a domain structure? Not according to Sir Charles’ viewpoint, but for many purposes I feel it is an adequate description to talk in terms of ‘domains’, each possessing a uniform orientation separated by a narrow ‘boundary’ or ‘wall’. The structures seen in this polymer can now be related to those described by Prof. Thomas. If the in-plane molecular orientations are drawn (fig. 5, shear direction vertical), they closely resemble the structure around an axial line in Prof. Thomas’ terminology. This structure leads to a large component of splay.As Sir Charles has pointed out, this splay energy may in thin films be relaxed by a ‘sucking in’, i.e. a local thinning, and this seems to be the case shown in plate 4 of Prof. Thomas’ paper. In contrast to this behaviour, B-N exhibits splay compensation by adopting a three-dimensional structure where locally a positive contribution to the splay energy in one plane is compensated by a negative contribution in an orthogonal plane. Two factors appear to encourage this kind of splay compensation in B-N: first the presence of the rocksalt substrate (veins do not develop in samples of B-N annealed without a substrate), and secondly, the presence of sufficient short chains to permit the homeotropy to develop. B-N, of a higher intrinsic viscosity than used in this study, does not exhibit a vein structure.As has previously been pointed out by Meyer, the elastic constants are going to be molecular-weight-dependent, and more data are urgently needed on this point.284 GENERAL DISCUSSION Y Fig. 6 In attempting to understand the structures and defects of these thermotropic polymers, it is dangerous to draw too many parallels between the rather rigid systems I have examined and the flexible-spacer type of Prof. Thomas' work. However, it is interesting to see where similarities exist, pointing to what must be common phenomena. Clearly the structures that are seen are determined to a large extent by the elastic constants. Both the TEM studies of Thomas and Wood and my own confirm the theoretical predictions that splay distortions are of high energy, but the relative magnitudes of bend and twist are not clear.It would be interesting to know if information on this point can be obtained from a detailed analysis of the decorated disclinations seen in the beautiful micrographs of Thomas and Wood. ' A. M. Donald and A. N. Windle, J. Mater. Sci., 1984, 19, 2085. ' Polymer Liquid Crystals, ed. A. Ciferri, W. R. Krigbaum and R. B. Meyer (Academic Press, New York, 1982), chap. 6. Prof. R. B. Meyer (Brandeis University, U.S.A.) said: The subject of splay compensation has been brought up, Le. the idea that splay in one two-dimensional cross-section of a structure can be compensated by splay in a second plane perpen- dicular to the first. This idea is not new.' My paper in this Discussion effectively used this concept, in describing the splay Frederiks transition.The two-dimensional twist structure which replaces uniform splay is essentially a splay-compensated structure, in that cross sections through it appear to be splay patterns. Once one realizes that apparent splay can be compensated to zero in a three-dimensional structure, the next step is to realize that the structure so created is actually composed of twist. Since the twist elastic constant in polymer nematics can be quite low compared with the splay elastic constant, the energy of splay-compensated structures is also low. ' Polymer Liquid Crystals, ed. A. Ciferri, W. R. Krigbaum and R. B. Meyer (Academic Press, New York, 1982), chap. 6. Dr A. H. Windle (University ofcambridge) (communicated): The issue of banded textures has threatened to surface on several occasions during this meeting.DrGENERAL DISCUSSION 285 Mackley showed us the bands actually forming on relaxation from shear deforma- tion, and Dr Zachariades, in his poster, distinguished between liquid-crystalline polymers which form bands during shear and those in which the bands only appear on relaxation from shear. Now the question of their structural significance has again arisen. The textures which can be seen in thermotropic copolyesters are rich in their variety. However, we have found that shear of the mesophase at rates in the approximate range 5 x lo3 to 5 x lo4 s-l (i.e. hand shear of 10 pm thick specimens), followed by reasonably prompt cooling to the solid, invariably produces a banded texture.In fact so general is the effect that we tend to use it as a first diagnostic test for a polymeric mesophase! The bands, which are perpendicular to the direction of shear and of period l-lOpm, can be seen between crossed polars (plate 6) and in dark-field TEM, (plate 7), the latter technique confirming that the molecules follow a periodic serpentine path about the shear axis. Prof. Thomas's comments have now focussed our attention on the possibility of out-of-plane components to the molecular trajec- tories. A scanning electron micrograph of sheared N-QT ([HNA],,, with [TA+ HQ]& obtained by Miss Golombok (plate 8), shows that although there is some detectable surface relief associated with the bands, it is very small compared with the in-plane molecular deviations clearly revealed by the path of the fibrillating crack.A. M. Donald and A. H. Windle, J. Muter. Sci., 1983, 18, 1143. Mr B. Reck (University of Mainz, West Germany) said: We prepared recently a new type of liquid-crystalline side-group polymer' having a polyester main chain instead of the commonly used poly( acrylate), poly( methacrylate) or poly( siloxane) structures: e.g. + (CHd 9 I 0 I c=o I 0 LC__Ix This new main-chain structure opens the way to chiral liquid-crystalline side-group polyesters by incorporation of optically active diols, e.g. ( R ) -3-methylhexane-l,6- diol or the diols used in Prof. Chiellini's work. What properties should one expect for such liquid-crystalline side-group polyesters with a chiral main chain? B.Reck and H. Ringsdorf, Makromol. Chem., Rapid Commun., 1985, 6, 291. Prof. E. Chiellini (University of Pisa, Italy) replied: The new series of polymers mentioned by Mr Reck appears interesting and should offer a valid alternative to the synthesis of a large variety of cholesteric polymers with tunable flexibility, length of helical pitch and selective reflections.286 GENERAL DISCUSSION It is hard to predict what will be the characteristic of the cholesteric array in terms of the twisting power related to a rather flexible main chain containing within the repeating chiral unit two asymmetric carbon atoms, of which the closest to the spaced mesogen may assume either one of two possible absolute configurations. By considering, however, that in some of our preferentially chiral liquid-crystal polyesters containing the mesogen in the main chain a cholesteric structure develops even at fairly low enantiomeric excess (10-20%), one may infer that distinct cholesteric structures should occur even with weak asymmetric perturbations.Dr C. Viney (University of Cambridge) said: I would like to add to Dr Gray's work by making some comments about cellulose nitrates (NC). These polymers are further examples of materials in which the molecules are semi-flexible, elongated and chiral, so that they would be expected to form cholesteric mesophases. Such behaviour has indeed been noted in a wide range of solvents.' Another solvent which should be added to the list is tetrahydrofuran (THF). A given volume dissolves a given weight of NC more quickly than acetone or ethanol, and its action is not restricted to a narrow range of polymer degrees of substitution, 0,.For example, acetone requires D, b 2.4 and ethanol requires D, G 2.4, before significant amounts of material can be dissolved. Using NC having M, = 3 x lo6, I obtained the following results for the critical concentration needed for any mesophase texture formation: polymer D, wt% polymer in THF 2.32 2.45 2.72 15 12 7 (Note that measurements of critical concentration can only be of value if the polymer 0, and molecular weight are quoted; this point is usually overlooked in the literature.) The decreasing critical concentration with increasing 0, is consistent with the corresponding expected increase in molecular rigidity.There is, however, another factor which may be important: as the polymer D, increases, so does the chance of neighbouring molecules having any common sequence of substituted monomers; this must increase the ability of the molecules to become ordered. The type of statistical consideration required to model this behaviour has been introduced (for solid-state ordering) in our paper at this Discussion, for the relatively simple case of a copolymer consisting of only two monomer types. In NC the substitution of anhydroglucose units is irregular. The existence of three hydroxy groups per unsubstituted unit means that nitration of the molecule may lead to a particular unit having any one of the following substitutions: unsubstituted; monosubstituted (3 ways) ; disubstituted (3 ways) and trisubstituted.The nitrated polymer therefore effectively consists of up to eight monomer types. However, within the 0, range 2.32-2.72 referred to above, it has been shown2 that increasing the D, has the following consequences: the partial D, for primary substitution [substitution on the C(6) site] tends to unity, while the number of 6-monosubstituted units tends to zero. This means that what is effectively a random copolymer of eight monomer types at 0,=2.32 becomes a random copolymer of only three monomer types at D, = 2.72. The increased likelihood of local longitudinalGENERAL DISCUSSION 287 register between sections of adjacent molecules must contribute to the lowering of the critical concentration required for lyotropic mesophase formation.D. G. Gray, J. Appl. Polym. Sci., Appl. Polym. Symp., 1983,37, 179. * D. T. Clark, P. J. Stephenson and F. Heatley, Polymer, 1981, 22, 1 1 12. Dr R. R. Luise (Du Pont, WiZmington, D.C., U.S.A.) said: Since anisotropic cellulosic solutions generally have low clearing temperatures, if one dares, one may try to heat one’s solutions above their clearing points, and then cool back to the liquid-crystalline state. In this way, one may obtain the undisturbed equilibrium state, which is more likely to be cholesteric. Dr M. R. Mackley (University of Cambridge) said: Concerning the spinning of mesophase carbonaceous pitch described in Dr Singer’s contribution, does the mechanism for the onion-skin effect have any relation to the skin-core effect observed for thermotropic polymers that we reported in our paper? Dr L. S.Singer ( Union Carbide, Parma, U.S.A.) replied: I do not think that there is any direct connection between the onion-skin mesophase pitch fibre structure which I showed and the skin-core structure discussed in the paper by Alderman and Mackley. Our structure was apparently uniform throughout. The mechanism for the development of the onion-skin structure is not fully understood, but U.S. Patents by Nazem (4376747) and Riggs (4504454) do discuss the matter of transverse fibre structure in considerable detail. Prof. G. C. Berry (Carnegie-MeZZon Uniuersity, U.S.A.) asked: Can Dr Singer provide any information on the dynamic shear moduli (i.e. G’ and G’’) for torsion about the axis of carbon fibres, especially as these might depend on the suprastructure described in his lecture? Dr L.S. Singer ( Union Carbide, Parma, U.S.A.) responded: The dynamic torsion behaviour has been studied by Fischbach and coworkers on a variety of carbon-fibre types, including PAN, rayon, isotropic pitch and mesophase pitch. The most recent detailed study of mesophase-pitch-based carbon fibres was made by Fischbach and Srinivasagopalan.’ They found that, qualitatively, all the fibres behave similarly ; however, the effective torsional moduli are consistent with differences in the micro- structure of the fibres. D. B. Fischbach and S. Srinivasagopalan, Proc. 5rh London Inr. Carbon and Graphite Con$, Imperial College, London, 1978 (Society of Chemical Industry), vol. I, pp. 389-397. Prof. E. L. Thomas (University of Massachusetts, U.S.A.) said: Can Dr Singer tell us how to tailor the spinning process so as to produce fibres with onion-skin texture and fibres with radial texture? Dr L. S. Singer (Union Carbide, Parma, U.S.A.) replied: Yes and no. The development of orientation and texture in fibres spun from liquid-crystal systems such as mesophase pitch is determined by the intrinsic structural and rheological properties of the pitch and the flow-orientation forces during spinning. As one might expect, the phenomena are either not well understood, proprietary, or both. Two recent U.S. patents by Nazem (4 376 747) and Riggs (4 504 454) are relevant.3 Bb molecular director fields. Plate 1. Bright-field electron micrographs illustrating some of the types of lamellar trajectories and their associatedPlate 2. ( a ) and ( 6 ) : Dark-field images of an as-sheared sample of B-N. The images were formed from the two wings of the equatorial arc and show complementary contrast. (c) The corresponding bright field image.Plate 3. Equatorial dark-field ( a ) and bright-field (6) images of B-N annealed for 10 min on a rock salt substrate at 300 "C.Plate 4. Bright-field images of B-N annealed for 10 min at 300 "C on a rocksalt substrate: ( u ) flat; (6) tilted through +24"; (c) tilted through -24" about the horizontal axis, which is perpendicular to the prior shear direction.Plate 5. Three bright-field micrographs showing changes in the apparent position of veins relative to rocksalt debris, on tilting the specimen about the horizontal axis (shear direction vertical): ( a ) 0"; ( b ) -24" and (c) +24". Plate 6. Banded texture in B-ET. The shear axis is horizontal (C. Viney).Plate 7. Dark-field TEM micrograph showing bands perpendicular to the shear direction. They cannot be seen clearly in bright field.' Plate 8. A sample of a copolyester (N-QT) sheared (bottom left to top right) to produce banded texture, seen in the SEM. The fibrillations reveal the sinusoidal trajectory of the molecules about the shear axis.

ISSN:0301-7249    

DOI:10.1039/DC9857900273

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

LIST OF POSTERS Molecular Motions in Thermotropic Rigid-chain Liquid-crystalline Polymers G. R. Mitchell, University of Reading, and F. Ishii, A. Odajimia and Y. Takase, Hokkuido University, Japan Rheology of Anisotropic Hydroxypropylcellulose Solutions P. Navard and J. M. Haudin, Centre de Mise en Forme des Matriaux, Valbonne, France Gaussian Approximation in Onsager-type Theories of Polymeric Liquid Crystals: Analysis of Bidispersity and Semiflexibility The0 Odijk, University of Leiden, The Netherlands Dynamics and Phase Relationships in Ternary Rod/ Matrix/ Solvent Systems Paul S. Russo, Louisiana State University, U.S.A. Dielectric Relaxation Behaviour of Liquid-crystalline Siloxan Polymers having Mesogenic Groups in the Side Chain G. S. Attard, B. Ashton, W. N. Jenkins and G.Williams, University College of Wales, Aberystwyth, and G. W. Gray, D. Lacey and P. A. Gemmel, The University of Hull Deformation Studies of Liquid-crystalline Polymers Patrick Navard, Ecole des Mines de Paris, Valbonne France, and Anagnostis E. Zachariades, IBM Research Laboratory, San Jose, U.S.A. The Viscoelastic and Elastic Constants of Mixtures of Side-chain Polymeric and Monomeric Liquid Crystals H. J. Coles, A. I. Hopwood and M. S. Sefton, University of Manchester Phase Behaviour of Copolymers containing Poly(ethy1ene oxide) with n-Decane C. J. Bowden and T. M. Herrington, University of Reading Deuteron N.M.R. Study of Liquid-crystalline Polymers and Polymer Model Membranes C. Boffel, R. Ebelhauser, T. Fahmy, U. Pechorn and H. W . Spiess, Universitut Mainz and Max-Planck-Institut fur Polymerforschung, Mainz, West Germany Computer Modelling of Polymer Liquid Crystals S.F. Edwards and D. A. Castelow, Cavendish Laboratory, Cambridge Effect of Screened Coulomb Interactions on the Isotropic-Nematic Phase Transition in Solutions of Rigid Polyelectrolytes A. Stroobants and H. N. W. Lekkerkerker, Vrije Universiteit Brussel, Belgium The Etch Micromorphology and Properties of an Injection-moulded Liquid-crystal Polymer H. Thapar, P. Allan and M. J. Bevis, Brunel University Liquid-crystalline Polyethylene D. C. Bassett, University of Reading Phase Diagrams, Nematic Order Parameters and Biphase Formation in Mixtures of Main- chain Polymeric Liquid Crystals with Model Compounds R. B. Blumstein, A. Blumstein, 0. Thomas and M.M. Gauthier, University of Lowell, U.S.A., and F. Volino, Centre d’Etudes Nucliaires de Grenoble, France Loss Processes in Aromatic Liquid-crystal Polyesters D. J. Blundell and K. A. Buckingham, ICI, Wilton Factors affecting Domain Formation in Thermotropic Polymers A. M. Donald, Cavendish Laboratory, Cambridge 289290 LIST OF POSTERS Structure and Properties of Liquid-crystal Side-group Polymers-Influence of the Spacer Group M. Engel and H. Ringsdorf, Universitd Mainz, West Germany A New Series of Polymer Liquid-crystal Model Compounds Anselm C. Griffin and Robert S. L. Hung, University of Southern Mississippi, U.S.A. Main-chain Mesogenic Polyesters A. D. Jenkins, A. H. Al-Dujaili and D. R. M. Walton, Uniuersity of Sussex Liquid-crystalline Polymers with Disc-like Mesogens in the Main Chain W.Kreuder, H. Ringsdorf and P. Tschirner, Uniuersitiit Mainz, West Germany Mesomorphic Behaviour of Conventional Polymers containing Small-molecule Liquid Crystals F. Kuschel, S. Oelsner and S. Diele, Martin-Luther- Uniuersitit, Halle, East Germany Orientational Order in Main-chain Liquid-crystal Polymers. An Experimental and Theoretical Investigation of Model Compounds J. W. Emsley, N. J. Heaton, G. R. Luckhurst and G. N. Shilstone, The University, Southampton Phase-transition Temperatures and Nematic Ranges of some Liquid-crystalline Aromatic Polyesters and Polyamides J. E. McIntyre and A. H. Milburn, The University of Leeds Dynamics of Magnetically Strained Nematic Polymers A. F. Martins, INIC, Portugal The Structure of Smectic D Mesophase Robert R.Matheson Jr, K. Gardner and P. Zoller, Wilmington, U.S.A. Defects in a Main-chain Nematic Polyester and Textures in a Side-chain Polyacrylate G. Mazelet and M. KlCman, Universiti Paris-Sud, Orsay, France Miscibility Requirements for Melt Blends of Thermotropic and Isotropic Polyesters R. R. Luise, Du Pont Experimental Station, Wilmington, U.S.A. Anisotropy of Mechanical Properties for Side-chain Mesomorphic Polymers P. Fabre, L. Leger and M. Veyssie, Collgge de France, Paris, France, and H. Finkelmann, Institut f u r Makromolekulare Chemie, Freiburg, West Germany Mechanical Properties of, and Molecular Motions in, Oriented Copolymers of 4-Hydroxyben- zoic Acid (HBA) and 2-Hydroxy-6-naphthoic Acid (HNA) J. Clements, G. R. Davies, R. Jakeways, M. J. Troughton and I. M. Ward, University of Leeds Synthesis and Properties of Cross-linked Liquid-crystal Polymers F. J. Davies, A. Gilbert, J. Mann and G. R. Mitchell, University ofReading Heterocyclic Polymers and Fibres: Synthesis Fabrication and Evaluation D. L. Brydon, J. Emans, I. S. Fisher and D. M. Smith, University of St. Andrews Combined Liquid-crystalline Polymers: Mesogens in Main Chains and Side Groups R. Beck and H. Ringsdorf, Uniuersitut Mainz, West Germany Miscibility of Dye-containing Liquid-crystalline Copolymers with Low-molar-mass Liquid Crystals G. Baur and H. Kiefer, Uniuersitut Mainz, West Germany, and H. Ringsdorf, H-W. Schmidt and R. Zentel, Fraunhofer Institut, Freiburg, West Germany

ISSN:0301-7249    

DOI:10.1039/DC9857900289

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

INDEX OF NAMES* Alderman, N. J., 149 Berry, G. C., 85, 141, 175, 180, 287 Biswas, A., 73 Blackwell, J., 73, 108, 117 Blumstein, A., 33, 88, 96, 97, 98, 99, 226 Blumstein, R. B., 33, 87 Blundell, D. J., 108, 184 Bruckner, S., 41 Chapoy, L. L., 86 Chiellini, E., 85, 241, 285 Chivers, R. A., 73, 116, 273 Coles, H. J., 90, 97, 175, 201 Donald, A. M., 55, 118, 281 Flory, P. J., 90, 176 Fraden, S., 125 Frank, F. C., 99, 102, 104, 274 Galli, G., 241 Gauthier, M. M., 33 Gray, D. G., 257 Griffin, A. C., 41 Griffin, B., 185 Golombok, R., 55 Gutierrez, G. A., 73 Hurd, A. J., 125 Jackson, W. J., 87 Jenkins, W. N., 191 Keller, A., 98, 186, 189 KlCman, M., 175, 181, 215 Krigbaum, W. R., 89, 133, 177, 178, 179 Laupretre, F., 191 Leadbetter, A. J., 107 Lee, S-D., 125 Lenz, R. W., 21, 94 Lonberg, F., 125 Luise, R. R., 287 Mackley, M. R., 149, 182, 184, 187, 286 Meyer, R. B., 86, 125, 175, 176, 284 Mitchel1,G. R.,55,91, 100, 107, 112, 115, 179, 181, Noel, C., 191, 225, 227 Reck, B., 285 Samulski, E. T., 7, 85, 86, 89, 99 Scott, J. C., 41 Singer, L. S., 176, 265, 287 Taratuta, V., 125 Thomas, E. L., 97,98, 188, 189,229, 274,280, 281, Thomas, O., 33 Viney, C., 55, 103, 105, 228, 286 Volksen, W., 41 Wissbrun, K. F., 96, 101, 161, 178, 179, 185, 189 Williams, G., 191, 226 Windle, A. H., 55, 90, 100, 102, 105, 107, 110, 118, Wood, B. A., 229, 274, 280 Yoon, D. Y., 41, 99 Zentel, R., 89, 100, 189, 225 182 287 186,281, 284 * The page numbers in bold type indicate papers submitted for discussion. 29 1

ISSN:0301-7249    

DOI:10.1039/DC9857900291

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

THE Date 1907 1907 1910 191 1 1912 1913 1913 1913 1914 1914 1915 1916 1916 1917 1917 1917 1918 1918 1918 1918 1919 1919 1920 1920 1920 1920 1921 1921 1921 1921 1922 1922 1923 1923 1923 1923 1923 1924 1924 1924 1924 1924 1925 1925 1926 1926 1927 1927 1927 1928 1929 1929 1929 1930 GENERAL DISCUSSIONS O F FARADAY SOCIETY/ FARADAY DISCUSSIONS OF THE CHEMICAL SOCIETY Subject Osmotic Pressure Hydrates in Solution The Constitution of Water High Temperature Work Magnetic Properties of Alloys Colloids and their Viscosity The Corrosion of Iron and Steel The Passivity of Metals Optical Rotatory Power The Hardening of Metals The Transformation of Pure Iron Methods and Appliances for the Attainment of High Temperatures in a Laboratory Refractory Materials Training and Work of the Chemical Engineer Osmotic Pressure Pyrometers and Pyrometry The Setting of Cements and Plasters Electrical Furnaces Co-ordination of Scientific Publication The Occlusion of Gases by Metals The Present Position of the Theory of Ionization The Examination of Materials by X-Rays The Microscope: Its Design, Construction and Applications Basic Slags: Their Production and Utilization in Agriculture Physics and Chemistry of Colloids Electrodeposition and Electroplating Capillarity The Failure of Metals under Internal and Prolonged Stress Physico-Chemical Problems Relating to the Soil Catalysis with special reference to Newer Theories of Chemical Action Some Properties of Powders with special reference to Grading by Elutriation The Generation and Utilization of Cold Alloys Resistant to Corrosion The Physical Chemistry of the Photographic Process The Electronic Theory of Valency Electrode Reactions and Equilibria Atmospheric Corrosion. First Report Investigation on Oppau Ammonium Sulphate-Nitrate Fluxes and Slags in Metal Melting and Working Physical and Physico-Chemical Problems relating to Textile Fibres The Physical Chemistry of Igneous Rock Formation Base Exchange in Soils The Physical Chemistry of Steel-Making Processes Photochemical Reactions in Liquids and Gases Explosive Reactions in Gaseous Media Physical Phenomena at Interfaces, with special reference to Molecular Atmospheric Corrosion. Second Report The Theory of Strong Electrolytes Cohesion and Related Problems Homogeneous Catalysis Crystal Structure and Chemical Constitution Atmospheric Corrosion of Metals.Third Report Molecular Spectra and Molecular Structure Colloid Science Applied to Biology Orientation 293 Volume Trans. 3* 3* 6* 7* 8* 9* 9* 9* 1 o* 11 12* 12* 13* 13* 13* 14* 14* 14* 14* 15* 15* 16* 16* 16* 16* 17* 17* 17* 17* 18* 18 19* 19 19* 19 19* 20* 20* 20* 20* 20* 21* 21 22 22 23* 23* 24 24 25* 25* 26* 26 10:294 FARADAY DISCUSSIONS OF THE CHEMICAL SOCIETY Date Subject 1931 1932 1932 1933 1933 1934 1934 1935 1935 1936 I936 1937 1937 1938 1938 1939 1939 1940 1941 1941 1942 1943 1944 1945 1945 1946 1946 1947 1947 1947 1947 1948 1948 1949 1949 1949 1950 1950 1950 1950 1951 1951 1952 1952 1952 1953 1953 1954 1954 1955 1955 1956 1956 1957 1958 1957 1958 1959 1959 1960 1960 1961 1961 Photochemical Processes The Adsorption of Gases by Solids The Colloid Aspect of Textile Materials Liquid Crystals and Anisotropic Melts Free Radicals Dipole Moments Colloidal Electrolytes The Structure of Metallic Coatings, Films and Surfaces The Phenomena of Polymerization and Condensation Disperse Systems in Gases: Dust, Smoke and Fog Structure and Molecular Forces in (a) Pure Liquids, and ( b ) Solutions The Properties and Functions of Membranes, Natural and Artificial Reaction Kinetics Chemical Reactions Involving Solids Luminescence Hydrocarbon Chemistry The Electrical Double Layer (owing to the outbreak of war the meeting was The Hydrogen Bond The Oil-Water Interface The Mechanism and Chemical Kinetics of Organic Reactions in Liquid The Structure and Reactions of Rubber Modes of Drug Action Molecular Weight and Molecular Weight Distribution in High Polymers (Joint Meeting with the Plastics Group, Society of Chemical Industry) The Application of Infra-red Spectra to Chemical Problems Oxidation Dielectrics Swelling and Shrinking Electrode Processes The Labile Molecule Surface Chemistry (Jointly with the SociCtC de Chimie Physique at Bordeaux) Colloidal Electrolytes and Solutions The Interaction of Water and Porous Materials The Physical Chemistry of Process Metallurgy Crystal Growth Lipo-proteins Chromatographic Analysis Heterogeneous Catalysis Physico-chemical Properties and Behaviour of Nuclear Acids Spectroscopy and Molecular Structure and Optical Methods of Investigating Electrical Double Layer Hydrocarbons The Size and Shape Factor in Colloidal Systems Radiation Chemistry The Physical Chemistry of Proteins The Reactivity of Free Radicals The Equilibrium Properties of Solutions on Non-electrolytes The Physical Chemistry of Dyeing and Tanning The Study of Fast Reactions Coagulation and Flocculation Microwave and Radio-frequency Spectroscopy Physical Chemistry of Enzymes Membrane Phenomena Physical Chemistry of Processes at High Pressures Molecular Mechanism of Rate Processes in Solids Interactions in Ionic Solutions Configurations and Interactions of Macromolecules and Liquid Crystals Ions of the Transition Elements Energy Transfer with special reference to Biological Systems Crystal Imperfections and the Chemical Reactivity of Solids Oxidation-Reduction Reactions in Ionizing Solvents The Physical Chemistry of Aerosols Radiation Effects in Inorganic Solids abandoned, but the papers were printed in the Transactions) Systems Published by Butterworths Scientific Publications, Ltd Cell Structure Volume 27 28 29 29* 30 30 31* 31* 32* 32* 33* 33* 34* 34* 35* 35* 35* 36* 37* 37* 38 39 40: 41 42* 42 A 42 B Disc.I* 2 Trans. 43* Disc. 3 4" 5 6 7 8* Trans. 46* Disc. 9 Trans. 47 Disc. 10 11 12* 13 14 15 16* 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 The Structure and PropeAies of Ionic Melts 32Date 1962 1962 1963 1963 1964 1964 1965 1965 1966 1966 1967 1967 1968 1968 1969 1969 1970 1970 1971 1971 1972 1972 1973 1973 1974 1974 1975 1975 1976 1977 1977 1977 1978 1978 1979 1979 1980 1980 1981 1981 1982 1982 1983 1983 1984 1984 FARADAY DISCUSSIONS OF THE CHEMICAL SOCIETY Subject Inelastic Collisions of Atoms and Simple Molecules High Resolution Nulcear Magnetic Resonance The Structure of Electronically Excited Species in the Gas Phase Fundamental Processes in Radiation Chemistry Chemical Reactions in the Atmosphere Dislocations in Solids The Kinetics of Proton Transfer Processes Intermolecular Forces The Role of the Adsorbed State in Heterogeneous Catalysis Colloid Stability in Aqueous and Non-aqueous Media The Structure and Properties of Liquids Molecular Dynamics of the Chemical Reactions of Gases Electrode Reactions of Organic Compounds Homogeneous Catalysis with Special Reference to Hydrogenation and Bonding in Metallo-organic Compounds Motions in Molecular Crystals Polymer Solutions The Vitreous State Electrical Conduction in Organic Solids Surface Chemistry of Oxides Reactions of Small Molecules in Excited States The Photoelectron Spectroscopy of Molecules Molecular Beam Scattering Intermediates in Electrochemical Reactions Gels and Gelling Processes Photo-eff ects in Adsorbed Species Physical Adsorption in Condensed Phases Electron Spectroscopy of Solids and Surfaces Precipitation Potential Energy Surfaces Radiation Effects in Liquids and Solids Ion-Ion and Ion-Solvent Interactions Colloid Stability Structure and Motion in Molecular Liquids Kinetics of State Selected Species Organization of Macromolecules in the Condensed Phase Phase Transitions in Molecular Solids Photoelectrochemistry High Resolution Spectroscopy Selectivity in Heterogeneous Catalysis Van der Waals Molecules Electron and Proton Transfer Intramolecular Kinetics Concentrated Colloidal Dispersions Interfacial Kinetics in Solution Radicals in Condensed Phases Oxidation 295 Volume 33* 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65* 66 67 68 69 70 71 72 73 74 75 76 77 78 * Not available; for current information on prices, etc., of available volumes, please contact the Marketing Oficer, Royal Society of Chemistry, Burlington House, London WI V OBN stating whether or not you are a member of the Society.

ISSN:0301-7249    

DOI:10.1039/DC9857900293

出版商:RSC    

年代:1985

数据来源: RSC