摘要:

Faraday Discuss. Chem. SOC., 1985, 79, 141-148 Rheological and Rheo-optical Studies of a Constitutive Equation for Nematogenic Solutions of Rod-like Polymers BY GUY C. BERRY Department of Chemistry, Carnegie-Mellon University, Pittsburgh, Pennsylvania 152 13, U.S.A. Received 3rd December, 1984 Rheological and rheo-optical studies on solutions of the rod-like poly( 1,4-phenylene-2,6- benzobisthiazole) are described for both isotropic and nematic solutions. The data include both steady-state and transient behaviour. For isotropic solutions it is found that a single- integral constitutive equation (a form of the B.K.Z. equation) represents the data available. For nematic solutions unexplained behaviour is observed for small K, perhaps related to unstable flow in simple shear predicted for such solutions by some mechanistic theories for the parameters in the Leslie-Erickson constitutive equation.For larger K the flow behaviour may be represented by a modified form of the single-integral equation. 1. INTRODUCTION The following describes rheological and rheo-optical studies 1-3 on nematogenic solutions of the rod-like poly( 1,4-phenylene-2,6-benzobisthiazole), PBT, carried out over a range of concentration c and chain length L encompassing the isotropic and nematic phases. The studies include measurements of the creep compliance J,( t ) determined from the strain y ( t ) obtained with a constant shear stress u: J a w = Y ( t ) / U (1.1) the recoverable R,(S, 8) determined after creep of duration S, with 8 the elapsed time for creep recovery: Y ( S ) - Y ( 8) U &(S, 8) = the stress-growth function qK ( t ) determined from the stress u( t ) obtained constant shear rate K : q K ( l ) = a(t>/ the viscosity relaxation function determined from the stress u8 following cessation of steady state flow at constant K : < K ( e ) = u ( e > / K ( 1-41 and the corresponding birefringence An('3) expressed below as the function k f K ( t ) = A n ' 1 3 ' ( t ) / ( ~ q K ) 2 GK( 8) = An('3)( 8 ) / ( K q K ) 2 where qK is the steady-state viscosity obtained as the asymptotic value of q K ( t ) for large t and An('3) is measured in the 1-3 flow plane, with flow in the 1-direction for a shear gradient in the 2-direction.In using these relations it is convenient to 141142 define RHEOLOGICAL AND RHEO-OPTICAL STUDIES lim R,(S, 0 ) = R,( 0) lim R,( e ) = R‘,S’.S=m e=oo Since the materials studied here are fluids, In general, J;’’ f R‘,S’ and vK < qo, but for a linear viscoelastic fluid J‘,“’ = I?:? = R r ) and 7, = rlK = qo, and (1.10) (1.1 1) (1.12) (1.13) With non-linear behaviour these relations are not obeyed. In the following the data will be compared with predictions made with a B.K.2.-type single-integral constitutive equation’ given by6 (1.14) with a(’’( t) and d 2 ) ( t ) equal to the shear stress a( t ) and first-normal stress difference z-J(*’( t), respectively. Here Ay( t, u ) = y ( t ) - y( t - u ) and F is a decreasing function of Ay( t, u ) for non-linear behaviour. [For F = 1, linear behaviour is recovered with eqn (1.14).] With nematic seolutions the rheological properties are inherently anisotropic.According to the constitutive equation of Leslie7 and Erickson: the steady-state shear viscosity q0 (determined for flow in a wide gap in a region for which the stress tensor is a linear function of the velocity-gradient tensor) may be expressed in the form TO’ q b + i ( l - A - ’ ) [ ( ~ c - ~ b ) + i a l ( l (1.154 2 V b = a3+ a4-k (1.156) r l c - rlb = - a 2 - a 3 (1.15~) A - ‘ = ( ( Y ~ - c T ~ ) / ( c Y ~ + a 3 ) (1.15d) where A and the ai are related to the order parameter S for the quiescent nematic fluid (see below). Stable simple shear obtains only if A > I , with the rod-like chains at an angle (arccos A-’)/2 with the flow direction. 2. EXPERIMENTAL The experimental methods used and molecular characteristics of the polymers studied Briefly, solutions were prepared in methanesulphonic acid (MSA).are reportedG. C. BERRY 143 Polymers 53, 62 and 72 have contour lengths L , of 120, 170 and 135 nm, respectively. Rheological data were obtained with a cone-and-plate rheometer constructed to permit measurement of y ( t ) with a specified stress history or a(t) with a specified strain history. The flow birefringence data were obtained with the fluid between glass (Pyrex) parallel plates, using measurement of the intensities I+( 4 ) and I!,( 4 ) transmitted between crossed and parallel polars, respectively, where 4 is the angle between the flow direction and the polarization direction of the incident beam. This arrangement permits measurement of An('3) for the birefringence in the 1-3 flow plane.' 3.DISCUSSION 3.1. ISOTROPIC SOLUTIONS Results2 for Ja( t ) and R,( t ) for isotropic solutions reveal linear behaviour Jo( t) for small enough a, but also show that J,( t ) = Jo( t) for t < t* and Ja( t ) > Jo( t) for t > t*, where t* decreases with increasing a. The strain y( t*) = oJ,( t*) is found2 to be nearly independent of (T and inversely proportional to c. This behaviour is consistent with eqn (1.14), with F( y ) = 1 for y( t ) < y*. Similar behaviour has been reported with flexible-chain polymers6 and analysed with eqn (1.14) with Go( t ) = C Gi exp (- t / r i ) (3.2) where rn is zero if Iyl< y' and unity otherwise. For example, with these expressions the steady-state parameters calculated with eqn (1.14) are given by6 where Ny' = v"'/2( ~ q , ) ~ , qi = Giri and, with gi = ?'/ T ~ K , (Y = y'/ y" and K' = 1 + a / g , qK,i = ( l + -f?> exp (-gi) (3.6) + ( -.h ) [ ( A + gi I2 +.f?l/ ( + (Y.h -.f?)' (3.7) PK,i = The needed parameters Gi and ri have been computed from the corresponding parameter Ri and hi, defined by by use of relations based on eqn (1.1 1).236 Typical results for Ro( t) are given in fig.1. As shown in fig. 2 both qK/qo and RK/Ro are fitted well by eqn (3.3) and (3.4) for the isotropic solutions. The ri distribution is found to be broader than that calculated by Doi and E d ~ a r d s , ~ for which there is essentially one relaxation time. The limiting viscosity qo is shown in fig. 3(a) in a form for comparison with the theoretical r e l a t i ~ n ~ ' ~ ' ' ~ RF'-R,(t)=C Ri exp ( - t / h i ) (3.8) q o = KNqsM[ q J(Y*3x3( 1 - Bx)-2 (3.9) where X = cL/MLo*, [q] is the intrinsic viscosity, M the molecular weight, qs the solvent viscosity, and K and a*/ B are treated as adjustable parameters.The data are fitted by eqn (3.9) with a* = c,L/ML and B slightly smaller than unity, where c, is the concentration for the appearance of nematic phase. These are in the range144 lo-' G *y,oo 1 0 5 h w v *o 0.1 0.01 0. O6 - = 0 6 6 1 I I I IIIII I I I , I I I I I I I I I , , I , I I I I , , , l 4 , , I - ' ' " " " 1 ' ' " 1 1 " ' ' ' ' l u l l i ' I ' 1 ' 1 ' ' 1 ' ""t Q Q r --- - c- 6 6 ' ' ' ' " ' I ' I ' ' ' " " 1 ' ' ' ' " ' I ' ' ' ' ' " " 1 ' ' ' - RHEOLOGICAL AND RHEO-OPTICAL STUDIES I--- I I I I 0.01 0.I I 10 100 t/ 7 c Fig. 1. R,( t ) / R o plotted against t / T= (with T~ = qORO) for an isotropic solution of PBT-53 (0.0294 weight fraction polymer); (-) R,( t ) / R plotted against t / q p R , for a nematic solution of the same polymer (0.0323 weight fraction polymer). For the latter R,( t), R, and qp were determined after steady-state flow with q p R , ~ L- 1. expected theoretically.2 The empirical value K = 1.5 x IOP4 is smaller than the original prediction,9y1o but in accord with subsequent treatments.' With a* = c,L/ ML the dependence of vo/ 7,c: is expected to be independent of temperature. This behaviour is observed for the system studied.2G. C. BERRY 145 log (BcL,/ M,a*) Fig. 3. qo/Mw[ . ~ ] ( a * / a $ ) ~ plotted against BcLw/MLa* for ( a ) isotropic and ( b ) nematic solutions of PBT-53 and PBT-62.3 With the nematic solutions, qo is replaced by qp (a; is a constant). In the range of cL with qo increasing markedly with increasing c, Rk" is found to be nearly independent of c, or to increase with increasing c.Similar behaviour has been predicted theoretically by Marrucci. l 2 With the stress-optic law in the form13 M, == 2CN'," (3.10) for either M , ( t ) or k K ( 0 ) , e n (1.14) can be used with flow birefrin ence data. Results in fig. 4 for M, are seen to be in satisfactory agreement with eqn (3.5). The results for Mo/RLs' are about thirty percent larger than values obtained at lower concentra- tion. The data on M, show that the rod-like molecules are well aligned in the flow direction, but that for q O R y ) ~ > 1 the orientation is not as great as would be expected hzd linear viscoelastic behaviour obtained.The relaxation function GK ( 6 ) / qK and M,(O)/M, shown in fig. 5 are in qualitative accord with eqn (1.14), for which2 j j K ( t ) = C qi(l-qK,i) ~ X P ( - t / T i ) (3.11) q;fi(l) K - - C qiTi(1- q K , i p K , i ) exp( - t / T i ) (3.12) The behaviour is similar to that obtained with flexible chain polymers in that the birefringenGe relaxes more rapidly than the tress.'^ Since N','' is expected to be R:' 9 for small K, it is seen that 2C = Mo/R$'. ? 3.2. NEMATIC SOLUTIONS Typical results shown in fig. 2 reveal an effect at small K different from that observed with isotropic solutions (at only slightly smaller c) and not included in146 RHEOLOGICAL AND RHEO-OPTICAL STUDIES Table 1.Leslie coefficient for rod-like polymers parametera ref. (15) ref. ( 16)b - S2 - rS2 -S(l+2S)/(2+S) -(s/2)[3(r- 1)+2(1+2S)]/[3(r- l)+2+S] = -S(4S- 1)/(5S-2) -(S/2)[3(r - 1 ) +2(1- S)J/[3(r- 1)+2+ S] =S(1 -S)/(5S-2) -S(1- S)/(2+ S ) (1 - 5)/3 S 0 1 - ( I -S)/3 - (7 - 5s - 2rS2)/35 = ( 1 - S)[1- (2/35)( 1 - S)(7 + 4S)]/3 S( 5 + 2rS)/7 = S [ 1 - (2/2 I ) ( 1 - S ) ( 3 - 4S)] -2S( 1 - rS)/7 = -2S( 1 - S)(3 +4S)/21 r - ( 1 - S)/3 = -1 -5(1- S)/3 1 - 4( 1 - S ) / 3 ~ ~~ ~~~~~~ a k = qbso( 1 - S)2; qbso given by eqn (3.9) with B = 0. Since a2+ a3 = a6 - a5, only five Based on asymptotic behaviour for a, given in ref. (16) and of the ai are independent. approximate relations for A and r. " A = ( a2 + a3)/( a2 - a3).eqn (1.14). Thus vK decreases with increasing K, reaching a plateau r ] , = qp for qpR',S)~ = 1. At the same time R',S) is independent of K for qpR!, < 1. It is tempting to attribute the behaviour at small K to effects of the anisotropy leading to eqn ( I . 15). In the latter, r]b and qC represent the ratio ( T / K for steady-state shear flow with the rod-like molecules held parallel to the flow direction and the direction of the shear gradient, respectively. Since r]= > qb, effects which decrease A toward unity cause a decrease in q0. In theoretical treatment^'^"^ based on the diffusion equation of Doi and E d ~ a r d s , ~ A and the Leslie coefficient ai are all functions of the order parameter S for the quiescent material: where 8 is the angle between the rod-like molecules with the average calculated using the equilibrium distribution of 0, and S depends on c / c c , increasing towards unity with increasing c/c,.If steady-state flow at finite K can be represented by eqn (1.15) with a modified order parameter S replacing the order parameter for the quiescent fluid, then q0 might be expected to decrease as K increases. With the ai and A given in ref. (15) (see table l), then (1 +2S)(2+3S) (2+S) TO= q b (3.14~) (3.14b) where qhSo is given by eqn (3.9) with B=O, so that qo decreases as S increases towards unity. With the ai and A given in ref. (16) (see table l), A < 1 and simple shear flow is predicted to be unstable. Since the calculation for the ai is delicate, the significance of this result is unclear for the observed behaviour, which results in stable steady-state shear stress at all K studied.With the ai given in ref. (16) (3.15)G. C . BERRY 147 00-0. I 10 100 710 R" K Fig. 4. Steady-state flow birefringence plotted against the reduced shear rate q0R0u for an isotropic solution of PBT-53 (0.0255 weight fraction) at several temperatures. The curve is calculated with eqn (3.5) and (3.10) using experimentally determined values of T~ and T ~ . I I I 1 I 0.0 I 0.01 0. I I 10 I00 t / P I Fig. 5. Reduced-flow birefringence relaxation function fi( t )/ M, and stress relaxation func- tion <( t)/v, for anisotropic solution of PBT-53 (0.0294 weight fraction) for several shear rates,* with p, = T,R,. which is smaller (ca. 20%) than v b given by eqn (3.2b) for given S.[Similarly, the elongational viscosity obtained with the ai in ref. (15) is about twice that for the ai in ref. (16) for comparable S.] Flow-birefrin ence data indicate a substantial degree of orientation' for shear of the quiescent nematic fluid is lost, and the fluid in flow is much like that for a well oriented isotropic solution, with similar characteristics for the transmission of polarized light. For smaller K (e.g. q p R h s ) ~ < 0.1) the transmitted intensity is smaller than expected and tends to fluctuate. In particular, with q p R r ) ~ > 1 the sum I++ I,, of the intensities I+ and Ill transmitted between crossed and parallel polars, respectively, is smaller than that for the quiescent fluid and the overall field appears optically homogeneous (albeit birefringent and oriented). For 71pRr)~ < 0.1 the flows with qpRo (ST K > 1.For example, at such K the strong turbidity characteristic148 RHEOLOGICAL AND RHEO-OPTICAL STUDIES sum I++ Ill is markedly depressed and the overall field is mottled in appearance. This is not consistent with the flow predicted in connection with eqn ( 3 . 2 ) [and the ai in ref. (1 5 ) ] and may suggest some kind of flow instability, perhaps similar to the effect predicted in connection with ai given in ref. (16). Values of qp plotted in fig. 3 ( b ) are much smaller than qo extrapolated for the isotropic fluid, in some cases being in the range expected for an isotropic fluid with cL/ML = a*. Based on the preceding discussion, it is not clear whether qp should be identified with Tb or qo in eqn (3.2) or with neither of these.In addition to the complicated flow birefringence behaviour of small K , it is found that qK determined in stress growth with shear rate K = dy,,/dt increases with decreasing shear rate and is larger than qp (see fig. 2 ) , whereas q a = ( d J a / d t ) - ' determined at creep stress u is about equal to q,,. This unusual behaviour may be related to the flow-birefringence behaviour reported above. In other respects the rheological behaviour is similar to that given in eqn (1.14). Thus, as shown in fig. 1, the value of R,( t ) determined after steady-state flow with q p R K ~ =: 1 is similar to Ro(t) for the nematic solutions. Values of the (apparent) hi and Rj calculated with eqn ( 3 . 8 ) and R,(t) so determined lead to (apparent) ri and qi that reproduce q K / ~ p reasonably well for q p R K ~ > 1 using eqn (3.3).I acknowledge partial support for the studies reported above from the Polymers Program, Division of Materials Research, National Science Foundation, and the Materials Laboratory, Wright- Patterson Air Force Base. S-G. Chu, S. Venkatraman, G. C . Berry and Y . Einaga, Macromolecules, 1981, 14, 939. S. Venkatraman, G. C. Berry and Y . Einaga, J. Polym. Sci., Polym. Phys. Ed., in press. Y . Einaga, G. C. Berry and S-G. Chu, Polym. J., in press. H . Markovitz, in Am. lnst. Phys. 50th Anniversary Physics Vade Mecum, ed. H. L. Anderson (American Institute of Physics, New York, 1981), chap. 19. B. Bernstein, E. A. Kearsley and L. J. Zapas, Trans. SOC. Rheol., 1963, 7 , 391. K. Nakamura, G. C. Berry and C-P. Wong, J. Polym. Sci., Polym. Phys. Ed., 1984, 22, 11 19. F. M. Leslie, Arch. Ration. Mech. Anal., 1968, 28, 265. ' J. L. Erickson, Arch. Ration. Mech. Anal., 1960, 4, 231. M. Doi and S. F. Edwards, J. Chem. SOC., Faraday Trans. 2, 1978, 74, 560. lo M. Doi, J. Phys. (Paris), 1975, 36, 607. l 1 J. A. Odell, E. D. T. Atkins and A. Keller, J. Polym. Sci., Polym. Lett. Ed., 1983, 21, 289. l 3 H. Janeschitz-Kriegl, Adu. Polym. Sci., 1969, 6, 170. l4 K. Osaki, N. Bessho, T. Kojimoto and M. Kurata, J. RheoL, 1980, 24, 125. G. Marrucci and N. Grizzuti, J. Non-Newtonian Fluid Mech., 1984, 14, 103. G. Marrucci, Mol. Cryst. Liq. Cryst. Lett., 1982, 72, 153. N. Kuzuu and M. Doi, J. Phys. Soc. Jpn, 1984, 53, 1031.

ISSN:0301-7249    

DOI:10.1039/DC9857900141

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

Faraday Discuss. Chem. Soc., 1985, 79, 149-160 Optical Textures Observed during the Shearing of Thermotropic Liquid-crystal Polymers BY N. J. ALDERMAN AND M. R. MACKLEY* Department of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge CB2 3RA Received 3rd January, 1985 We report optical observations of a number of main-chain thermotropic liquid-crystal polymers. In situ measurements were carried out at elevated temperatures using an apparatus that is capable of providing a controlled translational oscillatory shearing motion to samples of typically 1-10 pm thickness. In static samples we observe either a general birefringence with local scattering from line defects which we believe to be disclinations, or in thicker samples a texture dominated by light scattering due to the presence of dense disclinations.Superimposed shearing appears to result in both the multiplication of disclinations and the progressive decrease in the distance between individual disclinations. At high shear rates pure birefringence in the direction of flow is observed. Finally we report on the optical relaxation behaviour of oriented samples together with an explanation for the skin core effect observed when thermotropic liquid-crystal polymers are extruded through dies. When viewed in the optical microscope, main-chain thermotropic liquid-crystal polymers generally exhibit strong birefringence and or scattering where usually the light scattering is of sufficient intensity to limit transmitted-light microscopy to sample thicknesses < 15-20 pm.It is one of the primary objectives of this paper to characterize the parameters that influence these optical textures and to provide a structural interpretation of the observations. In addition we are particularly inter- ested in the effect of shear on the structural reorganization of liquid-crystal polymers. Our starting viewpoint is that if the molecular weight of the liquid-crystal polymer is reduced sufficiently, we would expect the material to behave in a similar way to that of small-molecule liquid crystals. On this basis our investigation can start in an area where materials have previously been fully optically characterized; see e.g. ref. ( 1)-(4). Whilst the textures and classification appropriate to small-molecule liquid crystals need not necessarily apply to liquid-crystal polymers our own optical observations suggest that for the systems we have examined there are indeed strong connections with nematic small-molecule liquid crystals and thermotropic liquid- crystal polymers.For static nematic small molecular liquid crystals there is ample evidence to support the view for the existence of line defects, which have been shown to correspond closely to Frank’s’ two-dimensional theoretical classification of disclina- tions. Meyer‘ observed what he called ‘thick’ and ‘thin’ disclination lines, where the ‘thicks’ generally had the characteristics of s = *l and the ‘thins’ s = *$, the strength of the disclination being defined as s = f (the number of extinction bands). In some instances it is possible to obtain a sample free from line defects.In particular, if the surfaces are clean, the director ,may orient with the director vertical to the glass plates bounding the fluid. When viewed between crossed polars and 149150 OPTICAL STUDIES OF THERMOTROPIC LIQUID CRYSTALS Fig. 1. spring- Schematic diagram of shearing apparatus: ( 1 ) motor, (2) gearbox, (3) cam, .loaded piston, (5) mechanical slider, (6) heater, (7) 13 mm diameter quartz glass and (8) hole for the thermocouple. (4) disc vertically through the plates, the field of view will be dark, and in bright field there will be no contrast. Wahl and Fischer’ conducted experiments starting from this situation and they observed the effect of a superimposed steady rotational displace- ment of one circular plate with respect to the other. They observed that at low shear rates (typically 0.1 - 10 s-’ depending on sample thickness) the shear had the effect of orienting the director in the direction of flow, an observation that was consistent with the predictions of the Erickson-Leslie theory’ for small-molecule liquid crystals.At higher shear rates they observed the appearance of line defects causing bright-field scattering which progressively obscured the field of view. Graziano and Mackleyg subsequently observed the occurance of these line defects in detail and showed that the line defects were due to a dynamic equilibrium of disclination loops for both ‘thick’ and ‘thin’ variety corresponding to loops of +1 and *$ strength. Disclination loops were being continually created, deformed, relaxed and lost depending in particular on the relative orientation at any instant of the loop with respect to the flow field.It will be the contention of the results which follow that thermotropic liquid- crystal polymers also possess disclinations and that on shearing substantial disclina- tion multiplication is observed. Typically the disclination density appears to be greatly enhanced over that found for small-molecule liquid crystals.N. J. ALDERMAN AND M. R. MACKLEY 151 APPARATUS Experiments were carried out in an apparatus shown schematically in fig. 1 and as a photograph in plate 1. The essential features of the device comprise a static-top quartz glass disc which can be accurately positioned using three outer micrometers with respect to a bottom glass disc mounted onto a precision horizontal mechanical slider.With experience we were able to position the top glass disc such that the gap between the optically flat glass discs was uniform and typically 1-15 pm in thickness. The temperatures of both the top and bottom glass discs were controlled independently using separate heaters in the temperature range 20-350 "C. At a temperature of say 300°C we estimate temperature variations of not more than *1 "C over the whole field of view. Controlled movement of the bottom glass disc was achieved by means of a piston-and-cam arrangement shown in fig. 1. In order to achieve different amplitudes of oscillation a range of cams were used varying from 0.1 to 0.5 mm displacement from the central position; in addition variable rotational speeds were explored ranging from 0.07 to 2 1 .O rad s-'.Experiments described in the paper were carried out using a modified 'Swift' polarizing-light microscope with a x20 long working-distance objective and the useful facility of coupled polarizer and analyser rotation. MATERIALS AND SAMPLE PREPARATION Over the past five years we have examined a wide range of thermotropic liquid-crystal polymers from a variety of sources. In this paper we will limit our observations to four different chemical compositions. Our knowledge of the molecular-weight averages and/or distribution is very limited, and only in one of the series to be discussed do we have low and high molecular-weight variants. The repeat units of what are thought to be random copolymers are shown below.POLYMER A L c1 JII This has molecular weights corresponding to the following inherent viscosities. Polymer A1 : inherent viscosity 0.056 x Polymer A5: inherent viscosity >0.12 X lop3 m3 kg-' m3 kg-' Polymer supplied by ICI. PLC.152 OPTICAL STUDIES OF THERMOTROPIC LIQUID CRYSTALS POLYMER B Polymer supplied by ICI PLC. POLYMER X7G f C-0-CH ,-CH 2-0 C-O-CH2-CH2-0 L J Polymer supplied by Eastman Kodak Co. POLYMER T, where rn = 4-12. This is a homopolymer. Polymer supplied by Prof. A. D. Jenkins, Dr D. R. M. Walton and Dr A. Al-Dujaili (Sussex University) The samples as received were usually in the form of a powder. Depending on the final thickness of the sample we wished to examine, between 2 and 5mg of sample was placed between Kapton films which were themselves cut out as 22 mm discs.The Kapton films were then placed between flat surfaces within a piston and die and the whole assembly heated to cu. 10 "C above the softening point of the polymer. A transmitted load of ca. lo4 N was applied to the piston for a period of cu. 30 s and on cooling this generally enabled a film of between 5 and 10 pm to be recovered between the Kapton discs. The film thickness was then accuratelyN. J. ALDERMAN AND M. R. MACKLEY 153 measured using a digital micrometer and samples of ca. 2-3 mm2 were cut for each optical observation. When the sample had been placed between the glass discs of the shearing apparatus a low-magnification photograph was taken of the sample. On subsequent heating the sample thickness might change and further low-magnifica- tion photographs were taken during the experiment.In this way the area of the sample was known, and for a given known starting area and thickness, the thickness at any subsequent stage could be determined to an accuracy of ca. *l pm. STATIC OPTICAL TEXTURES Optical observations in static thermotropic liquid-crystal polymers have been made by a number of worker^^^-^^ and our main objective in this section is to extend these observations showing clearly that textures are sensitive in particular to molecular weight, temperature and sample thickness. We have deliberately chosen samples in temperature ranges that do not obviously show any phase transitions, and in particular we are limiting observations to a temperature below that where the polymer may become isotropic.All observations shown are thermally stable and do not change significantly with time. We have observed certain optical features general to polymers Al, A5, B, X7G, T7 and others not cited in this paper. We list their general features below and then demonstrate their existence using specific examples from the chosen range of polymers tested. (1) Individual disclination lines and loops exist in thin-section samples when observations are generally made at high temperatures. (2) Increasing the sample thickness causes the background birefringence to be obscured owing to the presence of a dense disclination texture. ( 3 ) Increasing the molecular weight appears to result in an increasing disclination density.(4) Decreasing the tem- perature causes the disclination density to increase. We have found that the A series polymers are particularly suitable for optical observation as the working temperature range is broad and in the region of 200- 300 "C. The presence of individual line defects is clearly shown in plates 2(a) and (b), which corresponds to the lower-molecular-weight version of polymer A, viewed at a temperature of 230 "C with a specimen thickness of 2.0 pm. At this temperature the melt is mobile and birefringent as shown in plate 2(a) when viewed between crossed polars. When the analyser is removed the individual line defects are seen as in plate 2(b). We see predominantly loops, although line defects terminating at the top and bottom glass surface can also be observed. Whilst it is not possible from this side elevation view to prove conclusively that these line defects are disclinations, their characteristics bear a striking resemblance to the f 1 'thick' disclinations observed in previous work' for nematic small-molecule liquid-crystal polymers.When the sample thickness is increased as shown in plates 2( c ) and ( d ) , the disclination density increases. We note that the background spatial variation of the matrix birefringence seems largely unaffected by the presence of the line defects, suggesting that the line defects although present are only significantly affecting the director trajectory of the material very close to the line defect. With a further increase in sample thickness shown in plates 2(e) and (f), the birefringent contrast is gradually lost and an intense bright-field scattering is observed. The texture then appears very similar when viewed either between crossed polars or without an analyser.In a previous publication one of us13 named this contrast a 'worm texture'. After careful examination of many experiments we are now able to be more specific and state that this contrast corresponds to a dense disclination texture where we are observing the net optical effect of light that has154 OPTICAL STUDIES OF THERMOTROPIC LIQUID CRYSTALS been strongly refracted and scattered as it passes thraugh a sample that contains many disclinations of the type that were seen individually in the thinner sections. The effect of increasing molecular weight is shown in plate 3 .These photographs are for polymer A5 which has a higher molecular weight than polymer A1 used in plate 2. In order to observe individual disclinations shown in plate 3(a) it is now necessary to increase the temperature to ca. 280 "C and have a sample thickness of CQ. 1 pm. If at this temperature the sample thickness is increased to say 5 pm, the dense disclination texture occurs as shown in plate 3( 6). If at this sample thickness the temperature is reduced the scattering further increases, resulting in the material becoming nearly opaque at this sample thickness. The systematic effect of temperature on texture is clearly shown in plate 4 for polymer X7G. At a temperature of 350"C, which is close to the material's anisotropic-isotropic transition individual disclination lines can be seen.Also, in the middle of plate 4(a) a 'centred texture' can be seen where four extinction bands emanate from a point. These extinction bands rotate with the polar rotation indicat- ing that we are viewing a *I disclination along the axis of the line defect. If the temperature is reduced to 280 "C the number of disclinations increases as shown in plate 4( b). A further reduction in temperature to 220 "C at the same sample thickness results in the progressive appearance of the dense disclination texture where the birefringence becomes obscured by the intense scattering. Polymer B also shows a similar effect in terms of disclination density. In plate 5( a ) , at the higher temperature of 300 "C individual disclinations can again be seen.If the thickness remains constant a reduction in temperature will result in the disclination density increasing to such an extent that the dense disclination texture develops as shown in plate 5( 6). Again many of the T, series Sussex Polymers also show the characteristic dense disclination texture, and an example is given in plate 6 for the polymer T7. Plate 6(a) shows the dense disclination texture viewed between crossed polars and plate 6( b ) similar contrast with polarizer only. The T, series polymers are interesting in that they provide a system with a variable flexible unit. Generally up to the T7 polymer a dense disclination texture was observed; however, the T8 polymer has a striking and unusual mobile behaviour shown in plate 7. Initially, the mobile polymer at 180 "C shown in plate 7(a) and ( b ) is strongly birefringent with extinction bands that respond to the rotation of the polars.No line defects are apparent in this material, and this makes optical observations less complicated. On heating, as shown in plate 7 ( c ) and ( d ) , regions of the sample gradually become dark, indicating that these areas either have become isotropic or (as we believe) are areas where the director is vertical with respect to the glass plates. At a temperature of 225 "C all the sample appears dark [plate 7( e ) ] . If the sample is then sheared as shown in plate 7 ( f ) uniform and bright birefringence is seen, strongly suggesting that the previous orientation was director-vertical and that the shear had rotated the director in the direction of flow.The temperature effect is thermally reversible and has two plausible explanations. When the director is vertical the material must be optically uniaxial, as darkness is observed for all crossed polar orientations. With the lowering of temperature either the material in certain regions is reorienting towards director horizontal or alternatively (as pro- posed by Viney et aZL4 from separate experimental observations) the material is becoming optically biaxial with the principal director remaining vertical. EFFECT OF SHEAR Our starting point for optical observations of shear is either from the texture showing individual disclinations or the dense disdlination texture. In plate 8 weN. J. ALDERMAN AND M. R. MACKLEY 155 follow events for polymer A1 at a temperature of 230°C and sample thickness, 8, of 2 pm.At this temperature and thickness the static sample shows individual disclinations. We examine in detail a sinusoidal displacement of the top plate with an amplitude from the centre to maximum displacement of xo= 0.33 mm and a variable angular velocity w. Initially, at low angular velocities w < ca. 0.1 rad s-I the shear has the effect of only perturbing the individual disclinations. They are seen to move in the fluid; however, the shear does not cause any further changes. If the angular velocity is increased to w > 0.1 rad s-I but also kept below ca. 1 .O rad s-' a profound structural change occurs, and this is shown in plate 8(6) and (c). When the top plate is at mid-cycle and moving with its maximum velocity xow, a dense disclination texture is observed.The sudden and massive multiplication of disclinations occurs and this partially obscures the birefringence that appears to develop in the direction of flow. The scale of the structure observed within the fluid is typically on the pm level and is seen in bright field or when the polars are both at 45 and 0/90° to the flow. When the oscillation is at its maximum amplitude and the velocity gradient momentarily zero, the material attempts to relax back to its original low-disclination texture. If the angular frequency is further increased above w > 1.0 rad s-', at the central position where the velocity gradient is a maximum, pure birefringence is observed and the dense disclination texture is only seen as the material relaxes at the maximum-amplitude position. The onset of the dense disclination texture with shear occurs at a sharp transition and its detailed evolution is difficult to interpret.The relaxation process, however, occurs over longer timescales and can be readily followed as shown in plate 9 for polymer Al. Plate 9 ( a ) is a representative photograph of the dense disclination texture taken during shear. On the cessation of shear the texture relaxes to reveal disclination loops, and in the subsequent plates 9( 6)-( e ) the relaxation of individual loops can be followed over the minute time duration of the sequence. Shear is thus seen to cause two optical events in the material. With increasing angular velocity the onset of massive disclination multiplication is observed, together with indications of birefringence in the direction of flow.As the angular velocity is further increased the birefringence becomes uniform and the structure within the fluid disappears. On cessation of flow or at the maximum amplitude of oscillation the material relaxes towards its static equilibrium texture. Our preferred explanation of structural events seen in this sequence is that the occurrence and multiplication of disclination loops occur at a critical velocity gradient, and with an increasing velocity gradient the size of these loops progressively decreases until the optical microscope is unable to detect their presence. The idea of domains that diminish in size with shearing has been proposed independently by Marrucci I S and WissbrunI6 in order to explain certain rheological properties of thermotropic liquid-crystal polymers.The saturat- ing birefringence suggests that at quite modest velocity gradients the matrix orienta- tion of the material is essentially fully aligned in the direction of flow. Plate 10 shows the evolution of events for polymer A5 starting from a dense disclination texture. In this case the superposition of shear causes the material to transform from a dense disclination texture shown in plate lO(a) to a dominantly birefringent material as given in plate 10(b). When viewed with the polars at 0 and 90" as shown in plate 1O(c) it is still possible to see some very fine structure within the fluid, and at higher velocity gradients this structure gradually disappears, supporting the previous view that the size scale of the disclinations is progressively decreasing with increasing shear.Again when this material is at its position of maximum amplitude relaxation occurs, where in this case the material attempts to return to the dense disclination texture as shown in plate 10(d). As reported156 OPTICAL STUDIES OF THERMOTROPIC LIQUID CRYSTALS 1 I 1 1 l 1 1 l I I I I I 1 1 1 1 10 100 1000 Fig. 2. Polymer A5. T = 240 "C. Graph of maximum measured birefringence as a function of the maximum velocity gradient d, = xow/6 where 6 = 4.3 pm and xo = 0, 0.33, 0, 0.24 and A, 0.1 1 mm. (A) Dense disclination texture; (B) ordered texture only observed at maximum strain rate; (C) ordered texture. previously by Graziano and Mackley," on the cessation of flow the material may under certain circumstances relax to the dense disclination texture or alternatively, as shown in plate 11 straight at right angles to the direction of flow may occur.The development of birefringence measured using a rotary quartz compensator and plotted as a function of the maximum applied velocity x,w/6 is shown in fig. 2 for polymer A5 at a temperature of 240°C. The graph shows the approximate range in which the dense disclination texture is seen, together with the birefringence measurements which at high velocity gradients rapidly saturate corresponding pre- sumably to the fully oriented state. We noted a very slight effect on the magnitude of the maximum birefringence observed using cams of different amplitudes xo.PROCESSING OF FIBRES We can expect the development of orientation and textures reported in the previous sections to influence the rheology, processing and final solid-state properties of thermotropic liquid-crystal polymers, and by way of example we report on how the properties of fibres made from polymer A5 and polymer B can be significantly influenced by processing conditions. Polymer fibres were produced by ram extrusion of the polymer through a circular die of the form shown schematically in fig. 3. The extrudate, which showed little or no die swell, was wound up in air at ambient temperature on a take-up spool such that either a 'free-fall' fibre could be obtained or alternatively a differential draw could be applied between the die exit and take-up.Initially a series of experiments were carried out for different length-to-diameter ratios of dies and at different die temperatures with the mobile anisotropic phase of the polymer; however, no systematic effect on fibre properties could be established. A further series of experiments was conducted, examining the effect of the fibre draw ratio that was achieved during subsequent cooling and solidification of the fibre downstream of the die. Scanning electron micrographs of fracture surfaces of fibres formed from a 1 mm diameter die are shown in plate 12 for polymer A5 processed at T = 250 "C. Plate 12(a) corresponds to the free-fall fibre where no draw down occurs downstream of the die. The micrograph clearly shows what appears to be a two-phase structure.Near the outer surface the skin is highly fibrous with the fibres oriented parallel toN. J. ALDERMAN AND M. R. MACKLEY 157 Fig. 3. Schematic diagram of fibre extrusion system: ( a ) experimental arrangement, and ( b ) velocity-profile rearrangement. the fibre axis. X-Ray measurements1' also show that this material is highly oriented. In the central region the structure of the core material is less well ~ r i e n t e d ' ~ with an observed texture resembling that of a 'knotty' piece of wood. Plate 12(b) is an electron micrograph of a sample drawn to a draw ratio of 3 where the draw ratio is defined as the ratio of the original diameter of the fibre at the die exit to the final diameter of the solid fibre. In this fibre the material appears to have a uniform and highly oriented texture across the ~amp1e.l~ The mechanical properties reflect the ratio of skin core present in the fibre. Fig.4 shows the Young's molulus of polymer fibres A5 and B produced from a 0.4 mm die with different final draw ratios. As the draw ratio increases the amount of oriented skin increases and the amount of 'disoriented' core decreases. This leads to an increasing Young's modulus as a function of draw ratio until a limiting plateau modulus is reached for both polymers. An explanation for the variation of modulus and the presence of a sharp skin-core boundary low draw ratios can be envisaged in terms of an understanding of the velocity profile rearrangement at the exit of the die. Pressure-drop measurements of polymers A5 and B suggested that they both behaved as power-law fluids where the shear stress T is given by where K and n are constants for the fluid and v is the velocity at a radial position r within the die.We found that the power-law indices for polymers A5 and B were 0.50 and 0.72, respectively. If we assume the fluid behaves as a power-law fluid and the flow is viscometric at the exit, the velocity profile at the exit is given by where S is the pressure gradient in the die and ro the radius of the die.158 OPTICAL STUDIES OF THERMOTROPIC LIQUID CRYSTALS 30 2 5 "E -r 20 z 4: m 1 ln 3 'cf Y 15 z ln bD 10 * 5 0 -1 rA---.- - -- - $ 1 I I' i I t . i 1 2 3 4 5 draw ratio Fig. 4. Plot of Young's modulus as a function of draw ratio for polymers A5 (0) and B (A) extruded at 256 "C and 295 "C, respectively from a 0.4 mm die at a volumetric flow of 4.6 x m3 s-'.For the above velocity profile the mean velocity is is given by and the centre line maximum velocity v, is given by In the case of the free-fall fibre, the final velocity of the fibre at the take-up will be 5; this means that in outer regions of the fibres, where v, <6, fluid elements will be accelerated longitudinally and in the core region, where v, > is, these fluid elements are decelerated in an axial compression velocity fluid. The boundary between accelerated and decelerated fluid elements occurs at a radius r, where v,= 6 ; this yields On the basis that the skin comes from oriented fluid elements -that have been stretched and the core from fluid elements that have been axially compressed, weN.J. ALDERMAN AND M. R. MACKLEY 159 might expect from the above equation to have skin/core area ratios of 0.84 and 0.93 for free-fall polymers of types A5 and B where n = 0.5 and 0.72, respectively. When the fibre is drawn down after extrusion from the die, the final velocity v2 of the fibre increases and the fibre radius ratio is given from the mass balance: v2.nr: = fi.rrri where r2 is the final diameter of the fibre. Eventually, when v2 >urn, the maximum velocity at the die exit, all fluid elements leaving the die should be being accelerated with none decelerated. On this simple basis one might expect all the fibre to correspond to oriented skin when v2= urn, yielding a critical draw ratio R, where 3n + 1 This gives values of R, = 1.29 for polymer A5 and R, = 1.35 for polymer B.In fig. 4 we note that a draw ratio of ca. 2 is required in order to rech plateau modulus values where presumably all the fibre is fully oriented. The above predictions are therefore low; however, the trend is in the correct direction and this gives confidence that the sharp skin-core transition and its transfer to all-skin at high draw ratios is indeed due to the materials extreme shear sensitivity. We have shown in the previous section that thermotropic liquid-crystal polymers orient readily in shearing flows. The results described in this section suggest that these liquid-crystal polymers also readily orient in longitudinal accelerating flows but of possible equal importance they do not appear to be stable in decelerating axial flows, where a form of random anisotropy appears to develop.The stability of director orientation in small-molecule nematic liquid crystals subject to convergent flows, and their apparent instability in divergent flows was eluded to by Leslie in a recent address.8 This could be an important starting point for the further mathe- matical simulation of the flow behaviour for the more complex thermotropic liquid- crystal polymer. The additional role of disclinations, their multiplication and pos- sible relaxation is an additional area that demands future attention if the properties of these interesting materials are to be fully understood and exploited. We acknowledge the pioneering contribution of Diane Graziano to this work. Stimulating discussions with Sir Charles Frank, Kurt Wissbrun, Aubrey Jenkins, David Walton, Amar Al-Dujaili and Geoff Mitchell have aided our thinking.The skin-core work was largely carried out with the help of Chemical Engineering project students Fiona Macleod, Richard Fenner, Debbie Collins and Paul Chambers, and we are grateful for all their contributions. We also thank the various suppliers of material. Finally our thanks go to Alan Butcher, who expertly constructed the shearing apparatus. D. Demus and L. Richter, Textures of Liquid Crystals (VEB Deutscher Verlag fur Grundstoff Industrie, 2nd edn, 1978). S. Chrandraseker, Liquid Crystals (Cambridge University Press, Cambridge, 1980). P. G. de Gennes, The Physics of Liquid Crystals (Oxford University Press, Oxford, 1974). M. KICman, Points, Lines and Walls (Wiley Interscience, New York, 1983).F. C. Frank, Discuss. Furaday SOC., 1958, 25, 19. ti R. B. Meyer, Philos. Mag., 1973, 27, 405. ’ J. Wahl and F. Fischer, Mol. Cryst. Liq. Cryst., 1973, 22, 359. * F. M. Leslie, Philos. Trans. R. Soc. London, Ser. A 309, 1983, 155. lo M. R. Mackley, F. Pinaud and G. Siekmann, Polymer, 1981, 22, 437. D. J. Graziano and M. R. Mackley, Mol. Cryst. Liq. Cryst., 1984, 106( 1/2), 103.160 OPTICAL STUDIES OF THERMOTROPIC LIQUID CRYSTALS M. Kleman, L. Liebert and L. Strzelecki, Polymer, 1983, 24, 295. l 2 C. Viney and A. H. Windle, J. Muter. Sci., 1982, 17, 2661. l 3 D. J. Graziano and M. R. Mackley, Mol. Ctyst. Liq. Crysf., 1984, 106(1/2), 73. l4 C. Viney, G. R. Mitchell and A. H. Windle, Mol. Ctysf. Liq.Crysf., in press. I 5 G. Marrucci, paper presented at Int. Congr. Rheology (Acapulco, 1984). l 6 K. F. Wissbrun, Faraday Discuss. Chem. SOC., 1985, in press. G. R. Mitchell, personal communication.Plate 1. Photograph of shearing apparatus. [facing page 160Plate. 2. Polymer Al: t = 230 "C, horizontal field of view 350 pm. Sample thickneqs 6 given for each photograph, crossed polar orientation shown in each photograph by orientation of cross wires. ( a ) Crossed polars, S = 2.1 pm; ( b ) polarizer only, S = 2.1 pm; (c) crossed polars, S = 7.9 pm; ( d ) polarizer only, S = 7.9 pm; (e) crossed polars, 6 = 15 pm; (f) polarizer only, S = 15 pm. Plate 3. Polymer A5: horizontal field of view 350 pm. ( a ) Crossed polars, S = 1.2 pm, T = 280 "C; ( b ) crossed polars, S = 6.0 pm, T = 280 "C ; (c) crossed polars, S = 6.0 pm, T = 240 "C.Plate 4. Polymer X7G: horizontal field of view 350 pm. ( a ) Polars crossed, 6 = 4.0 pm, T = 350 "C; ( b ) polars crossed, S = 4.0 pm, T = 280 "C; ( c ) polars crossed 6 = 4.0 pm, T = 220 "C. Plate 5. Polymer B: horizontal field of view 350 pm. ( a ) Polars crossed, S = 5 pm, T = 300 "C ; ( b ) polars crossed, S = 5 pm, T = 280 "C. Plate 6. Polymer TI: horizontal field of view 350 pm. ( a ) Polars crossed, S = 10 pm, T = 140 "C; ( b ) polarizer only, S = 10 pm, T = 140 "C. Plate 7. Polymer T8: horizontal field of view 350 pm. (a) Polars crossed, T = 180 "C; ( b ) polars crossed, T = 180 "C; [rotated w.r.t. ( a ) ] ; ( c ) polars crossed, T = 200 "C; ( d ) polars crossed, T = 210 "C; (e) polars crossed, T = 225 "C; (f) polar crossed, T = 225 "C, shear in 45" direction. Plate 8. Polymer Al: horizontal field of view 350 pm, T = 230 "C. Horizontal shear, w = 1.27 rad s-', x, = 0.33 mm, S = 1.5 pm. ( a ) Polars crossed, 0 and 90", no shear; ( b ) polars crossed, 45" shear at maximum i, w = 1.27 rad s-'; ( c ) polars crossed, 0 and 90", shear at maximum i, w = 1.27 rad s-' ; ( d ) polars crossed, 45", shear at maximum E, w = 10 rad s-'. Plate 9. Polymer Al: horizontal field of view 350 pm, T = 235 "C, 6 = 5 pm. ( a ) Polarizer only during shear, w = 1.27 rad s-', xo = 0.33 mm; ( b ) polarizer only, 10 s after cessation of flow; ( c ) polarizer only, 20 s after cessation of flow; ( d ) polarizer only, 30 s after cessation of flow; ( e ) polarizer only, 60 s after cessation of flow. Plate 10. Polymer A5: horizontal field of view 350 pm, T = 240 "C, S = 3.7 pm. Horizontal shear w = 1.27 rad s-', xo = 0.33 mm. ( a ) Crossed polars, no shear; ( b ) crossed polars, 45" shear at maximum i ; ( c ) crossed polars, 0 and 90" shear at maximum i ; ( d ) crossed polars, 45" shear at maximum amplitude. Plate 11. Polymer A5: horizontal field of view 350 pm, T = 220 "C, S = 15 pm. Striated texture observed after cessation of flow.Plate 2 -*Plate 3. Plate I;.Plate 5. Plate 8.Plate 6. Platelo.Plate 7.Plate 9. Platell.Plate 12. Scanning electron micrograph of fracture surface for polymer A5, extruded from a 1 mm die: ( a ) free-fall fibre of 1 mm diameter and ( b ) drawn-down fibre of 0.3 mm diameter.

ISSN:0301-7249    

DOI:10.1039/DC9857900149

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1985,79, 161-173 A Model for Domain Flow of Liquid-crystal Polymers BY KURT F. WISSBRUN Celanese Research Company, Summit, New Jersey 07901, U.S.A. Received 14th November, 1984 Liquid-crystal polymers often exhibit a low-shear-rate region of shear thinning of viscosity, accompanied by lack of net orientation and by a dependence of texture and rheology on shear history. A phenomenological model to account for this region is presented. The model uses Marrucci's description of stable domains whose size is a function of stress. The interaction of these domains is estimated from the continuum theory of liquid crystals. The shear-rate- dependent viscosity and normal stress of the polydomain fluid are calculated from the interaction energy and size of the domain by analogy with non-equilibrium molecular- dynamics calculations for small-molecule fluids.The shear-rate dependences are similar to those observed experimentally. Other tests of the model, including optical observations and measurement of dynamic viscosity, are proposed. One of the remarkable phenomena exhibited by liquid-crystal polymers is that in many, if not all, instances there is a region of flow behaviour at low shear rates in which they are shear thinning. Also, in this region of flow liquid-crystal polymers orient poorly or not at all in a simple shear flow. They exhibit transmission of light between crossed polars, but there is no strong net birefringence of extinction direction.'-1° The viscosity of a liquid-crystal polymer in this state is also affected by its shear history prior to testing.2 Flow appears to occur in this so-called region I' by the relative motion of 'domains' which retain their identity in flow.That is not to say that the domain texture is unaffected by the shear flow. On the contrary, the domain texture, especially the size scale, is strongly altered by the application of shear.*.9 However, the viscosity in this region of flow reaches a steady state (in most cases) after an initial transient period, and, presumably, also the domain texture. After cessation of shear the domain texture reverts to its quiescent state on a timescale not related to the stress-relaxation time.599 Of the three causes proposed' ' to account for this region I flow, the most plausible is that of the 'polydomain' flow originally proposed by Onogi and Asada' on the basis of their rheo-optical observations.Another mechanism considered was the existence of a yield stress due to the occurrence of a heterophase structure. Crystalliz- ation of sequences of crystallizable comonomers in copolyesters8-12 or the association of strongly polar molecules such as polyamides, especially in the presence of ~ a t e r , ' ~ ? ' ~ could lead to such structure. In some cases a yield stress due to phase separation does appear to be present, as evidenced by failure to relax to zero stress after cessation of flow, or grossly distorted stress response to a small-amplitude sinusoidal strain. However, in many cases these phenomena are not observed. In addition, the negative slope of the logarithmic plot of viscosity against shear rate is often less than the value expected for a true yield stress.Even the occurrence of a slope of unity is not proof of the existence of a yield stress. In one such case16 it was found that flow occurred at the lowest stress possible to impose, although the slope was unity over several decades of shear rate. 161162 DOMAIN FLOW OF LIQUID-CRYSTAL POLYMERS The proposal of Onogi and Asada’ that region I flow was caused by the polydomain texture that is almost always found with liquid-crystal polymers must therefore be considered seriously. The first attempt to model such a flow was recently presented by Marru~ci.’~ His model predicts a region I with a slope of -1/2 up to a critical shear rate, after which there is a transition to a constant viscosity, such as is observed in the so-called region I1 flow.’ Marrucci’s model has the defect that it predicts the progressive increase of orientation as the viscosity decreases in region I.The reason for this prediction is that he assumes that the number of domains remains constant with increasing shear rate and that only the size of the domains is affected by the shear stress. This assumption is also contrary to the experimental which shows that the size scale of the domain texture is reduced in shear flow but that there is no net macroscopic orientation and that the sheared material is entirely occupied by polydomain texture. In this paper Marrucci’s ingenious model is adopted as a starting point, but the development is modified in a way to be consistent with the experimental evidence cited above.Another advantage of this approach is that it permits the slope to vary from -1/3 to -1, a range that embraces the experimental observations, rather than restricting it to Marrucci’s result of -1/2. Also, the present model predicts the behaviour of the first normal stress difference, N1, in a manner consistent with experiment. The price paid for these advantages is the introduction of a different assumption for the flow mechanism and the introduction of an empirical adjustable parameter. The calculation of the viscosity, the normal stress and their shear-rate dependen- ces rests upon an analogy with the results of calculations of these quantities by the methods of non-equilibrium molecular dynamics (NEMD) for dense fluids com- posed of spherical particles.18 ASSUMPTIONS OF THE MODEL We adopt Marrucci’s suggestion that a ‘domain’ is a configuration of the director field of the liquid crystal comprised of sets of disclinations such that there is no net orientation averaged over a domain, and of such a shape that an assembly of domains is space filling.An example of such a configuration, adapted from Marruci, is shown schematically in fig. 1. The representation shown may be considered to be a distorted version of the domain structures proposed previo~sly.’*~ Further, it is assumed that the domain is stable in the sense that although it is not a global minimum-energy configuration, it is locally in an energy minimum. Any path to a lower-energy configuration requires passage over a high-energy barrier. It is also assumed, with Marrucci, that upon application of a shear stress the domain texture will equilibrate with the stress in accordance with the continuum model of liquid However, unlike Marrucci’s model it is not assumed that the number of domains remains constant.Rather, as the domains decrease in size in response to the applied stress, their number increases in order to keep the total domain volume equal to that of the sample. The mechanism of the domain break-up and multiplication is not addressed here ; their occurrence is postulated on the basis of the experimental observations. There is a limit beyond which the model must break down. This occurs when the domain size approaches the size of the rigid-rod molecules of the liquid-crystal polymer.Presumably, when this happens there will be a transition to a new region of flow. The consequences of this transition will be explored in subsequent work.K. F. WISSBRUN 163 Fig. 1. Schematic diagram of single domain, showing no net orientation. The principal new assumption of this model is that the form of the equations describing the rheology of the polydomain texture is identical to those obtained by the NEMD computations for fluids of molecular particles. The validity of this assumption is certainly open to question. The molecules treated by NEMD are rigid, in the sense that their shape does not change in flow. The polydomains, on the other hand, are deformable assemblies of molecules. Furthermore, the domains are assumed to be cube-shaped, so as to be space-filling, rather than spherical.In the NEMD calculations the intermolecular potentials are well established, spheri- cally symmetric functions such as the Lennard-Jones and related potentials. In this model the potential energy associated with the relative motion of domains is derived by arguments from the liquid-crystal continuum theory. This last point may not be too serious, as the NEMD results are not sensitive in form to the details of the assumed potential function.20 DEVELOPMENT OF THE MODEL DOMAIN SIZE Following Marrucci12 we assume that, in the quiescent state, the volume of material is filled with domains of radius (or, considering the domains as cubes, half the length of a side) Ao. According to the continuum model, the average energy of the curvature distortion of the director field per unit volume is then (1) K E --8AiNo=8KNoAo "A; where No is the number of domains per unit volume of material and K is an average curvature distortion modulus of the material.(The average is used for the sake of simplicity, although it is recognized that for a liquid-crystal polymer the three moduli may have very different values.") When the material undergoes a shear flow at a shear rate 7, the shear stress causes break-up of the domains, and in the steady state the radius becomes A. The inceased distortion energy per unit volume is AE=8K(NA-NoAo) (2) 8NoAi=8NA3= 1. (3) where N is the number of domains per unit volume of radius A. By conservation of volume164 DOMAIN FLOW OF LIQUID-CRYSTAL POLYMERS Substituting eqn (3) into (2) Eqn (4) is identical to Marrucci's result, although he made the assumption that the number of domains per unit volume remained constant, rather than the assumption of eqn (4). Again following Marrucci, the increased distortion energy per unit volume A E is equated to the shear stress r, which in turn is equal to the product of viscosity q and shear rate y : AE = r= ~ y .( 5 ) VISCOSITY From the NEMD computations18 the viscosity of a fluid is given approximately by an equation of the form where rl0 is the viscosity at zero shear rate and A is the factor that makes the shear rate non-dimensional, i. e. a characteristic time.22-24 The constant C depends upon the fluid density and temperature.20 It is ca. 0.3 for argon at its triple point.I8 The approximately linear dependence upon the square root of the dimensionless shear rate Ay has been found to be valid for values of Ay as large as nine.At higher values the dependence levels off, as it must do to avoid prediction of a negative viscosity. qo depends upon the mass M of the fluid particles and upon the parameters e and a of the function relating the potential energy of the particles of the fluid to their separation, as follows: qo = q*[( ~ e ) O . ' ] / a ~ . (7) T* is a dimensionless function of reduced temperature and pressure, increasing with increasing density.20922 Eqn (7) has been derived from a corresponding-states principle by statistical mechanics.23i24 In this paper q* is taken as constant. This is equivalent to assuming that the temperature dependence of viscosity is negligible compared with the dependence upon shear rate.In order to adapt eqn ( 6 ) and (7) to the problem of the polydomain flow, it is necessary then to estimate the parameters analogous to M, e, A and a of those equations. We do this by the following argument, illustrated schematically in fig. 2. Consider two adjacent domains at rest. Their energy is at a minimum when the directors in the planes of contact are parallel, as shown in fig. 2. (Note that the domain directors are shown schematically and are not intended to represent the detailed configuration within a domain, a separate question not considered here.) Under a shearing deformation, as one domain slides over the other, non-parallel directors are brought into contact.From continuum theorylg there is a stress associated with the deviation from the parallel-contact configuration. The stress will vary linearly with displacement (for the schematic domains of fig. 2) and becomes a maximum when the displacement is equal to half the length of the sides of the domains. (It is assumed that the displacement occurs in a time shorter than that required for the domains to reorient their configurations.)K. F. WISSBRUN 165 1 L f Fig. 2. Schematic diagram for computation of interaction energy and modulus of domains in relative shearing motion. According to the continuum model the stress is inversely proportional to the square of the radius of curvature of the distortion. For a fluid composed of rigid rods of length L, the smallest possible radius of curvature must be on the order of L, and we therefore write r = K I L ~ .(8) (For semi-rigid liquid-crystal polymers L may be more properly identified with a persistence length.) The energy associated with this stress is r multiplied by the contact area A2 to give the force and by the distance A over which the force acts: E = K A ~ / L ~ . (9) Therefore the motion of one domain over another is associated with a potential whose maximum depth is given by eqn (9) and occurs at a separation A. This argument then establishes the association of E and A of eqn (9) with the potential function parameters of eqn (7). The mass M is the density D of the liquid-crystal polymer times the volume of the domain, 8A3. This argument also supplies an estimate of the relaxation time A.The stress corresponding to the displacement A is, from eqn (8), K / L2. The corresponding strain is on the order of A / L. The compliance J is then the strain divided by the stress J = ( A / L)*( L2/ K ) = AL/ K (10) A = qAL/K. (1 1) and the relaxation time A is the product of compliance and of the viscosity q:25 [Note that the corresponding-states provides a dimensionless time in terms of the mass, radius and depth of the potential well, and that this might be used instead of the value from eqn (11). Also, in eqn (11) one could consider166 DOMAIN FLOW OF LIQUID-CRYSTAL POLYMERS various estimates of the viscosity. Rather than q, one might use qo, the viscosity of the system at zero shear rate, or the viscosity of the hypothetical suspending medium, i.e.the monodomain nematic fluid. The consequences of these different assumptions will be explored in future work.] This then completes the identification of the parameters required to use eqn (6) and (7) for the present model. Making the necessary substitutions, we arrive at the following set of equations to describe the shear-rate dependence of the viscosity: One other result from the NEMD computations is that the normal stress difference Nl is very nearly proportional to the 3/2 power of N , == ( (14) The set of simultaneous equations [eqn (6) and (1 1)-( 13)] was solved numerically by an iterative method (IMSL routine ZSCNT, based on a procedure described by W01fe~~). The results are discussed below. Some limiting values of these equations derived analytically are also discussed.RESULTS The model parameters that must be prescribed in order to solve the equations are the density D, the minimum or molecular length L, the average elastic constant K , the quiescent domain radius Ao, the multiplier q* and the shear-thinning constant C. The density was chosen as 1.25 g ~ m - ~ , a reasonable value for a molten aromatic polyester. The length L was taken as 1 x cm, corresponding to a degree of polymerization of an aromatic polyester of ca. 150, again a reasonable estimate for a condensation polymer. The radius of a quiescent domain A. was taken as 1 x and 1 x cm, which is about the size scale of quiescent domains visible in an optical microscope. The choice of K was more difficult.Small-molecule liquid crystals usually have values of K of cu. 10-7-10-8 dyn. Bend and twist moduli of polymeric liquid crystals have been measured and are of comparable magnitude,28 but measurements of the splay modulus have not yet been published. However, theoretical consider- ations indicate2' that the splay modulus may be on the order of 1 OP7 L/ d dyn, where d is the rod diameter. With the above length of L and a diameter of ca. 10 A, one arrives at a value of K of 1 x dyn. Calculations were made with this value and with one of 1 x dyn, which is closer to the typical value for small-molecule liquid crystals. Very little was known to guide the choice of C, so it was treated as an adjustable parameter and the calculations carried out for different values of C.That leaves q* to be chosen. In the limit of zero for the constant C, the set of four equationsK. F. WISSBRUN 167 103 lo2 A 0 c.) Cn ._ 2 .- 10' loo 10 3 1 0.3 1 I I I I -6 -4 -2 0 2 4 log (shear rate) Fig. 3. Logarithmic plot of viscosity against shear rate, showing effect of varying parameter C. Assumed values of other parameters are A. = dyne and cm, L = lop5 cm, K = q* = lo3. can be solved analytically (when A<< A,) to give q* was chosen then as 1000 which, from eqn (15), leads to a reasonable value of 200 P at y = 1 s-'. This value of q* is appreciably greater than the values of the order of unity found in the NEMD calculations; possible explanations for this are discussed below. The effects of varying C on the shear-rate dependence of viscosity are shown graphically in fig.3 and 4. For all values of C, the limiting viscosity as the shear rate goes to zero is 1000 P, the value of q*. With increasing shear rate the viscosity decreases. The higher the value of C, the earlier the onset of appreciable shear thinning. As the shear rate increases, the viscosity approaches power-law behaviour over an appreciable number of decades of shear rate. The slope in this region is a sensitive function of C. At C = 0.3 the slope is close to - 1/3 ; for C = 1-3 the slope is a little less or more than -1/2 and at C = 10 it is very close to - 1. The model is capable then, by adjustment of one parameter, of reproducing the range of slopes found experimentally for liquid-crystal polymers. IJ' Further, as discussed below, it may be possible to do experiments to make a measurement of C independent of the rheological data, thereby providing a test for the model.which according to the NEMD results is proportional to N1, is plotted logarithmically against shear rate in fig. 4. As with the viscosity, the results for different values of C converge in the limit of low shear rates. However, in the region of shear rates in which the viscosity approaches power-law behaviour, Nl is very sensitive to C. When C = 0.3 or 1 the slope of the logarithmic plot in this region is ca. 1/2; it is considerably lower at C = 3 and close to zero at C = 10. The very small slope of plots of N1 against y has been observed e~perimentally,'~*~~ The quantity168 DOMAIN FLOW OF LIQUID-CRYSTAL POLYMERS -7 -5 -4 -3 -2 -1 0 2 3 4 log (shear rate) Fig.4. Logarithmic plot of the quantity (Ay)”l’, proportional to the first normal stress difference N , , against shear rate. Parameters as in fig. 3. so this result also is in at least qualitative agreement with experiment. (Note that, for the range of data of fig. 3 and 4, the product hy, the dimensionless shear rate, did not exceed a value of unity. It is therefore well within the limits of linearity of the NEMD computation.’8) The asymptotic levelling-off of N , at high shear rates is closely connected to the observation from the calculations that the ratio AIL approaches a limit at high shear rates. This may be shown analytically as follows. Consider the behaviour of eqn (6) in the limit of high shear rates, as q/qo approaches zero.In this limit and also k A [ 1 -(;)1+ c2. Relation (16) can be verified directly from fig. 4. Relation (17) is a quadratic in A and can be solved analytically. When C is not too large, A/Ao is small compared with 1, and AIL approaches C-2 as a limiting value. Relation (17) provides an independent test of the model alluded to above. The rheological data, specifically the slope of the power-law region of the viscosity, can be used to estimate C. Optical observation can then determine for a given material whether, in fact, the domain size approaches a limiting value consistent with the rheological estimate of C. For a lyotropic liquid-crystal polymer that is not so turbid as to cause excessive problems of multiple scattering, it may be possible to do this directly during flow by light- scattering measurements.Thermotropic liquid-crystal polymers are probably too turbid, but for these it may be possible to quench the domain texture of the sheared polymer by rapid cooling and then make microscopic observations on thin sections. Qualitatively, of course, the domain size reduction by shear has already been observed,’ and in fact represents one of the corner stones of the present model.K. F. WISSBRUN 169 10: h U .* u, .- 0 ? lo’ 2 e L: T N 1 0’ log (shear rate) Fig. 5. Logarithmic plot of ‘zero-shear’ viscosity against shear rate. Parameters as in fig. 3. However, these observations were made on very thin films whose textures may have been unduly influenced by surface forces. From eqn (12) another consequence of an approach of L/A to a limiting value is that the ‘zero-shear-rate’ viscosity, q0, will also become a constant, whose magni- tude depends upon C, at high shear rates.Possible experimental methods for measuring q0 are discussed below. Fig. 5 shows the behaviour of qo as a function of shear rate and of C. The prediction of a limiting value of A is important for another reason. Clearly this model must become invalid as the value of A approaches L or some small multiple of L. Presumably the flow mechanism must then change, perhaps to that proposed by Onogi and Asada,’ in which there is a gradual transformation of the polydomain texture to a monodomain, with increasing degree of net orientation while maintaining a nearly constant viscosity (region I1 flow in their nomenclature.) If C has such a high value that the condition A+ L cannot be met, the occurrence of region I1 flow and of shear-flow orientation would be inhibited.This possibility, if true, obviously has significant consequences for the processing of a liquid-crystal polymer. Consideration of the limiting values of y and q as A+ L also suggests a speculative explanation of the order-of-magnitude larger value of r ) * compared with that found with simple molecular fluids. Although the argument may be done generally from the numerical solutions of the equations, it is most easily carried out analytically for the case when C = 0. In that case q = qo and eqn ( 1 2 ) becomes where qc is the viscosity when A = L. For the parameters used in the computations above, r)* is then equal to 100qc, where qC is the viscosity of the polydomain fluid near the transition to region I1 flow. However, since the transformation to the170 DOMAIN FLOW OF LIQUID-CRYSTAL POLYMERS Fig.71*- log (shear rate) 6. Logarithmic plot of viscosity against shear rate, showing effect of parameters K and ( a ) K = lo-’, q* = lo3; ( b ) K = cm, q* = lo4. Other parameters are-A,= L = lo-’ cm and C = 0.3. monodomain flow (region 11) occurs at nearly constant viscosity, it is also close to the viscosity of the nematic monodomain fluid. The monodomain flow may be considered as occurring by the slippage of one rod past its neighbouring ones. Now, if one thinks of the rod as consisting of P beads (monomer segments) the translation of one rod past its neighbour by the length of one bead might be expected to require P times the energy of one such isolated bead undergoing a similar translation.By this crude reasoning a value of q* that is of the order of P times the simple molecular-fluid case does not seem unreasonable. More rigorously one might wish to establish the connection with the monodomain rheology via Doi’s molecular theory.30 The use of this theory could impose more stringent conditions on the permissible parameters of the present model, e.g. of their molecular-weight dependence. Another requirement to establish a connection between monodomain flow and the region I model is a theory for the region I1 flow connecting the two. A modification of the present model to perhaps accomplish this is being considered.Finally, the effects of two other parameters of the model, A, and K, are shown graphically in fig. 6 and 7. In fig. 6 K was varied by a factor of 100; q* was adjusted at the same time in order to maintain the same limiting viscosity at low shear rates for purposes of comparison. Variation of K has essentially only the effect of shifting the viscosity with respect to the shear-rate curve. This result could have been anticipated from eqn (13), which determines the shear rate at which A. becomes sufficiently large compared with A for its effect on the rheology to become insig- nificant. Similarly, fig. 7 shows that the assumed value of A, has no interesting effect on the rheology in the power-law region of interest. An assumed larger value of A, merely increases the length of the power-law region.K.F. WISSBRUN "-6 171 -: -; b h A 1c 10 h 0 c) v1 .- 3 .- lo DISCUSSION The model presented here is obviously not developed in a rigorous scientific fashion. Some of the assumptions made, e.g. the existence of stable domains whose size is affected by shear, are founded in observation. Others, such as the assumed form of the interaction potential between domains, are derived from well founded theory, but the applicability of that theory to liquid-crystal polymers, although plausible, has not yet been proved unequivocally. Still other assumptions of approxi- mations are made for want of better approaches. The use of a pair-potential function, neglecting three-body interactions, and some of the other assumptions described in the development of the model fall into this category.The application of a theory developed for molecular fluids to a system of microscopically observable domains may raise some concern. It is some comfort to note that the applicability of NEMD to a system of colloidal particles of comparable size scale has in fact already been suggested. 18922,26 With these reservations in mind, the possible usefulness of the model is judged on its ability to predict phenomena or observations that were not explicitly built into the model. The ability to predict a range of slopes of the shear-rate dependence of viscosity consistent with the range observed experimentally is certainly encourag- ing. Similarly, the qualitative prediction of the shear-rate dependence of the normal stress may be evidence for the possible utility of the model.Since the model predicts a correlation of viscosity with domain size, and the domain size is known to be affected by shear, the model also then predicts qualitatively the sort of thixotropy or shear-history dependence of viscosity that has been observed.2172 DOMAIN FLOW OF LIQUID-CRYSTAL POLYMERS As has been discussed above, direct observation of the domain texture during flow and relating its changes to the rheology would provide an experimental test of the model. Another approach might be to conduct in a controlled fashion measurements of the thixotropy of liquid-crystal polymer melts and solutions, to try to quantify the sorts of observations reported by Cogswell.’ It would be desirable to carry out such experiments in a rotational instrument in order to study the shear-history dependence without any complications caused by thermal history.Because of the difficulty of obtaining steady-shear data over a wide shear-rate range it would be helpful if one could use small-amplitude dynamic data as a measure of the rheology. Observations of thixotropy of dynamic viscosity by application of either high-frequency oscillatory shear or of steady-shear flow have already been reported.’273’ It is possible that a theoretically sound connection between dynamic-viscosity and steady-shear measurements can be found. The NEMD calculations26 suggest that the dynamic viscosity in the limit of zero frequency approaches the steady-shear viscosity in the limit of low shear rates. If such a relation is generally valid, and specifically is applicable to liquid-crystal polymers, the low-frequency dynamic viscosity would be a direct measure of the ‘zero-shear-rate’ viscosity, qo, of the model.The quotation marks are used as a reminder that the shear rate serves two functions in this model. First, it is the cause of the shear stress that determines the steady-state domain size. The domain size in turn determines the ‘zero-shear vis- cosity’, i.e. the viscosity at zero shear rate that the system would have if that domain size were stable in the absence of shear. The actual measured viscosity then depends upon the qo and upon how much shear thinning occurs, which also depends upon the domain size. The calculations shown in fig. 3 and 5 suggest that there should be a correlation, predictable by the model, of the slope of the viscosity against shear-rate curve with the ratio of rlo to 7 at any given shear rate.If the low-frequency dynamic viscosity is, in fact, a good measure of q0, then its measurement after application of steady shear provides another possible test of the model. A theoretically sound calculation of the dynamic rheology in terms of the parameters of the model is required for this purpose. A welcome by-product of such theory would perhaps be an explanation of the observation that in many liquid-crystal polymer systems the first normal stress difference is appreciably greater than twice the dynamic storage modulus.12731 I am most grateful to Prof. Marrucci, not only for a preprint of his paper but also for a most enlightening explanation of its basis in continuum theory.Discussions with F. N. Cogswell of various aspects of liquid-crystal polymer rheology have also been most helpful. The rheo-optical observations of Diane Graziano and Malcom Mackley were most important for formulating the model. Many of my colleagues at Celanese have made contributions to my studies on liquid-crystal polymers; in the present context, Yoshiaki Ide, Linda Sawyer and John Flint were especially helpful. Finally, I am grateful to Celanese Corporation for the opportunity to work in this area and to publish this work. ’ ( a ) S . Onogi and T. Asada, Roc. 8th. Int. Congr. Rheology, ed. G. Astarita, G. Marrucci, and L. Nicolais, (Plenum, New York, 1980), vol. 1, pp.127-147; ( b ) T. Asada, in Polymer Liquid Crystals, ed. A. Ciferri, W. R. Krigbaum, and R. B. Meyer (Academic Press, New York, 1982), chap. 9. V. G. Kulichikhin, V. A. Platonov, L. P. Braverman, T. A. Belousova, V. G. Polyakav, M. V. Shablygin, A. V. Volokhina, A. Y . Malkin and S. P. Papkov, Vysokomol. Soyed. Ser. A, 1976, 18, 2656 (English translation by E. 0. Phillips in Polym. Sci. USSR, 1976, 18, 3031). * F. N. Cogswell, Br. Polym. J., 1980, 12, 170.K. F. WISSBRUN 173 M. Horio, Annu. Rep. Res. Inst. Chem. Fibers, Jpn, 1978, 35, 87. Y. Onogi, J. L. White and J. F. Fellers, J. Non-Newtonian Fluid Mech., 1980, 7 , 121. J. L. den Otter and J. L. S. Wales, unpublished report CL801104 to Celanese Research Co., October 23, 1980. A. E. Zachariades and J. A. Logan, Polym. Eng. Sci., 1983, 23, 797. D. J. Graziano and M. R. Mackley, Mol. Cryst. Liq. Cryst., in press. Plast. Eng., New Orleans, 1984. K. F. Wissbrun, J. Rheot., 1981,25, 619. ' Y. Ide and 2. Ophir, Polym. Eng. Sci., 1983, 23, 261. lo D. G. Baird, E. Joseph, R. Pisipati, G. Viola and G. L. Wilkes, ptepr. Annu. Tech. Conf;, SOC. l 2 K. F. Wissbrun, Br. Polym. J., 1980, 12, 163. l 3 C.-P. Wong, H. Ohnuma, and G. C. Berry, J. Polym. Sci., Polym. Symp., 1978, 65, 173. l4 D. G. Baird and R. L. Ballman, J. Rheol., 1979, 23, 505. l6 G. C. Berry, personal communication. l7 G. Manucci, paper.presented at Int. Congr. Rheology, Acapulco, 1984. " For a recent review see D. J. Evans, in Nonlinear Fluid Behauior, ed. H. J. M. Hanley (North- l 9 P. G. deGennes, The Physics ofLiquid Crystals (Clarendon, Oxford, 1975). 2o D. J. Evans and R. 0. Watts, Chem. Phys., 1980,48, 321. 21 R. B. Meyer, in Polymer Liquid Crystals, ed. A. Ciferri, W. R. Krigbaum and R. B. Meyer 22 H. J. M. Hanley and D. J. Evans, J. Chem. Phys., 1982, 76, 3225. 23 M. J. Tham and K. E. Gubbins, Ind Eng. Chem, Fundam., 1969, 8, 791. 24 E. Helfand and S . A. Rice, J. Chem. Phys., 1960, 32, 1642. 2s J. Frenkel, Kinetic Theory of Liquids (Dover, New York, 1955), p. 198. 26 D. J. Evans, Phys. Reo. A, 1981, 23, 1988. 27 P. Wolfe, Commun. ACM, 1959, 2, 12. 28 D. B. DuPre, in Polymer Liquid Crystals, ed. A. Ciferri, W. R. Krigbaum and R. B. Meyer, 29 K. F. Wissbrun, paper presented at AIChE Meeting, Houston, March 27-30, 1983. 30 M. Doi, J. Polym. Sci., Polym. Phys. Ed., 1981, 19, 229. 3' K. F. Wissbrun and A. C. Griffin, J. Polym. Sci., Polym. Phys. Ed., 1982, 20, 1835. H. Sugiyama, D. N. Lewis, J. L. White and J. F. Fellers, to be published. Holland, Amsterdam, 1983). (Academic Press, New York, 1982), chap. 6. (Academic Press, New York, 1982), chap. 7.

ISSN:0301-7249    

DOI:10.1039/DC9857900161

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

GENERAL DISCUSSION 7 Prof. M. K16man (Universite' Paris-Sud, Orsay, France) said: I would like to mention that other kinds of walls than those presented by Prof. Meyer have long been observed in the Frederiks transition in small-molecule liquid-crystal nematics.' They arise, not from a dynamical instability as is the case in the work presented today, but from the fact that the molecules can tilt towards two different but equivalent directions in the magnetic field: hence the existence of domains separated by walls. In fact, such domains, which have a typical elliptical shape, can also be observed (and their dynamical behaviour followed) in polymer nematics. In the K3 geometry the ratio of the ellipse axes provides an estimation of the ratio K , / K Z . We have obtained K 1 / K 2 =r 10 in a polyester synthesized by Strzelecki and Van Luyen.' This ratio seems to be in agreement with separate measurements of K, and K2.3 We have also observed the walls discussed by Prof.Meyer. * F. Brochard, J. Phys. (Paris), 1972, 33, 607; L. LCger, Solid State Commun., 1972, 11, 1499. ' L. Strzelecki and D. Van Luyen, Eur. Polym. J., 1980, 16, 299. Dr H. J. Coles (University of Manchester) said: I address Prof. Meyer. In your paper you mention the determination of ratios of elastic constants and viscosities using quasi-elastic depolarised light scattering for planar samples. If you used homeotropic alignment, would it not be possible to measure the splay and twist elastic constants independently on the application of electric fields in the light- scattering experiment? Could a similar technique using either electric or magnetic fields not be applied in your existing experimental arrangement, with the added advantage that this would allow several of the individual viscotic constants to be determined? Sun Zheng Min and M.KlCman, Mof. Cryst. Liq. Cryst., 1984, 111, 321. Prof. R. B. Meyer (Brandeis University, U.S.A.) replied: In principle, an external field can be used to suppress fluctuations and thereby determine absolute values of elastic constants, assuming one knows the coefficient for the coupling to the field. In the case of PBG nematics there are very serious electrohydrodynamic effects up to frequencies of over 100 kHz, above which the dielectric anisotropy is negative. Using parallel boundary conditions with the sample contained between electrically conducting glass slides, and splay/twist geometry (1) described, the field would suppress the twist fluctuations. The only technical difficulty in carrying out this experiment might be the higher optical reflectivity of the electrically conductive glass, which makes the clean observation of the scattered light signal more difficult.With a negative dielectric anisotropy, the use of a homeotropic sample between parallel-plate electrodes would result in a Frederiks transition, which would be inappropriate for these experiments. Prof. G. C. Berry ( Curnegie-Mellon University, U.S.A.) (communicated): Accord- ing to the Manguin, or adiabatic approximation, with the geometry used in your studies of the twist Frederiks transition, a light beam linearly polarized parallel to the director at the entrance side emerges from a slab with polarization parallel to the director at the exit side. Apparently this approximation does not apply with Prof.Meyer's system - could he explain why? t Plates 1-10 face p. 190. 175176 GENERAL DISCUSSION Prof. R. B. Meyer (Brandeis University, U.S.A.) replied: The twist structures which I discussed are visible because the sample is not very birefringent. The adiabatic limit for optical propagation depends on both the wavelength of the twist and the birefringence being large; although the wavelength is large in the case studied, the low birefringence more than makes up for this. Even for more birefrin- gent samples, by using highly convergent illumination or even oblique illumination one can still see twist structures.Dr L. S. Singer (Union Carbide, U.S.A.) said: I thought it might be of interest to mention our results of a similar determination of the elastic constant for bend ( K 3 ) for our flat-molecule liquid-crystalline pitches. From plate 3 in our paper later in this Discussion, it is apparent that the magnetic coherence length 6 in the mesophase spheres is ca. 7 pm. Using the formula by de Gennes, (( H) = (5) 1'2 H with H = lo4 G and xa determined by Delhaes et aL,' we determined K3 to be 3 x lop5 dyn (at 300 "C). The corresponding values for PAA (at 120 "C) and MBBA (at 22 "C) are as follows. PAA: xa = 0.12 x dyn; MBBA: xa = 0.12 X dyn. It is of interest that these K , values for rod-like molecule mesophases are more than an order of magnitude smaller than the value determined for our disc-like carbonaceous mesophase.emu, K3 = 1.7 x emu, K3 = 0.7 x ' P. Delhaes, J. C. Rouillon, G. Fug and L. S. Singer, Carbon, 1979, 17, 435. Prof. P. J. Flory (Stanford University, U.S.A.) said: Drastic distortion of bond angles in the ester groups of poly(p-phenylene terephthalate) by substituents on the phenylene members seems unlikely. Even in the case of a phenyl substituent attached to the terephthalate residue, steric repulsion may be largely relieved by rotation of the ester group about its axis and rotation of the phenyl substituent out of the plane of the phenylene to which it is attached, as occurs in biphenyl. Thermal destabiliz- ation of the polymer by substitution may take precedence over distortion of the structure of the ester group.If distortion of the chain backbone is deemed to be a significant consequence of substitution, this possibility should be explored through determination of persistence lengths from radii of gyration evaluated using appropri- ate measurements on dilute solutions. The nematic-isotropic transition depends significantly on the free volume."* The depression of the transition on this account becomes marked at elevated temperatures where the free volume is large.',2 Substitution should be expected to obstruct efficient packing of chains in both the nematic and the isotropic liquid states. The free volume may thereby be enhanced by substitution. This effect, rather than the postulated flexibilization of the chains, is the more likely cause of the lowering of the glass-transition temperature by substitution. The effects of substitu- tion on the free volume could readily be ascertained from determinations of thermal expansivities.Additionally to be noted is the action of substituents as 'diluents' in the lattice theory of the liquid-crystalline state.3 The increase in free volume caused by the presence of a substituent and the simultaneous dilution of the semi-rigid cores may plausibly account for the effects observed.GENERAL DISCUSSION 1-77 In offering the Kuhn model as a basis for treating semiflexible chains4 we were well aware of the effect of temperature on the Kuhn segment. Recognition of the necessity of taking account of this temperature coefficient is recurrent throughout studies of the statistics of chain configurations.In focussing immediate attention on the isotherms describing biphasic equilibria,4i5 we have certainly not been oblivious of the role of temperature, as Prof. Krigbaum implies. The strong tem- perature dependence of the transition in solutions of cellulose derivatives was explicitly attributed to the temperature dependence of the equivalent Kuhn segment in a recent re vie^.^ ' P. A. Irvine and P. J. Flory, J. Chem. Soc., Faraday Trans. 1, 1984, 80, 1807; 1820. * M. Ballauff, P. J. Flory and E. M. Barrall 11, Ber. Bunsenges. Phys. Chem., 1984, 88, 524, 530. R. R. Matheson Jr and P. J. Flory, Macromolecules, 1981, 14, 954. P. J. Flory, Macromolecules, 1978, 11, 1141. P.J. Flory, Adv. Polym. Sci., 1984, 59, 1 . Prof. W. R. Krigbaum (Duke University, U.S.A.) replied: We agree that ring substitution is unlikely to cause drastic bond-angle distortion in the ester group. We propose, instead, that the high chain extension of poly(p-phenylene terephtha- late) is due to adoption of the planar trans conformation by a preponderance of the ester groups. So long as the carbonyl group is nearly coplanar with the ring, the partial double bond character (shown below in the structure on the left) can be stabilized by resonance with the aromatic T electrons. If substituent Y forces rotation of the carbonyl group out of the plane of the ring (illustrated on the right), the resonance stabilization and the partial double-bond character are eliminated, permitting rotation about the central C-0 bond and increasing chain flexibility.Such a substituent effect might be exhibited by aromatic polyamides or polyesters, but it could not occur in poly(p-phenylenes). This hypothesis will be tested by an experimental study of the persistence length and order parameter of the nematic phase of several substituted poly(p-phenylene terephthalates). We have explored the effect of temperature-dependent unperturbed molecular dimensions by a modification of Flory's treatment' of Kuhn chain polymers. Although substitution may lead to less efficient packing and a larger free volume, and free volume may play an important role in the thermotropic transition, it is not evident how this effect can be incorporated into that modified treatment.Ballauf et aZ.,2 in recognition of the artificiality of the lattice model for the treatment of free volume, confined their investigation to the effect on the orientational factor in the partition function. The magnitude of the orientation-dependent interactions will also be reduced. Flory's treatment of Kuhn chain polymers, in common with his earlier treatment of semi-flexible chain polymer^,^ does not derive an explicit relation for the orientation distribution. For this reason orientation-dependent interactions can not be incorporated in either of these treatments. Thus, precisely those factors which might be affected by free volume were omitted in treating the Kuhn chain model.178 GENERAL DISCUSSION Prof. Flory has, of course, been a leader in recognizing the effect of temperature on the unperturbed dimensions of coiling macromolecules.Nevertheless, so far as I am aware, this dependence has not been explicitly considered in any of his theoretical treatments of polymeric thermotropic or lyotropic mesophases. For example, his treatment of the Kuhn model is an isothermal one, and does not foresee the possibility of a thermotropic transition. By modifying that treatment through incorporation of a temperature-dependent Kuhn segment length, we predict a first-order thermotropic nematic-isotropic transition with a calculable enthalpy change. We have recently used d.s.c. data to measure these enthalpy changes for poly( n-hexylisocyanate) (PHIC) and hydroxytropylcellulose (HPC), as shown in the following table.AH,,/cal mol-' repeating unit polymer observed predicted4 PH IC 145 300 HPC 520 500 The experimental values are uncertain, owing to the possibility of degradation at the transition temperature. Despite this reservation, it appears that the predicted AHN, values are of the correct order of magnitude. P. J. Flory, Macromolecules, 1978, 11, 1141. M. Ballauf, P. J. Flory and E. M. Barrall 11, Ber. Bunsenges. Phys. Chem., 1984,88, 530. P. J. Flory, Proc. R. SOC. London, Ser. A, 1956,234,60. W. R. Krigbaum, H. Hakemi, A. Ciferri and G. Conio, Macromolecules, 1985, 18, 973. Dr K. F. Wissbrun (Celanese Corporation, N.1, U.S.A.) said: Prof. Krigbaum concludes from his studies of substituted poly(p-phenylene terephthalates) that substitution reduces not only the melting temperature but also the rigidity of the polymer chain.Would he speculate as to the effect upon chain rigidity of other methods of reducing the melting point, e.g. copolymerization with moieties such as naphthalene, to introduce a jog into the chain, or with biphenylene, to disrupt chain registration? Prof. W. R. Krigbaum (Duke University, U.S.A.) replied: We know from ther- modynamics that a terpolymer having a random sequence distribution must have a lower crystalline melting temperature than the homopolymer. However, we have had no experience with this type of polymer. I would speculate that extended-chain polymers, whether rod-like or having some jogs in the chain, would exhibit similar mechanical properties at low temperatures. At some higher temperature the latter polymers will begin to undergo crankshaft-type motions, and their modulus will decrease.There is anothLI difference which might be significant. The crystalline phase of the substituted poly(p-phenylene terephthalates) has three-dimensional order, despite the random placement of the ring substituents. The crystalline perfection of the random terpolymers appears to be of lower order. An interesting question is how these different degrees of crystalline order will affect the modulus, and its temperature dependence, for extended-chain polymers.GENERAL DISCUSSION 179 Dr G. R. Mitchell ( University of Reading) (communicated): There has been some discussion on the possible variation of the flexibility of various liquid-crystal-forming polymer molecules (in particular the length of the Kuhn link or its axial ratio) with temperature in polyester molecules.It has already been suggested in the discussion that available rotations of the phenyl rings within the polyesters will have no effect upon the chain trajectory because their rotation axes are collinear with the chain direction. A consequence of this is that substitution into the phenyl rings and the resulting change in the rotations possible, as described by Prof. Krigbaum, will have no effect upon the chain trajectory or the length of the Kuhn link size. Furthermore, there will be no variation in the Kuhn link size with temperature. I would like to draw attention to the particular geometry of the ester units. Structural investigations of model compounds of esters [ e.g.ref. (1)-(3)] show that the valence angle about the oxygen atom differs from that about the carbon atom in the ester unit by 6-8”. If we consider a chain in which the esters (more strictly the carbon-oxygen double bond) are normal to the planes of the phenyl rings (thus minimizing non-bonded steric interaction), the trajectory of the chain will be circular, the radius of the possible circle being related to the differences in the valence angles within the ester units. A similar curved chain configuration is exhibited by poly( methylmetha~rylate)~ and by poly( dimethyl~iloxane),~ and it occurs for the same reason, namely unequal skeletal valence angles. The reason why the co- polyester chains are straight and comprise potential liquid-crystal material is that the phenyl units are rotated away from that ‘all-trans’ position to enhance the conjuga- tion effects.This rotation of ca. 30°, which is limited by steric interactions, will occur randomly in opposite senses and will lead to an overall ‘straight’ molecular trajectory. Substitution into the phenyl units will naturally effect the preferred level of twisting and hence the nature of the chain trajectory. However, this effect would be limited, with the possible variation in Kuhn-link aspect ratio being directly related to the difference in valence angles within the ester unit. J. M. Adams and S. E. Morsi, Acta Crystallogr., Sect. B, 1976, 32, 1345. J. Kaiser, R. Richter, H. Lemke and L. Golic, Acta Crystallogr, Sect. B, 1980, 36, 193. W. L. Bencze, B. Kiss, R.T. hckett and N. Finch, Tetrahedron, 1970, 26, 5407. R. Lovell, G. R. Mitchell and A. H. Windle, Faraduy Discuss. Chem. SOC., 1979,68,46. G. R. Mitchell and A. Odajima, Polym. J., 1984, 16, 351. Prof. W. R. Krigbaum (Duke University, U.S.A.) (communicated): I agree that the mechanism which Dr Mitchell suggests would offer only a limited variation in the unperturbed dimensions of the substituted poly(p-phenylene terephthalates) with temperature. Our conjecture concerning the temperature dependence has been offered in my reply to Prof. Flory’s comments, and I have nothing further to add at this point. In my view it will be more fruitful to pursue this discussion after we have accumulated data on the persistence lengths of some of these substituted polyesters. Dr K.F. Wissbrun (Celanese Corporation, N.J., U.S.A.) said: I address my comments to Prof. Berry. I would like to comment on your discussion concerning the low-shear-rate upswing of the viscosity with decreasing shear rate of the nematic solution. You suggested that this behaviour could be explained by an increase of the order parameter S with increasing shear rates because the viscosity calculated from the Doi molecular theorv decreases with S.180 GENERAL DISCUSSION Your arguments are based on the linearized version of the Doi theory applicable in the limit of zero shear rate. Prilutski and Metzner have recently obtained numerical solutions of the Doi equations without linearization. They do in fact find that the order-parameter tensor increases with shear rate and that the viscosity decreases, in qualitative agreement with your argument.However, in the limit of low shear rates the viscosity is constant. When the shear rate (normalized to the rotary-diffusion coefficient) increases above unity, the viscosity decreases with a concave downward shape on a doubly logarithmic plot. Their results suggest that your hypothesis may account for the high shear rate shear-thinning at reduced shear rates above unity on your fig. 2, but not for the concave-upward shear-thinning that you see at low shear rates. It is worth noting that the equations solved by Priiutski and Metzner were derived using the decoupling approximation originally employed by Doi and subsequently by Marrucci. As you point out, Kuzuu and Doi later found that not making this approximation results in the prediction of unsteady shear flow.Is it possible that such unsteady flow is a cause of the domain texture observed in the low-shear-rate (Region I) flow? Prof. G. C . Berry (Curnegie-Mellon University, U.S.A.) replied: I agree with you that the rheological behaviour observed with nematic polymer solutions at low shear rate in which the steady-state viscosity qK increases with decreasing shear rate K is yet to be definitively explained. Continuing studies in our laboratory on the flow birefringence of such solutions, mentioned briefly in our contribution to this meeting, have given results that indicate that a layer of strongly anchored polymer chains exists at the surfaces of the platens used in the rheometer. We postulate that the rod-like chain axes of such chains are in the plane of the platen, and that the chains tend to be parallel locally.However, there is no global, preferred orientation of the chains over the entire platen surface, with departures from linearity occurring either gradually ( i e . on a scale long compared with the chain length L ) or as disclinations near to or bound on the surface. This texture would propagate for some distance into the quiescent fluid, perhaps from one platen to the other, producing local twists in the director orientation. In a shearing deformation, the flow birefringence data are interpreted in terms of well oriented chains in the fluid interior far from the platens, but with an essentially stagnant boundary layer near each platen, produced by the anchored chains.The thickness I of these layers decreases with increasing K, so that qK calculated from the observed torque and deformation velocity decreases with increasing K until K is large enough to make 2 negligibly small. This range corresponds to K for which qK = qp. Similar behaviour has been reported for capillary flow of small-molecule liquid crystals [J. Fisher and A. G. Frederickson, MoZ. Cryst. Liq. Cryst., 1969, 8, 2671. With the polymeric fluids studied here, the presence of any disclinations bound to the surface may enhance the effect observed. Thus, we suggest that with the PBT nematic polymer solutions the decrease of qK with increasing K is caused by neglect of a boundary layer of variable thickness in the estimation of these parameters from observed torques and angular velocities, with the boundary being stabilized by rod-like chains strongly anchored to the platens.Work to assess this postulate is in progress in our laboratory. It does not appear that the decrease of qK with increasing K is accompanied by an inherently unsteady flow, in that the shear stress becomes independent of time after a sufficiently long time.GENERAL DISCUSSION 181 Dr G. R. Mitchell (University ofReading) said: I would now like to address my comments to the paper of Alderman and Mackley, and in particular to add further experimental evidence relating to the problem of skin/core variation. The variation of orientation through the extrudate of a rigid-chain thermotropic copolyester may be considerable, and thus correlations drawn between mechanical properties and molecular orientation averaged over the complete sample volume will possibly be misleading.Plates 1 and 2 show wide-angle X-ray scattering patterns for extrudates of a random copolyester which were prepared by Dr Mackley. Each figure contains three patterns: (A) relates to scattering from the complete extrudate, (B) is a pattern obtained from a thin slice taken from the skin or surface of the extrudate, while (C) relates to sample taken from the central core of the extrudate. The principal features of the scattering patterns are the peaks, which are most intense in the equatorial section (perpendicular to the extrusion direction) and arise from correla- tions between chain segments. The degree of arcing of these peaks is a measure of the molecular orientation.The scattering patterns of plate 1 are for an extrudate for which there was no post extrusion drawing. The difference between the high orientation in the skin and the low orientation in the core section is most pronounced. Plate 2 shows the results for an extrudate with draw-down. There is now very little difference between the orientation in the skin and core sections. The draw-down appears to have only a limited effect upon the level of molecular orientation reached in the extrudate. Its particular effect is to produce an extrudate of uniform orientation and therefore presumably uniform physical properties. The presence of uniformity of orientation may well limit the capacity for delamination. Prof.M. Kleman (Univeriste' Paris-Sud, Orsay, France) said: Alderman and Mackley claim that the 'dense disclination texture' they observe is thermally stable: the density of defects is reversible and depends only on temperature. This result, if it is confirmed, raises interesting questions. Thermal disclinations have been advocated recently in a number of situations with 'frustrated' molecular structures, in particular in disordered systems' (liquids, glasses and blue fog, which is a disordered blue phase); periodic disclination arrays can also relieve frustration in some cases (Frank and Kasper phases2 and blue phases3). In all these cases disclinations separate domains inside which the frustration is weak. Why an uniaxial nematic phase should present frustration is hard to imagine.I would like therefore to consider seriously the possibility that these copolyesters are biaxial, like those of Windle et uZ.,~ with which they have much in common. This possibility suggests some comments. (1) Disclinations in biaxial nematics do not obey the same rules of interaction as those which apply in uniaxial nematics; while in the latter case disclinations can easily cross without obstruction (this is expressed by the fact that, in the topological theory of defects, disclinations are classified by the elements of the commutative group with two elements Z2), it is predicted that there is obstruction to crossing in the former case.5 (In a biaxial nematic disclinations are classified by the quaternion non-commutative group Q.) Fig. 1 represents a typical crossing of two disclinations belonging to two non-commuting elements of Q: a third disclination segment appears between both and the total density of disclinations is increased.If such crossings occur when the temperature of the sample is changed, and are at the origin of the change in density with T, then the fact that this density is smaller when T is higher (as stated by Alderman and Mackley) implies that crossings with T going downwards are more effective than those with T going upwards. If this is the case, it means182 GENERAL DISCUSSION Fig. 1. Typical crossing of two disclinations (see text). probably that there is some activation energy against annealing which is in the temperature range considered. (2) Frustration should be a usual characteristic of biaxial nematics if biaxiality is due to some anisotropic coiling of the long molecules one around another.As discussed briefly in ref. (6) and in more detail in a forthcoming publication of this discussant, one would expect in such a case that domains made of coiled-together molecules would have a transverse characteristic size (a correlation length for coiling) and would be separated by disclinations or some sort of wall. A hierarchy of domains might also occur (supercoils). The analogy with the blue fog (if disclinations are at random) or the blue phase is evident. We expect that any reasonable theory of random disclinations relieving frustra- tion would predict a density of defects increasing with T. This is not the case here. Therefore both frustration (leading to a thermal distribution of defects) and obstruc- tion to crossing with activated annealing have to be considered. Of course, all our comments are of an heuristic nature, and should be taken into consideration only if the basic experimental facts have been correctly interpreted above.If this were the case, even more could be said! M. KlCman and J. F. Sadoc, J. Phys. (Paris) Lett., 1979, 40, L-569. F. C. Frank and J. S. Kasper, Acta Crystalfogr., 1958, 11, 1984; 1959, 12, 483. See for example S. Meiboom, M. Sammon and W. F. Brinkman, Phys. Reu. A, 1983, 27, 438. A. H. Windle, C. Viney, R. Golombok, A. M. McDonald and G. R. Mitchell, Faraday Discuss. Chem. Soc., 1985, 79, 55. G. Toulouse and M. KlCrnan, J. Phys. (Paris) Lett., 1976,37, L-149; G.Toulouse, J. Phys. (Paris) Lett., 1977, 38, L-67. M. KlCman, Faraday Discuss. Chem. Soc., 1985, 79, 215. Dr M. R. Mackley (University of Cambridge) said: Prof. KICman raises an important issue concerning the temperature dependence of the disclinations observed in our samples. Temperature cycling of samples presents two experimental difficul- ties. A temperature change can lead to a change in sample thickness and of perhaps greater importance to local flow within the sample. From our observations so far, it is not possible to say that the disclination density changes without any local flow occurring. We also note that the greatest change in the disclination density appears to occur in the temperature ranges where there are changes in the melting or cooling endotherms of the material’s d.s.c.traces. This itself suggests that the overall observed decrease in disclination density with increasing temperature is not related to thermal activation in a simple way. Dr. G. R. Mitchell (University of Reading) said: I would like to comment on the remarks of Prof. Kl6man relating to the level of local orientation in liquid-crystal polymers. We may describe the orientation of molecules, or perhaps more realisti- cally (for polymers) the orientation of chain segments, with respect to some external axes, or relate the orientation of the chain segments to each neighbouring chain segment. The former mode, which describes the distribution of individual chainGENERAL DISCUSSION 183 segments, I will term global orientation. The alternative mode of description specifies the interactions or correlations between chain segments.Since it is the correlations which occur in the immediate environs of each chain segment which are of interest, I shall term this specification local. The difference between these terms is principally one of scale, and a knowledge of both is a prerequisite for any satisfactory model of liquid crystals. The wide-angle X-ray scattering observed for an aligned liquid-crystal polymer, of which fig. 2 of our paper is an example, contains both scattering which arises from correlations from individual chain sequences and scattering which may be related to the correlations between chain segments. Thus it should be possible in principle to extract both a measure of the global orientation and the level of local orientation correlation.The scattering which occurs at scattering vectors > 2 A-' results almost entirely from correlations within separate chain segments. The scatter- ing observed at one such scattering vector is then simply the convolution of the scattering for a perfectly aligned chain segment I,,( a) and the orientation distribution function D(a). If we express these functions in terms of a series of Legendre polynomials we may express the observed anisotropy ( P 2 n ( ~ ~ ~ as (PZn(C0S 4 ) I = (P2n(COS 4)lu(P2n(COS 4 ) D (1) where the subscripts D and I,, relate to the distribution function and the scattering from a perfectly aligned unit. For the rigid-chain copolyesters under consideration (see our paper at this Discussion) we may calculate ( P2n (cos a ) ) I , from a knowledge of the chemical configuration and thus obtain a measure of the global orientation (Pzn(cos a)), as a series of quotients.In fact it is possible to determine the correctness of the conformation of the molecule used in the analysis by exploiting the spherical-harmonic analysis as described elsewhere.273 Values of ( P2( cos a)) obtained in this way for typical melt-extruded copolyester pellets are ca. 0.5-0.6. The scattering which occurs at s = 1.5 A-' results from spatial correlations between chain segments, and as such its anisotropy results from the orientation of those correlations. If it were a crystal reflection, and hence if it were assumed that for a perfectly aligned system the intensity of the reflection would be limited to the equatorial plane, we would be able to extract the molecular orientation functions using eqn (1) with ( P 2 n ( ~ ~ ~ a ) ) I , set to values given elsewhere.' Such a method involves obvious approximations (although it is in widespread use), for it ignores for a liquid-crystal system the effects of intrachain scattering and the finite correlation length of the molecules.An ap roach will now be described which utilizes the interchain scattering at s = 1.5 I-' to provide values for the local orientation function. If we ascribe some local orientation (P2n(c0s a)), to a small volume of the material, and also describe the orientation of those volumes using (Pzn(cos a))", we may write the global orientation as (&"(COS 4 ) D = (P2n(COS 4>L(PZn(COS a h .(2) For a liquid-crystal sample in which the local orientation is relatively high the arcing of the peak at s = 1.5 A-' is related to the anisotropy of those local volumes. In other words, (Pzn(cos a))v is equivalent to ( P 2 n ( ~ ~ ~ a!))* for s = 1.5 A-'. Since we can obtain, independently, values of (P2n(c0s a)), the values of ( P 2 n ( ~ ~ ~ a))L are available as a series of quotients (P2n(COS a))DI(Pzn(COS a >>v where the divisor is obtained from the anisotropy of the peak at s = 1 . 5 A - ' .184 GENERAL DISCUSS I 0 N However, before performing these manipulations it is necessary to correct the ( P2n (cos term for the effects of finite-length segments and the inclusion of some intrachain scattering. Those corrections reduce the anisotropy of ( P2,, (cos to 86-92% of their recorded value^.^ Values were recorded for the copolymer of hydroxybenzoic and hydroxynaph- thoic acids (70/30) of (P2(cos a))D = 0.53 and (P2(c0s = 0.55, which give a local orientation parameter ( P2(c0s of 0.95.Thus the local correlations between chain segments are considerably enhanced above the general level of global orienta- tion. The extent of this enhanced orientation is currently unknown. It is only possible from the X-ray analysis to suggest a correlation volume with a radius of at least 30A. ' R. Love11 and G. R. Mitchell, Acta Crystallogr., Sect. A , 1981, 37, 135. G. R. Mitchell and A. H. Windle, Colloid Polym. Sci., 1982, 260, 754. G. R. Mitchell and A. H. Windle, Polymer, 1983, 24, 1513. G. R.Mitchell, in preparation. Dr D. J. Blundell (ICI, Wilton) said: I wish to remind the meeting of the evidence of the flow regime in main-chain thermotropics under high stress that is revealed by looking at the microstructure of injection mouldings. This was well exemplified by the excellent microscopy shown in the poster from Brunel University by H. Thapar, P. Allan and M. J. Bevis. After sectioning longitudinally along injection-moulded bars, one can often see with the naked eye a series of light and dark bands closely related to the flow history during mould filling. By making thin sections and observing by light microscopy my colleague A. D. Curson has shown that there is a substructure which usually appears as elongated layered structures typically 0.25 p m thick and elongated by several p m in the flow direction. The Brunel SEM pictures reveal a similar picture.Taking a simple view of what is seen, it is difficult to avoid the conclusion that these entities are related to the units that have sheared or flowed passed each other during the mould filling process. Are these entities perhaps related to the cells modelled by Dr Wissbrun? There is a close resemblance to the phenomena presented by Prof. Meyer. As Prof. Meyer has suggested earlier, these flow entities may be the products of a mechanical instability under the influence of a high melt stress. This would suggest that it is the regions between them where flow occurs and where disclinations may presumably play a role. In our studies we have examined longitudinal sections with a 100 p m microbeam X-ray camera.This gave an assessment of the molecular chain orientation in the flow entities within the region probed by the beam. We found the net degree of orientation varied from region to region. The orientation tended to correlate both with the size and disposition of the flow entities and with the flow regime in the mouldings. Similar conclusions were drawn by the Brunel poster. We also believe that the gross dark/light banded appearance seen in the mouldings is the result of differences in the light scattering between different bands, resulting from the differen- ces in the size and orientation of the component birefringent units making up the microstructure. Dr M. R. Mackley (University of Cambridge) replied: Dr Blundell raises a point concerning instabilities within thermotropic liquid-crystal polymer injection mould- ings.In my view this could be an important factor and deserves both theoretical and experimental attention. The results presented in our own paper suggest that the stability of the director trajectory is dependent on whether the fluid is beingGENERAL DISCUSSION 185 axially accelerated or decelerated. The injection moulding process is complex and will contain flow regimes of these types although their occurrence will be a function of both time and position. Our results also illustrate the extreme shear sensitivity of these materials. Thus flow instabilities within injection-moulded articles should certainly be expected. Dr B. Griffin (ICI, Welwyn ) said: Tennessee Eastman's 60% p-oxybenzoate-40% PET copolyester, known as X7G, was the first freely available thermotrope and as such has now received detailed attention at a number of centres.Because of its historical significance I feel it is important to draw attention to its untypical character. In particular, the synthesis by melt acidolysis with p-acetoxybenzoic acid of preformed PET polymer, followed by melt and solid-state repolymerization, gives rise to chemical inhomogeneity. This is in addition to the normal molecular-weight distribution found in most thermotropes prepared by conventional condensation of AB or AA plus BB type monomers. Work in the ICI laboratory during the late 1970s for example has shown, using graded solvent extraction, that X7G samples contain a continuous range of copoly- mer compositions varying from ~ 2 0 % to 280% p-oxybenzoate units.Under the polarizing microscope the PET-rich fractions furnish only isotropic melts at all temperatures. Intermediate fractions give mesomorphic melts similar in appearance to unfractionated X7G but with a narrower biphasic temperature interval near TI. Fractions with ca. 80% p-oxybenzoate show no TI below the decomposition point (350-400°C). Thus it is important to realise that the isotropic character revealed by n.m.r. studies below TI and the more obvious biphasic separation observable at TI owe their origin to both the flexible PET-rich fractions as well as normal TN-,I transitions observable for each molecular weight (axis-ratio) species present in conventional, chemically homogeneous, completely rod-like thermotropic melts and lyotropic solutions. The fact that rapid demixing of the phases between T, and TI has not been reported is believed to be a reflection that the low-shear-rate viscosity of the extremes of composition present are in fact closer together than they are to that of the mean composition.Certainly at ca. T, + 40 "C slow separation of isotropic material was evident in the ICI work. Prof. Lenz's proposal to form an IUPAC study group for liquid-crystal polymers would now seem most timely. Apart from X7G there are a number of development materials available which would now repay more careful characterization. Dr K. F. Wissbrun (Celanese Corporation, N.J., U.S.A.) said: I have two brief comments to make on this very interesting work.One is to correct a possible misimpression, namely that the effect of shear upon domain size was proposed independently by Marrucci and by myself. Speaking for myself, and I believe also correctly for Prof. Marrucci, the concept was based on the pioneering observations of Graziano and Mackley, and the quantitative relation of domain size to stress was formulated by Marrucci. Secondly, the idea that the velocity rearrangement of a power-law fluid upon exiting from an extrusion die was the cause of a skin-core morphology has also been presented by Ide and Ophir.' I do not believe that they considered the effect of subsequent draw-down as Dr Mackley did to explain the increasing growth of the skin fraction with increasing stretching. * Y.Ide and 2. Ophir, Polym. Eng. Sci., 1983, 23, 261.186 GENERAL DISCUSSION Dr A. H. Windle ( University of Cambridge) said: I have a comment and a question. ( 1 ) Dr Mackley has shown us the dynamic response of the microstructure of a thin sample under shear. We have been concerned whether the proximity of static glass slides might also influence the microstructure of a thin section, and have compared the textures of thin samples (ca 2-3 pm) prepared as a melt between glass slides with samples of much the same thickness microtomed from the bulk. The textures observed, whether fine Schlieren or coarsened by annealing, were closely comparable. On the other hand, the annealing on rock salt of samples only ca. 0.1 p m thick (i.e. of the order of the molecular length), produced domain textures of a type not observed in 2-3 p m samples.The domains in such thin specimens also appeared exceptionally rapidly, and were well formed after a second or two. (2) Do the disclinations observed by Dr Mackley play any significant role in the mechanism of shear deformation, or should we regard them simply as markers going along for the ride? Prof. A. Keller (University of Bristol) said: I would like to clear my own mind as to what is meant by ‘domains’. I note that such ‘domains’ are being referred to in several papers and discussion remarks and this in different contexts. In some instances I get the impression that they are supposed to be separate entities similar to grains in a polycrystalline material, while in others they refer merely to compara- tively defect-free regions with directors all parallel which, while delineated by disinclinations, do not possess materially distinct boundaries.Neither do references to the role of these ‘domains’ in flow and/or deformation help me to appreciate what they are supposed to stand for. Further, are these ‘domains’ meant to be equilibrium structures under the prevailing constraints imposed on the system or are they, in analogy to polycrystalline grains, the results of a nucleated growth process? I would be most anxious to have some clarification on this matter. Dr A. H. Windle (University of Cambridge) replied: In taking up Prof. Keller’s challenge to define a ‘domain’ we at once find Marrucci’s particular description restrictive, although Dr Wissbrun has shown it may be a useful structural model when discussing deformation.In keeping with previous usage of the term ‘domain’ or ‘grain’ in metal crystals to describe regions in which a particular type of order is preserved (such as crystal orientation, solid solution ordering or magnetic orienta- tion), we see a domain, when it exists in the liquid-crystalline polymer context, as a region within which an orientation parameter varies no more than slowly with position, compared with its rapid variation at the delineating boundaries. For a number of reasons, including some observations of the structure of the boundaries themselves, we are drawn to the analogy with ferromagnetic domains as being the most useful. For example, the orientation of the extinction directions in a fine Schlieren texture (plate 3) varies comparatively rapidly with position, and yet, as far as can be seen, the variation is continuous.We do not call any part of this microstructure a ‘domain’, even though it might be possible for some local areas to satisfy Marrucci’s description. On the other hand, polymers such as ClQT-QG (polymer 111 of our paper), in which the microstructure of the mesophase coarsens very rapidly with time, show regions of fairly uniform orientation with clear-cut boundaries. We call these regions ‘domains’ (plate 4). Domains also form very rapid1 during the one of Dr Donald’s dark-field electron micrographs. It shows a fairly uniform annealing of samples thin enough to be observed in TEM (ca. 1000 x ). Plate 5 isGENERAL DISCUSSION 187 orientation within the domains and comparatively localized boundaries, which on analysis show features similar to either Bloch or NCel walls in ferromagnetic materials.''2 Prof.Keller also enquires as to the mechanism of domain formation. Starting with a fine Schlieren texture in a 60/40 HBA/ET, we have followed the coarsening sequence on annealing at 290 "C. The fine Schlieren textures themselves are formed when the liquid-crystalline polymer is either: (i) cooled from the isotropic phase, (ii) subjected to complex flow situations in the mesophase, e.g. extruded, or (iii) cooled across the transition from uniaxial to biaxial. The initial stages of coarsening occur with the appearance of circular zones with a comparatively coarse radial texture.Three such coarsening centres can be seen in plate 6 . The contrast between crossed polars is sometimes a four-fold brush and is very similar to the Maltese cross characteristic of spherulites in crystalline systems. During the anneal the circular regions increase in number and size (plate 7), until the remaining fine-scale Schlieren texture appears as isolated 'knots' in the otherwise coarsened microstructure. The general appearance is then as in plate 8, while the fully coarsened texture is shown in plate 9. It has not yet been possible to follow the complete sequence in thicker specimens, where the fine microstructures are superimposed leading to a confused texture. ' A. M. Donald, C . Viney and A. H. Windle, Philos. Mag., Part A, in press. ' A. M. Donald and A.H. Windle, Polymer, 1984, 25, 1235. Dr M. R. Mackley (University of Cambridge) said: I am grateful to Prof. Keller and Dr Windle for their remarks. Our own optical observations are concerned with both birefringence and scatter- ing, from which we believe that the scattering originates from line defects within the material. For the samples we have examined we do not have any experimental evidence to support the view that sharp domain walls are present. Plausibly it could be envisaged that the disclinations represent boundaries in the fluid; however, this does not mean that there are well defined surfaces separating different domains. Concerning flow, we envisage the roll of disclinations in liquid-crystal polymers to have similarities with that of disclinations previously studied in small-molecule liquid crystals.' In the latter case, material that started defect-free in the director vertical state was initially oriented by low shear rates with the director horizontal along the direction of shear.At higher shear rates, disclination loops were nucleated and they could subsequently both multiply and relax. During flow, the presence of the disclinations will only locally modify the conditions around the line defect. On cessation of flow, the relaxing disclination loops influence the director trajectory over extended distances within the fluid. In terms of liquid-crystal polymers, similar events occur. Flow causes both matrix orientation and disclination multiplication. Unlike the role of dislocations in the plastic deformation of metals, disclinations in thermotropics do not appear to be essential for the material to have fluidity.However, the presence of disclinations does depend on the past shear history of the material, and the line defects act as boundary constraints that will effect the director trajectory within the material, particularly during relaxation after shear. In concluding the discussion on my paper I would like to emphasize that it is important to appreciate the factors that thermotropic main-chain liquid-crystal polymers have in common with both conventional flexible thermoplastics and small-molecule liquid crystals. In relation to thermotropics, common factors are ( 1)188 GENERAL DISCUSSION Fig. 2. Schematic diagram of isotropic phase (top) and nematic phase with orientation domains (bottom) [taken from ref.(2)]. a molecular weight distribution and (2) melt-processible, high-viscosity viscoelastic behaviour with an associated broad range of relaxation time constants. In relation to small-molecule liquid crystals common factors are (1) anisotropic elastic and viscous behaviour (2) line defects that occur as disclinations which can multiply as a consequence of shear. Thermotropic liquid-crystal polymers do not appear to have as simple solid/liquid boundary conditions as small-molecule liquid crystals can have. The elastic properties of thermotropic liquid-crystal polymers also appear to have greater complexity than the splay, twist and bend distortions present in nematic small- molecule liquid crystals. Fluid and solid properties of thermotropic polymers can be expected to depend on both the anisotropic behaviour of the material, together with the defect structure within the fluid or solid.D. Graziano and M. R. Mackley, Mol. Crysr. Liq. Cryst., 1984, 106, 103. W. G. Miller, C . C . Wu, E. L. Wee, G. L. Santee, J. H. Rai and K. G. Goebel, Pure Appl. Chem., 1979, 38, 37. Prof. E. L. Thomas (University of Massachusetts, U.S.A.) said: I would like to raise the question of domains as a key microstructural feature in liquid-crystalline polymers since Dr Wissbrun indicated an important role for such structures in determining rheological behaviour. Fig. 2 is often cited in the literature as a schematic illustration for what is meant by an orientation domain structure in the liquid-crystalline state. The domains are bounded by surfaces where the molecular director changes discontinuously from one domain to the next.As we will show in our contribution, the presence of disclinations and bands give rise to various textures in the field of lamellar trajectories, but we have found no evidence for orientation domain boundaries as such. The concept espoused by Marruccil and presently further developed by Wissburn of defining an ‘effective domain’ as a stable array of disclinations such that the ‘domain’ has no net overall orientation is appealing, but it is not clear to me whether the grouping of the disclinations provides any unique domain morphology appropriate to rheology. Furthermore, how such domains (really disclinations) interact and change their number, size and shape during flow is still an open question.Dr Mackley’s flow-visualization experimentsGENERAL DISCUSSION 189 seek to answer this question, but it appears that the size scale is beyond the resolution of his optical technique. Finally, as a note on the value of preprinting and circulating the Faraday contributions, I would like to remark that Dr Wissbrun previewed our paper and used a slide in his oral presentation today to illustrate the concept: ‘domain = stable array of disclinations’. We saw Dr Wissbrun’s paper, re-examined our images, and found a frequently occurring disclination array (plate 10). The molecular director field suggested by the lamellae closely resembles Dr Wissbrun’s schematic representation of a single domain; nevertheless we are unable to reconcile images of larger areas with the stipulation that assemblies of the ‘domains’ must be space-filling.’ G. Marrucci, paper presented at Int. Congr. Rheology, Acapulco, 1984. W. Miller et al., J. Polym. Sci., Polym:Symp., 1978, 65, 91. Prof. A. Keller (University of Bristol) asked Prof. Thomas: Are disclinations equilibrium defect structures? Prof. E. L. Thomas (University of Massachusetts, U.S.A.) responded: I think the analogy between dislocations in crystalline materials and disclinations in liquid- crystalline materials is relevant here. In crystalline materials the dislocation density is lowered by annealing. This is because the increase in internal energy of the crystal by the presence of the dislocation more than offsets its entropic contribution.Such is not the case for vacancies, which are equilibrium point defects. I expect the trade-off between internal energy and entropy for disclinations will be similar to dislocations, i. e. disclinations are not equilibrium defects. This has been verified in our experiments in which the disclination density of a film was observed to decrease with long-time annealing. Dr R. Zentel (University of Mainz, West Germany) said: I am rather astonished about the different approaches which people interested in low-molar-mass liquid crystals and in polymer liquid crystals seem to use. For low-molar-mass liquid crystals one starts with the definition of five different viscosity coefficients and three elastic constants,* which can be determined indepen- dently for well oriented samples without disclination lines.Starting from this one hopes to understand more complex behaviour also. In polymeric liquid crystals it is recognized that this must be the case,2 but people normally start with unoriented samples and use only one viscosity coefficient. This may be the right way if one is only interested in processing properties, where one also works with unoriented material. However, if one tries to gain a better understanding of the behaviour, would it not be better to start with five coefficients? See for example: P. G. de Gennes, The Physics of Liquid Crystals (Clarendon Press, Oxford, 1975). * See for example: P. G. de Gennes, S. Chandrasekhar or W. R. Krigbaum, in Polymer Liquid Crystals, ed. A. Cifem, W.R. Krigbaum and R. B. Meyer (Academic Press, New York, 1982). Dr K. F. Wissbrun (CeEanese Corporation, N.J., U.S.A.) responded: It would be very desirable in principle to measure the three elastic constants and the five Leslie coefficients, as Dr Zentel suggests. In practice this is very difficult for the main-chain thermotropic polymers, especially the all-aromatic polymers that I have been prin- cipally concerned with. The difficulties arise from the strongly persistent domain texture of these polymers and their high melting temperatures. A beginning has been made at studies of the sort Dr Zentel suggests with lyotropic main-chain polymers. Prof. Meyer’s work presented at this conference is the most advanced that I am aware of; DuPre has also measured elastic constants of poly(benzy1190 GENERAL DISCUSS ION glutamate). Some measurements of anisotropic viscosity of poly(p-benzamide) solutions have been reported by Kulichikhin et al.' I do agree with Dr Zentel that eventually it will be necessary to make such measurements in order to understand the flow behaviour of polymer liquid crystals. It may be necessary to devise new measurement techniques for special requirements of polymeric liquid crystals. This is one of the problems that I proposed at a workshop on orienting polymers.2 Dr Zentel then continued: This is right for liquid-crystalline main-chain polymers, but liquid-crystalline side-group polymers can be well oriented by surface effects and in electric and magnetic fields3 V. G. Kulichikhin et al., Vysokomol. Soyed., 1979, 21, 1407. ' K. F. Wissbrun, in Orienting Polymers (Lecture Notes in Mathematics), ed. A. Dold and B. Eckman (Springer, Berlin, 1984). See for example: H. Finkelmann and G. Rehage or V. P. Shibaev and N. A. Plate, in Adv. Polym. Sci. (Springer, Berlin, 1984), vol. 60/61.Plate 1. Plate 2. Plate 1. Wide-angle scattering patterns for different sections of a melt-extruded sample of a rigid-chain thermotropic copolyester. The extrusion direction is vertical, and the principal feature of the patterns corresponds to a real-space correlation of CQ. 5 A. (A) Complete extrudate, (B) thin section taken from skin of extrudate and (C) thin section taken from the central core of the sample. Plate 2. Wide-angle scattering patterns for different sections of the same copolyester as plate 1 but extruded with a post extrusion pull-off. Key as plate 1. [facing p g e 190Plate 3. Fine Schlieren texture in B-ET cooled rapidly from 340 "C. Plate 4. Domains in a specimen of ClQT-QG viewed between crossed polars; sheared and subsequently coarsened at 350 "C.'Plate 5. A thin film of polymer B-N sheared and annealed for 20 s at 320 "C on rock salt. TEM dark-field image. Plate 6. Fine Schlieren texture in B-ET showing coarsening centres. Samples rapidly cooled from 340 "C and annealed at 290 "C (T. J. Lemmon).Plate 7. Growth of coarsening centres. B-ET annealed at 290 "C (T. J. Lemmon).Plate 8. Predominantly coarsened texture with isolated 'knots' of remaining fine Schlieren texture (T. J. Lemmon). Plate 9. Fully coarsened texture after annealing at 300 "C for several hours, showing domains (C. Viney).Plate 10. ( a ) Stable array of disclinations defining an ‘effective domain’ (Wissbrun). ( b ) Image of array of pairs of S = +$ and S = -4 disclinations (Thomas and Wood).

ISSN:0301-7249    

DOI:10.1039/DC9857900175

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1985, 79, 191-199 Dielectric, Nuclear Magnetic Resonance and Electron Spin Resonance Studies of Relaxation Processes in a Liquid-crystalline Polyester BY FRANCOISE LAUPRETRE AND CLAUDINE NOEL* Laboratoire de Physico-Chimie Structurale et Macromoleculaire, E.S.P.C.I., 10 rue Vauquelin, 75231 Paris Cedex 05, France AND W. N. JENKINS AND GRAHAM WILLIAMS Edward Davies Chemical Laboratories, Aberystwyth, Dyfed SY23 1 NE Received 10th December, 1984 The molecular dynamics of the polyester (-OC-[ Ph]3-CO-O-[CH2-CH2-O-]4)n, which has a liquid-crystalline smectic C (S,) phase, have been investigated by the e.s.r. spin-probe, dielectric relaxation and carbon-13 solid-state n.m.r. techniques. The low- temperature, high-frequency y relaxation preserves the characteristics of the local in-chain motions of polyethers.The apparent activation energy corresponds to a very simple flexible structure. The most likely mechanism of this y relaxation might be the local tg+t - tg-t transition in the central 'ether' units. Two /3 relaxation processes were found: the PL relaxation is believed to be caused by diffusional segmental motion of the flexible spacers located in isolated amorphous isotropic regions of the material and the flu process is associated with the 'ether' sequences located in the glassy Sc domains. The increasing rate of rotation of the phenyl rings about their CI-C4 axis is related to the crystal * Sc transition. Note that this motional process of the phenyl rings does not alter the mean orientation of the mesogenic groups.Considerable research effort has been expended in trying to gain a better understanding of the properties of thermotropic liquid-crystalline polymers. So far, however, little is known about the relaxation behaviour exhibited by these materials.'-" Since the detection and evaluation of polymer motions may aid in correlating polymer structure with mechanical properties, it seemed of interest to investigate the dynamics of the polyester TO1 1: ( -0C-Ph-Ph-Ph-CO-O-[CH2-CH2-0-]4),, which exhibits the following thermotropism: 12-14 115-125°C 245-255°C crystal - smectic C - isotropic liquid. This polymer is partially crystalline and, in addition to the melting and clearing endotherms, the d.s.c. curves show two transitions at low temperature: a small increase in heat capacity between -40 and 0 "C ( TgL) and a larger increase between 20 and 60 "C (TgU).l4 Two distinct relaxation processes are observed in the tem- perature dependence of the dynamic mechanical properties.l4 Recent e.s.r. spin- probe studies2 and high-power proton decoupled 13C solid-state n.m.r. measure- ments* have suggested that TgL, the lower of the glass transitions, is due to the flexible 'ether' sequences located in the amorphous isotropic regions of the material, while Tgu is associated with the flexible spacers located in the glassy smectic domains. 191192 DYNAMICS OF A LIQUID-CRYSTALLINE POLYESTER contact Fig. 1. Cross-polarization pulse sequence used in the n.m.r. experiments. It is the purpose of the present work to obtain new information on the molecular motions which occur in polyester TO11 using dielectric relaxation and to correlate these results with e.s.r.and n.m.r. results already reported for this polymerlS2 or reported in this paper. Indeed, the e.s.r. spin-probe technique provides information about any molecular motion which may occur in the polymer, dielectric relaxation measures the backbone motions of the ether and ester electric dipole groups and C solid-state n.m.r. is a powerful tool for studying the intramolecular motional processes which affect either the aromatic or the aliphatic part of the polymer. 13 EXPERIMENTAL The polyester TO1 1 was prepared at the Centre de Recherches des Carrikres de Rh6ne- Poulenc, Saint-Fons, France by standard methods, as described elsewhere.I2 Details of the properties of the polyester are given in ref.(12)-( 14). The dielectric measurements were made in the temperature range -80-36 "C using a three-terminal parallel-plate dielectric cell together with a General Radio 16 15-A capacitance- measuring assembly. The disc sample was prepared by compression-moulding the powdered material at room temperature and ca. 150 bar pressure, giving a sample of 22 mm diameter and 1.55 mm thickness. The temperature of the sample in the dielectric cell was controlled to *O.l "C by use of liquid circulated from a Lauda Ultrakryostat. The e.s.r. samples were prepared by addition of nitroxide radical to the polyester, taking care that the concentration of spin probe did not exceed 0.01 wt%. Samples were sealed under dynamic vacuum in e.s.r.tubes after repeated freeze-pump-thaw cycles. The e.s.r. measurements were performed on a Varian E-4 X-band spectrometer. The temperature in the active region of the cavity was controlled by a Varian E-257 variable-temperature control unit to a long-term stability of *O-1 "C. The e.s.r. spectra were analysed according to the method previously described.2 Carbon-1 3 cross-polarization, proton dipolar decoupling and magic-angle-spinning n.m.r. experiments were performed at 75.47 MHz on a Bruker CXP 300 spectrometer, employing quadrature detection and a single r.f. coil, which was double-tuned for both 13C and 'H. The cross-polarization pulse sequence used in the experiments is shown in fig. 1. Matched spin-lock cross-polarization transfers employed 13C and ' H magnetic-field strengths of 64 kHz.The rises in I3C polarization were obtained from a plot of the relative magnitude of the carbon magnetization as a function of the time of carbon-proton contact for very short contact times (10-200 ps). In all the spectra, spin-temperature inversion techniques were employed to minimize base-line noise and r011.15 Flip-backI6 was also used systematically to shorten theF. LAUPRETRE, C. NOEL, W. N. JENKINS AND G. WILLIAMS 193 delay time between two successive pulse sequences. Spinning experiments at the magic angle using boron nitride and [2H8] PMMA rotors were performed with spinning speeds of 3.5 kHz. THEORETICAL BACKGROUND In the domain of fast motions, information can be derived from the rise in carbon polarization in spin-lock experiment^.^'-^* When some protons are strongly coupled to carbons, as for example the protons of a methyne or a methylene group, the cross-polarization dynamics can no longer be described by a single cross- relaxation time.The initial step of the contact is coherent energy transfer between the strongly coupled carbons and protons. This oscillatory transfer is damped by the coupling of the carbon and its directly attached protons to the remote protons. At the same time, this coupling induces (i) the establishment of a quasi-equilibrium state within the tightly coupled group and (ii) a cross-relaxation energy transfer from the tightly coupled carbon and protons to the remote protons viewed as a thermal bath. At long times the rise in polarization is well described by an exponential dependence on contact duration.Motional information is provided by the short-time behaviour (first tenths of ps), which is governed by the coherent energy transfer. In the case of a powder sample, the short-time olarization rise can be approximated by a quadratic function of contact duration.P9p22 The rise time finf to half of the total polarization depends on the strength of the dipolar interaction (M,,)' of the carbon under interest and its bound protons. Values of tl12 as short as 20 ps for a CH2 group or 28 ps for a CH group are indicative of rigid-lattice behaviour. Longer tl12 values are evidence for motional reduction of (M,,)'. RESULTS AND DISCUSSION ')' RELAXATION From the representative e.s.r. spectra in fig.2 it can be seen that the motion of the probe varies from the rigid limit to the fast region over the temperature range from -160 to +150 "C. At low temperatures, the shape of the slow-motion spectra remains unaltered: the separation between the extrema is experimentally indisting- uishable from the rigid-limit value. However, at a temperature that depends on the probe size but is in the range from -70 to -4O"C, the separation of the outer hyperfine extrema decreases slightly with increasing temperature, indicating the onset of slow motion of the spin probe caused by local main-chain motion. If correlation times for probe tumbling in this region are plotted again reciprocal temperature in Arrhenius fashion,2 then an activation energy of 12 * 1 kJ mol-' is derived.Note that although the frequency and energetics of probe tumbling are intimately affected by the dynamics of the host polymer, it must not be assumed that the frequency and energetics of motion of the probe are strictly equal to those of the host polymer. A dielectric dispersion and absorption region is also seen above -80 "C in the frequency range 1 02- l O5 Hz (fig. 3 and 4). The loss curve clearly narrows and moves to higher frequencies as the temperature is increased. If the frequency of the maximum of dielectric loss for given temperatures is plotted against the reciprocal temperature, then an activation energy of 16.5 kJ mol-' is found, which is similar in magnitude to the activation energy determined by e.s.r. The present dielectric data indicate that, in the frequency and temperature ranges under investigation, local motions do occur: dipoles have limited freedom.Note also that the loss occursI94 Fig. 2. DYNAMICS OF A LIQUID-CRYSTALLINE POLYESTER /--- /---/&----. 0 --/ \ \ /- - /’ ---- -/ 4- // : /’ ‘ / / ‘.,I ‘ 1 \ I E.s.r spectra at different temperatures (in “C) of polyester Toll doped with: in the frequency and temperature ranges where loss is also observed for poly(oxy- methylene),23-’’ poly( ethylene oxide)26-28 and poly(tetramethy1ene oxide).” According to Wetton and Williams? the y process found for these materials must arise from a special kind of main-chain motion which is different from the cooperative micro-brownian motions of the chain responsible for the p relaxation. A local twisting motion was suggested to explain the y processes: the dipoles undergo a damped torsional oscillation within potential-energy minima prescribed by theF. LAUPRETRE, c.NOEL, w. N. JENKINS AND G. WILLIAMS 195 4 .O- E' - - - - 80 2.5- ! I I I I 4 5 l o g ( f l W 3 Fig. 3. Plot of the dielectric constant E' as a function of frequency for given f 36 16 0 -20 -40 -60 -80 temperatures 0 3 4 5 log(flHz) ("0 Fig. 4. Plot of the dielectric loss factor E" as a function of frequency for given temperatures196 DYNAMICS OF A LIQUID-CRYSTALLINE POLYESTER 0 10 20 30 40 50 60 70 tcdw Fig. 5. Plot of the variation of carbon magnetization in a spin-lock experiment as a function of contact duration (0, a, 0, b, and 0, c p s ) for polyester Toll. conformation of the chain.3* Dielectric and dynamic mechanical measurements carried out on multiblock copolymers composed of poly(ethy1ene oxide) and bis- phenol-A polycarbonate have clearly indicated that PEO segments as small as a dimer or a sequence of five or six bonds can accommodate the intrachain motions involved in the y relaxation of the PEO homo polymer^.^^ A small-scale motion, i.e. a two-site model similar to Monnerie's three-bond explains the low-temperature-high-frequency relaxation of these copolymer^.^' The y-process mechanism might be mainly due to local tg+tc*tg-t transition in the PEO blocks. To a certain extent polyester TO1 1 can also be considered as an alternating (AB).-type multiblock copolymer composed of soft (PEO) and hard (p-terphenyl) segment blocks, and our dielectric data suggest that the y relaxation is occurring via a local-model process, the mechanism of which might be the flip-flop transition between tg+tt* tg-t conformation in the PEO segments.Support for this local-mode model has been obtained using I3C solid-state n.m.r. techniques. From proton-decoupled 13C n.m.r. lineshape analysis it appears that the spectra of the aromatic carbons of the polymer are those of the rigid limit below 120 OC.l As regards the carboxy groups, measurement of the principal values crll and c33 of the C=O chemical-shift tensor from the side-band intensities of the carboxy 13C line in a sampie spinning at the magic angle34 leads to a u33-u11 difference of 124 ppm at room temperature. This is close to the value of 128 ppm reported for the same carbon in a single crystal of benzoic acid,35 which indicates that, at 25 "C, there exist no fast motions of the carboxy groups able to reduce the carboxy chemical-shift anisotropy.The increase of the magnetization in a spin-lock cross-polarization experiment as a function of the contact duration is shown in fig. 5 for the various aliphatic carbons of polyester TO1 1. The tl/Z value for the methylene carbon a, which is next to the carboxy group, is 21 ps, which is the expected valueF. LAUPRETRE, c. NOEL, w. N. JENKINS AND G. WILLIAMS 197 for the rigid lattice. The carbons a appear as frozen on the timescale of the experiments, lo5 Hz. By contrast, the tl12 of the methylene carbons 6 and c are longer, indicating substantial but incomplete motional averaging of the dipolar interaction between the carbon of interest and its directly bonded protons. In the case of carbons b the carbon-proton second moment averaged by the motion is 60% of the rigid-lattice value.Such a reduction is too low to be interpreted in terms of three rotational states on the valence cone or in terms of jumps between two equilibrium conformations. It corresponds to oscillations on the valence cone of ca. 20" about one equilibrium conformation. In the case of carbons c, the reduction is larger and indicates either oscillations of larger amplitude or jumps between two equilibrium conformations. From these results we conclude that the y process is associated with small-scale motion of the flexible central -CH2-CH2-0 units. A two-site model similar to Monnerie's three-bond might reasonably explain the low-temperature-high-frequency y relaxation of polyester TO1 1, in agreement with the models proposed for the y relaxation of PE03' and bisphenol-A polycarbonate-polyoxyethylene system^.^' Note, however, that the activation ener- gies determined from e.s.r.(ca. 12 kJ mol-') and dielectric (16.5 kJ mol-') data are much smaller than the values of ca. 35-50 kJ mol-' reported for the y relaxation of these polymers .2733 1,36 Several explanations could be offered for this low apparent activation energy, possibly in combination, since it is known that when an activation energy is determined for an overall composite process, it may not be representative of the average of the activation energies of the various processes involved: (i) At low temperatures the loss peaks are very broad (fig.4), which suggests that there are a wide variety of local environments for the dipoles and that the environment places great constraints on the motion.A given 'ether' group can move only in cooperation with that environment, this being overall a slow process. As the temperature is increased, the whole system cooperatively moves faster and the overall loss peak is narrowed. As a consequence, at high enough temperatures a reference group moves in an average environment. (ii) The y relaxation may consist of two main components associated with a small amount of an isolated disordered structure and a 'glassy' smectic C phase. (iii) Above TgL (ca. -20 "C), overlapping loss processes PL and y may be present.Assuming the same frequency against temperature dependence for the PL process of polyester TO11 and the /3 process of PEO, the PL and y processes could merge in the temperature range 16-30°C. Note that marked narrowing of the loss curve occurs in this region (fig. 4). p RELAXATION AND THE CRYSTAL * SMECTIC c TRANSITION Above a critical temperature, which depends on the nature of the spin probe but which is in the range 0-40 "C, complex e.s.r. spectra are observed (fig. 2), which can be resolved into a mobile component and a solid-state component. The narrow component shows insignificant dependence of its outer peak resonance positions on temperature in contrast to the inward shift of the outer peaks of the broad line spectrum with increasing temperature.As a result, at 45-85 "C, depending on the size of the spin probe, the broad-line and narrow-line spectra coalesce and at higher temperatures the spectra show the motionally narrowed three-line pattern. However, the e.s.r. spectra are asymmetric. The low-field and the high-field lines are broader than the centre line. These features are characteristic of a spin probe undergoing anisotropic rotational reorientation. In this region, the correlation times T~ have the temperature dependence expected from WLF theory. At the crystal-smectic C198 DYNAMICS OF A LIQUID-CRYSTALLINE POLYESTER transition ( 1 15-125 "C), sharpening of the low-field and high-field lines occurs and the probes rotate isotropically.2 The superimposed spectral feature is believed to arise from two different environ- ments for the probes, i e .both rigid and rubbery regions are present. Such an assumption is consistent with a system exhibiting a double glass transition with the TgL transition occurring ca. 40-50 "C below the Tgu transition. Evidence in support of this assumption comes from d.s.c. and dynamic mechanical investigations." Two glass transitions, TgL and Tgu, separated by ca. 50 "C are detected. However, d.s.c. data for a series of samples submitted to different cooling cycles have shown that only the samples which have been rapidly quenched from the isotropic state ( T > 250 "C) exhibit a marked TgL. This suggests that the PL process is due to diffusional segmental motion of the flexible 'ether' sequences located in isolated amorphous isotropic regions of the material.This assignment seems reasonable since the temperature and frequency ranges at which the mobile component appears in the e.s.r. spectra of polyester TO1 1 also correspond to a marked change in the nitroxide- doped PEO spectrum: the broad-line spectrum, typical of a solid-state nitroxide, assumes the three-line rotationally narrowed form, which has been assigned to the onset of the p relaxation.37i38 On the other hand, the flu process seems associated with the flexible spacers located in the 'glassy' smectic C domains. At this point it seems interesting to recall that earlier proton-decoupled 13C solid-state n.m.r. studies' have shown that the linewidth of the methylene carbons regularly decreases from 40°C up to the crystal-smectic C transition and then remains unchanged.On the other hand, for the aromatic carbons agreement with the rigid lattice is observed below the crystal-smectic C transition and with a rapid rotation about the CI-C4 axis in the smectic C phase. Note that this motional process of the phenyl rings does not alter the mean orientation of the mesogenic groups. ' P. Sergot, F. Laupretre, C. Louis and J. Virlet, Polymer, 1981, 22, 1150. P. Meurisse, C. Friedrich, M. Dvolaitzky, F. Laupretre, C. Noel and L. Monnerie, Macromolecules, 1984, 17, 72. K. Mueller, K. H. Wassmer, R. W. Lenz and G. Kothe, J. Polym. Sci., Polym. Lett. Ed., 1983, 21, 785. K. Mueller, B. Hisgen, H. Ringsdorf, R. W. Lenz and G. Kothe, in Recent Advances in Liquid Crystalline Polymers, ed.L. Chapoy (Elsevier, Barking, in press). K. H. Wassmer, E. Ohmes, G. Kothe, M. Portugal1 and H. Ringsdorf, Makromol. Chem., Rapid Commun., 1982, 3, 281. C. Boeffel, B. Hisgen, U. Pschorn, H. Ringsdorf and H. W. Spiess, Isr. J. Chem., 1983, 23, 388. H. Ringsdorf, H. W. Schmidt, G. Strobl and R. Zentel, Polym. Prepr. Am. Chem. SOC., Div. Polym. Chem., 1983, 388. * N. A. Nikonorova, T. I. Borisova, L. L. Burstein and V. P. Shibaev, in 12th Europhysics Conference on Macromolecular Physics: Molecular Mobility in Polymer Systems (Leipzig, 198 I), p. 223. H. Kresse, S. Kostromin and V. P. Shibaev, Makromol. Chem., Rapid Commun., 1982, 3, 509. l o H. Kresse and R. V. Talrose, Makromol. Chem., Rapid Comrnun., 1981, 2, 369. l 1 H. Kresse and V. P. Shibaev, 2. Phys.Chem. (Leipzig), 1983, 264, 161. l 2 P. Meurisse, C. Noel, L. Monnerie and B. Fayolle, Br. Polym. J., 1981, 13, 55. l 3 L. Bosio, B. Fayolle, C. Friedrich, F. Laupretre, P. Meurisse, C. Noel and J. Virlet, in Liquid Crystals and Ordered Fluids, ed. A. C. Griffin and J. F. Johnson (Plenum Press, New York, 1984), vol. 4, p. 401. l4 C. Noel, C. Friedrich, L. Bosio and C. Strazielle, Polymer, 1984, 25, 1281. l 5 E. D. Stejskal and J. Schaefer, J. Magn. Reson., 1975, 18, 560. l6 J. Tegenfeldt and U. Haeberlen, J. Mugn. Reson., 1979, 36, 453. l 7 T. T. P. Cheung and R. Yaris, J. Chem. Phys., 1980, 72, 3804. '' D. Demco, J. Tegenfeldt and J. S. Waugh, Phys. Rev. B, 1975, 11, 4133. 2o F. Laupretre, L. Monnerie and J. Virlet, Macromolecules, 1984, 17, 1397. L. Muller, A. Kumar, T. Baumann and R. R. Ernst, Phys. Rev. Lett., 1974,32, 1402. 19F. LAUPRETRE, c. NOEL, w. N. JENKINS AND G. WILLIAMS 199 21 F. Laupretre and J. Virlet, to be published. 22 G. C. Chingas, A. N. Garroway, R. 0. Bertrand and W. B. Moniz, J. Chem. Phys., 1981,74, 127. 23 B. E. Read and G. Williams, Polymer, 1961, 2, 239. 24 Y. Ishida, Kolloidn. Zh., 1961, 174, 162. 25 G. Williams, Polymer, 1963, 4, 27. 26 T. M. Connor, B. E. Read and G. Williams, J. Appl. Chem., 1964, 14, 74. 27 Y. Ishida, M. Matsuo and M. Takayanagi, J. Polym. Sci., Part B, 1965, 3, 321. 28 K. Hikichi and J. Furuichi, J. Polym. Sci., Part A, 1965, 3, 3003. 29 R. E. Wetton and G. Williams, Trans. Furuduy SOC., 1965, 61, 2132. 30 K. Yamafuji and Y. Ishida, Kofloidn. Zh., 1962, 183, 15. 31 T. Suzuki and T. Kotaka, Macromolecules, 1980, 13, 1495. 32 B. Valeur, J. P. Jarry, F. Geny and L. Monnerie, J. Polym. Sci., Polym. Phys. Ed., 1975, 13, 667. 33 B. Valeur, L. Monnerie and J. P. Jarry, J. Polym. Sci., Polym. Phys. Ed., 1975, 13, 675. 34 J. Herzfeld and A. E. Berger, J. Chem. Phys., 1980, 73, 6021. 35 J. Kemf, H. W. Spiess, U. Haeberlen and H. Zimmermann, Chem. Phys., 1974, 4, 269; Chem. Phys. Lett., 1972, 17, 39. K. Se, K. Adachi, Y. Ishida and T. Kotaka, Rep. Prog. Polyrn. Phys. Jpn, 1977, 20, 375. 37 M. C. Lang, C. Noel and A. P. Legrand, J. Polym. Sci., Polym. Phys. Ed., 1977, 15, 1329. P. Tormala, J. Mucromol. 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ISSN:0301-7249    

DOI:10.1039/DC9857900191

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1985, 79, 201-214 Electro-optic Effects in Side-chain Polymer Liquid Crystals BY HARRY J. COLES Liquid Crystal Group, Schuster Laboratory, Department of Physics, University of Manchester, Manchester M 13 9PL Received 15th January, 1985 A brief background to electro-optic effects in side-chain polymer liquid crystals has been given. Electro-optic effects in dyed and undyed smectic polysiloxanes are described and it has been shown that these high-contrast optical effects may be written on the CQ. 100 ms timescale and are stored in the smectic phase above T'. It has also been shown that ultra-high contrast and resolution laser writing may be achieved with both aligned and scattering textures. Finally, it has been demonstrated that the siloxane polymers may be added to monomeric liquid crystals to control their viscoelastic properties.This has led to a desirable improvement in these properties and the polymer characteristics likely to lead to further improvements have been discussed. Polymer liquid crystals, as a class of materials, were probably first recognised in the classic work of Robinson some thirty years ago.' However, this early work was concerned with lyotropic mesophases, Le. those formed as a function increasing polymer concentration, and thermotropic polymer mesophases (formed as a function of temperature) were only discovered during the last decade. Of the thermotropic materials it is polymers of the main- and side-chain that have excited the most interest because of their potential for technological applications.Such materials, which do not require solvents, combine the viscoelastic properties associ- ated with polymers with the electro-optic properties characteristic of monomeric or low-molecular-mass liquid crystals. This combination has led to the possibility of new display devices involving novel fabrication techniques. For example, rather than the thin, hermetically sealed sandwich-type cells commonly used for monomeric liquid-crystal displays, thin surface films or coatings of polymer liquid crystals may be used to provide devices of great constructional simplicity.6 The combination of properties is particularly important for side-chain polymer liquid crystal^^*^ where the relatively mobile mesomorphic side groups may be reoriented in an applied field whilst the polymer backbone provides a suspending matrix.The polymer viscosity then allows the induced order to be stored on removal of the field, thus giving rise to optically bistable materials. Such materials will form the basis of the current paper. The main emphasis of previous work on side-chain polymer liquid crystals has been on the synthesis of nematic and cholesteric polymers that mimic monomeric liquid-crystal displays. As observed by Ringsdorf and Zentel,7 Finkelmann et al.' and Shibaev and Plat&: such materials may be used to exhibit a variety of electro- optic effects. However, the operating parameters for such effects are generally less agreeable than those found for equivalent monomeric systems. Although fabrication 201202 ELECTRO-OPTIC EFFECTS IN POLYMER LIQUID CRYSTALS of the device may be easier, the high bulk viscosity, caused by the polymer backbone, slows down the electro-optic response time by one or two orders of magnitude.Furthermore, the existence of the glass transition in such materials tends to imply operating temperatures typically ca. 100 "C above ambient. Besides the slow response times for such materials, the threshold fields for electro-optic effects in the polymer mesophases are higher than those for structurally equivalent monomeric liquid crystals. It would, therefore, appear that the electro-optic device performance of a nematic or cholesteric polymer liquid crystal will always be inferior to that of the equivalent monomeric material. The question then arises as to how these materials might be useful.It has been pointed out1o911 that the operating parameters of the polymer devices are not impossible and that, if electric or magnetic fields can be used to induce changes in the order in the system, then this induced order (or optical information) may be stored by rapidly cooling the sample to a temperature below the glass transition (Tg). However, if Tg is to be above ambient temperatures, the high polymer viscosity still implies that the material has to be heated to very high temperatures (ca. 150-200 "C) before field alignment on a reasonable timescale can take place. Besides their potential use in optical storage devices, polymer liquid crystals have been used in the so-called guest-host effect.'* For such polymers guest dye molecules are included as pendent side groups in a copolymer system.Under an applied electric field cooperative motion between the mesogenic and dye side groups causes realignment of the absorption dipole of the dye. In this way the polymer may be transformed from an absorbing to a clear state, or vice versa, in a way totally analogous to monomeric liquid crystals. l 3 Although the polymer response time will be slower than that of the monomeric system, the inclusion of the dye in the polymer structure allows greater dye concentrations to be used and, therefore, higher optical-contrast ratios to be achieved. It is believed that one of the main uses of side-chain polymer liquid crystals will be in the storage of optical information. As indicated above, such information corresponds to a change in the absorption, birefringence or scattering properties of the liquid-crystal polymer because of the order (or disorder) induced in the system by the applied field.The principle experimental problem to be overcome is the high operating temperature required for fast response times if T' is to be above room temperature. A different and novel approach has been adopted for side-chain polymer liquid crystals6 by using materials with Tg below ambient and then looking for storage effects in the smectic phase exhibited by certain of these polymers. It has been shown that such phases are at least bistable and may be used to store various optical textures of very high contrast induced by both electrical and laser fields. These textures may be clear, dyed or scattering and are written either within a few tens of degrees above ambient in the case of electric-field addressing, or at ambient temperatures in the case of laser addressing. Although the main use of these side-chain polymer liquid crystals appears to be in the storage of optical data, their use as additives to monomeric liquid-crystal materials has recently been exploited.In this way it has been possible to use the elastic properties of the polymer to influence the elastic constants of the bulk monomeric material as well as use the side-group mesogen's dielectric properties to adjust the bulk dielectric constants of the monomer. The implications of this work for the performance of electro-optic devices will be discussed. Finally, after considering each of the above electro-optic phenomena in turn, the physico-chemical parameters likely to lead to a greater understanding of the behaviour of side-chain polymer liquid crystals will be discussed.H. J.COLES CH3 I CH3-Si-CH, I 0 I 203 T I 0 CH3-Si -(CH2)"--0 I CH 3-Si- CH I CH3 n 0 OH Fig. 1. Molecular structures of (a) the copolymer polysiloxane system, (b) the azo dye and ( c ) the anthraquinone dye used in these studies. EXPERIMENTAL MATERIALS The essential elements of a side-chain polymer liquid crystal are a flexible or semi-flexible polymer backbone to which mesogenic side-chain species may be attached by a suitable chemical reaction." The most common backbones currently used are based on alkylacrylates, methacrylates and linear polysiloxane copolymers.The mesogenic side groups have been composed of Schiff's base compounds, esters of alkoxybenzoic acids, cyanobiphenyls etc. With such a vast range of materials and combinations available it is evidently possible to synthesise an almost infinite variety of polymer liquid-crystal structures. In collaboration with Professor Gray and colleagues at Hull University, a study based on one family of these compounds, ie. the polysiloxanes, has been initiated. Although a variety of homopolymer, copolymer and terpolymer systems have been studied, the majority of the work has concentrated on a copolymer system containing both cyanobiphenyl and benzoic ester side groups [fig. 1 (a)]. By synthesising a system (PG296) with x = y = 25, n = 6 and rn =4, a smectogenic polymer has been p r ~ d u c e d ' ~ with Tg=4"C and Ts-I=860C.204 ELECTRO-OPTIC EFFECTS IN POLYMER LIQUID CRYSTALS I I 1 I I I I I I 1 I Ti' Tm Tc I I I I 1 1 1 I 1 I 30 50 7 0 90 110 T/"C Fig.2. Thermo-optic analysis of PG296. The curve was obtained by measuring the light transmission through the sample on heating in a microscope hot stage with the optics set for crossed-polarisation planes of the input and transmitted light. T, is the clearing temperature, T, is the temperature at which the isotropic phase first appears and T, is the temperature at which the smectic phase becomes fluid. Details of typical microscopic textures have been given elsewhere.'' Although there are now data on some fifty or so polymers of the siloxane type, it has been found that PG296 appears to behave in a typical manner and this paper will concentrate on results obtained with this material.For the guest-host and laser-addressed research, commercially available azo or anthraquinone dyes (B.D.H. Ltd, Poole, Dorset) have been used [fig. l(6) and ( c ) ] . APPARATUS The electro-optic measurements were carried out using an optical polarising microscope adapted to give direct sample observation, photodiode detection and photographic (or video) recording facilities. Suitable electronics were included to allow pulsed voltages up to 400 V r.m.s. at frequencies up to 100 kHz to be applied across thin polymer films. For the purpose of monitoring the electro-optic response behaviour, the samples were contained between In/SnO,-coated conducting glass slides with a spacing of between 6 and 40pm.This configuration allowed fields to be applied in the direction of viewing ( i e . perpendicular to the plane of the electrodes). The samples were held at constant temperature (within ltO.1 "C) using a thermostatically controlled hot stage (temperature range from -20 to 600 "C). The apparatus and its use have been described in more detail elsewhere.I6 All cells were filled by capillary action. RESULTS ELECTRO-OPTIC STORAGE EFFECTS It is a feature of monomeric liquid crystals that, before electro-optic effects may be studied in them, the materials have to be aligned at their surface boundaries,H. J. COLES 205 90 95 d " " I I I I ' I ' 85 T/"C 80 Fig. 3. Electro-optic switching time, T,, for PG296 as a function of temperature.The applied voltage was 84 V r.m.s. at 2.5 kHz (sine wave) and the cell thickness was 21 Fm. This precondition also applies to nematic and cholesteric polymer liquid crystals. In the case of smectic polymers this is not the case as the existence of the polymer main chain appears to be sufficient to give a highly scattering non-aligned texture. In the following work on pure polymers no alignment techniques were used. Thermo-optic analysis (t.0.a.) of the smectic polymer as a function of temperature was carried out (fig. 2). T.0.a. is normally carried out using crossed polarisers in the microscope and then recording the intensity of transmitted light. At low tem- peratures ( T < T,) the low transmission is caused by the high scattering power of the polymer.At T, the scattering texture becomes highly birefringent and mobile resulting in an increase in light transmission. At T, 'black' isotropic regions appear in the mesophase. These correspond to regions in the polymer that have transformed into the isotropic phase. As the temperature is further increased more of the polymer in this biphasic region is transformed into the isotropic phase and the transmitted intensity decreases until at T, all residual birefringent structure is lost. This fluid- biphasic behaviour (between T, and T,) appears to be typical of these smectic polymer liquid crystals. It is in this temperature region that the majority of the electro-optic measurements were carried out. 1 5 , 1 7 For a fixed voltage and frequency, the electro-optic response time decreases markedly with increasing temperature (fig.3). In this work the switching time, r,, is defined as the time for the light intensity to decrease to 50% of its initial value on application of the pulsed electric field. As shown in fig. 4, this switching time also decreases with increasing voltage [fig. 4( a)], decreasing sample thickness [fig. 4( b)] and optimisation of the a.c. frequency [fig. 4(c)]. Fig. 4 shows that, despite the smectogenic nature of the polymer, the switching times are of the order of seconds or less in the fluid-biphasic region. Such response times are easily comparable with those observed in nematics at lessI I I ' I 0 ' O 0 V/V r.m.s. 200 Fig. 4. Electro-optic switching time, T,, as a function of (a) applied voltage, V, ( b ) voltage and thickness and ( c ) frequency of the applied voltage, j For (a) T = 86.2 "C, d = 21 pm and f= 2.5 kHz, for ( b ) f= 2.5 kHz and T = 85 "C and for (c) T = 84.6 "C and d = 21 pm, 0, 84 and 0, 140 V r.m.s.10 r n bU 5 H.J. COLES I 1 I I I I I I I I I I turbulence I I I homeotropic alignment 207 30 vl \ b* 2 0 I I 1 I 1 1 r r r l I ' I 1 , , . . I 1 . 102 1 O?requency/ Hz lo4 Fig. 4. continued accessible temperatures. The frequency response shown in fig. 4( c ) merits further comment. At low frequencies ( f < f c ) the predominant electro-optic effect is field- induced dynamic scattering. Thus the field induces a chaqge from one scattering texture to another of differing optical density. At frequencies near to fc or above, the electric field induces a homeotropic or clezr texture, the optical clarity (or grey scale) of which depends on the applied voltage.If the sample is cooled back to room temperature this clear texture is stored in the region of the electrodes (plate 1). The sample may be cooled with or without the field applied. Generally the relaxation time of the induced texture is orders of magnitude slower than the switching or cooling times. We have established from optical conoscopy that the clear texture is optically isotropic, uniaxial and positive. This means that the highly optically anisotropic mesogenic side groups are oriented predominantly perpen- dicular to the glass surfaces in the electrode region. These clear textures are durable and remain stored, apparently indefinitely since we have observed no deterioration in their optical properties over the last two years.It is instructive to compare the contrast obtained with a monomeric liquid crystal with that achieved with the polymers. This is shown for PG296 and octylcyanobiphenyl (8CB) in fig. 5 , where the PG296 data refer to a 6 pm thick cell whilst those for 8CB refer to a 23 pm thick cell under otherwise identical experimental conditions. Obviously the contrast ratio is at least an order of magnitude greater for the polymer than the monomeric material. GUEST-HOST STORAGE EFFECTS In the guest-host effect a guest dye is dissolved in a liquid-crystalline host. Through cooperative motion the absorption direction of this dye molecule is altered208 ELECTRO-OPTIC EFFECTS IN POLYMER LIQUID CRYSTALS electrode - I , region - distance/mm Fig.5. Comparison of the transmission characteristics of the polymer PG296 and the monomer smectic material octylcyanobiphenyl (8CB). For PG296 the sample thickness was 6 pm and for 8CB it was 23 pm. In the field ‘on’ region the transmission of the stored textures is identical for each system. on application of a field to the mesomorphic host. Thus a liquid crystal may be switched from an absorbing (or coloured) state to a clear te~ture.’~ Although this effect has been known for over a decade in monomeric liquid crystals, it appears only to have been recently exploited for polymer liquid crystals. With polymers two routes are possible. First, the dye may be included as a side group in the polymer structure6”* or, secondly, the dye may be dissolved into the polymer matrix.I8 Although the former method has been successfully used to produce a coloured switching device, this paper concentrates on the second method since it allows the use of the dye molecule as a probe of the reorientational motion.The guest-host system used herein was ca. 3 wt% anthraquinone dissolved in PG296. The polymer was written in an electric field as described above. The contrast achieved for this system is evident from plate 1. Other dyes have been used to obtain a variety of colours including a black-dyed cell; the results for these will be presented elsewhere. The important features of these cells, apart from their use as storage devices, is the use of the dye to probe the liquid-crystal motion.The switching times for a dyed and undyed cell are shown in fig. 6. Evidently the switching times are identical. This is not the case for monomeric liquid crystals, where the inclusion of dye molecules causes a marked increase in electro-optic switching times. As the dye in both cases is ordered cooperatively by the motion of the mesogenic groups this must imply that the polymer backbone is coupled to the response time. Indeed it seems likely that the polymer viscosity dominates the reorientation process. We will report at a later date electro-optic Raman measurements that appear to confirm this statement. LASER-INDUCED OPTICAL EFFECTS It has recently6 been shown that moderate laser powers may be used to induce textural changes in smectic polymer liquid-crystal systems.Although for an effective storage display laser addressing is normally combined with the guest-host effect,Plate 1. Stored optical texture at room temperature of PG296 for ( a ) the azo-dyed (blue) polymer (dye concentration ca. 3 wt%) and ( b ) the undyed polymer. [/.Cing page 208Plate 2. Comparison between the laser-written track, L, on a clear texture, C, and the scattering texture, S, when viewed through crossed polarisers. For the homeotropic texture, C, the laser track appears to be highly scattering against a clear background whilst for the scattering texture it appears to light against an opaque background when the polarisers are removed. EF denotes the electrode edges, and the beam diameter in the clear texture was 20 pm.H.J. COLES 209 1 ' 1 I i 1 I 1 1 I I 1 I I I I I I I I 1 20 50 100 200 V/V r.m.s. Fig. 6. Comparison of the electro-optic switching time, T,, for the pure polymer PG296 (*) and the anthraquinone-dyed polymer (*). Cell thickness was 25 fim in each case. this does not always have to be the case. Laser-induced realignment has been observed in the direction of polarisation of the laser in undyed samples, although in this case the experiments have to be carried out with the sample held at a temperature between T, and T,. For the guest-host systems the sample is held at room temperature and the temperature jump caused by the absorption of the laser beam gives rise to the induced textural change. By using clear electrically aligned regions of the polymer as well as randomly aligned scattering textures, it has been possible to write either positive or negative high-contrast displays (plate 2).These effects are stored unless the sample is heated back to the fluid-biphasic region or unless low-frequency a.c. fields are applied for a long time. Note that for the laser-addressed systems the response times are governed not by director reorientation mechanisms but by the absorption coefficients of the dyes and by the thermal heat spread within the sample. The measurement of such parameters suggests a fruitful research area since write times of the order of 20ps have been achieved for ca. 10 p m spot sizes and a 4 nJ pm-* energy density. POLYMER- MO NOM ER LIQUI D-CRY STAL SOLUTIONS As mentioned above, the nematic or cholesteric polymeric liquid-crystal materials appear to have a worse performance when included in electro-optic displays than their structurally equivalent monomeric counterparts.Generally the operating tem- peratures, viscosity and elastic constants of the polymers appear to be higher than for the monomers. However, the polymer property of exhibiting a glass transition does allow texture and optical information to be stored below Tg. It would seem that this is the area of electro-optics where such material will find the greatest210 ELECTRO-OPTIC EFFECTS IN POLYMER LIQUID CRYSTALS I I I I I 1 I I I 16 12 8 T, - T/"C 4 0 Fig. 7. (a) Splay, k l l , and ( b ) bend, k33, elastic constants as a function of temperature for various concentrations of PG296 in pentylcyanobiphenyl: 0, 0 wt% ; 0, 4.65 wt% ; A, 9.05 wt% ; *, 22.5 wtQ/O ; *, 38.3 wto/Q.usefulness. Recently a different approach has been adopted. Recognising that the slow response times in the polymers were linked to the ff exibility or fluidity of the polymer backbone, this viscoelastic property was used to modify the behaviour of monomeric liquid crystals. By using both Freedericksz transition methods and photon correlation spectroscopy, the dependences of k , , k33, k22/ y1 and A&/ E~ on polymer concentration were studied. In this notation kll, k22 and k33 are the splay, twist and bend elastic constants, y1 is the twist viscosity, and E , are the dielectric constants parallel and perpendicular to the nematic director direction and AE = - E,. The solute was PG296 and the monomeric solvent was pentylcyanobiphenyl (5CB).In the pure state, 5CB is nematic between ca. 22 and 35 "C. All of the solutions were nematic over the concentration range studied (Le. up to 40 wt'h).H. J. COLES I I I I I I I I 1 I 12 a 6 4 2 lo T,-T/"C 21 1 I I 1 I I I I I I 11 I I I I I I I 1 I 0 4 a 12 16 20 concentration (wto/o) Fig. 8. ( a ) Variation of the viscoelastic ratio ( k22/ yl) with reduced temperature for 0, pure SCB; 0, 2 wtoh and A, 6.5 wt% solutions of PG296 in 5CB. ( b ) Variation of A E / E ~ with polymer concentration (PG296 in 5CB) for different reduced temperatures T,- T : A, 4; 0, 8; 0, 12°C. The data for kll and k33 are shown in fig. 7, from which it can be seen that the polymer causes a noticeable decrease in the elastic constants.Similarly k22/ y1 decreases with increasing polymer concentration (fig. 8). This marked change is caused primarily by an increase in yl. Finally, A&/ cl also decreases with increasing polymer concentration as A& decreases and E, marginally increases. If the polymer solutions were used in a twisted nematic electro-optic cell, then with the exception of the behaviour of k22/ y l , the trends in k3J k, and A&/ E, would lead to a superior multiplexing capability and performance." The decrease in k22/ yl would lead to212 ELECTRO-OPTIC EFFECTS I N POLYMER LIQUID CRYSTALS an increase in response time. However, this increase in response time would still be c 100 ms and so acceptable in a complex display. Therefore, these polymers would appear to be an advantageous additive for monomeric liquid-crystal devices.DISCUSSION The above has summarised the results of four research areas in which side-chain polymer liquid crystals exhibit electro-optic effects. Attention has been concentrated on the smectogenic polysiloxanes, as they appear to exhibit desirable switching effects in both electric and laser fields and these effects may readily be stored, above T', in the smectic phase. Throughout the previous sections the importance of the polymer backbone flexibility has been stressed. It is important to state that the siloxane system was chosen because of its inherent high flexibility in comparison with the acrylate and methacrylate systems.' However, it is still possible to increase this flexibility further by changing the degree and type of substitution and the tacticity of the system.Furthermore, the length of the spacer group can be increased to decouple further the mesogenic side group from the main chain. These are all changes to the polymer structure that could be carried out to study the influence of backbone flexibility and coupling on the viscosity and response time of the system. A second polymer characteristic that would seem self-evidently important for the performance of the smectogenic systems is the poldispersity of the backbone chain. It is evident from the early work of Finkelmann" that different polymer fractions exhibit phase transitions at different temperatures below ca. 50 siloxane units. Since the polymer system of this work has a polydispersity factor (M,/M,) of ca.2 this suggests a reasonable content of both high- and low-molecular-weight fractions in this sample. This presumably explains the relatively broad biphasic region, highlighted by the thermo-optic analysis, since the lower-molecular-weight components will undergo a phase transition at slightly lower temperatures than the higher M , components. In a fraction recently prepared at Hull University, a much narrower thermo-optic curve was observed with T, - T, = 5 "C. Such fractions are being examined with the aim of gaining better understanding of the importance of polydispersity in the polysiloxane materials. This problem does not seem to have been considered in any of the previous studies on liquid-crystal polymers yet it is fundamental to polymer physics and chemistry and must also be fundamental for polymer mesophases.Liquid-crystal polymer dispersions containing both high- and low-molecular-weight components in differing proportions will lead to further optimisation of the thermal and electro-optic properties of these new materials. As an extreme of this reasoning, small percentages of monomeric materials have been added to various polymer liquid crystals and marked changes have been observed in the solution properties. Generally T, and Tg are both reduced and the response times are improved.6 This work emphasises the importance of the polymer polydis- persity and will be reported in greater detail at a later date. Whilst it appears from a polymer viewpoint that backbone flexibility and polydis- persity will have a major influence on the response behaviour of the polymer mesophase, it is also evident that the chemical structure and dielectric properties of the mesogenic side groups will play an important part.In the materials used in this work, the strongly dipolar cyano group has led to large electro-optic effects both in the pure material and in the solutions. The materials reported herein all have a positive A& and it would be possible to observe other electro-optic effects using negative materials. This could lead to novel two-frequency switching devices. Using strongly dipolar side groups the miscibility problems encountered in low heH. J. COLES 213 compounds have not been observed. Following Ringsdorf and Schmidt,12 it is possible to use polymerisable azo dyes as side groups on acrylate polymer backbones and obtain highly coloured guest-host systems with response times of the order of seconds.If this principle is extended to the polysiloxane systems a further decrease in response times should be observed. Such materials should have implications for both the guest-host effect and for the laser-addressed cells. It should, for example, be possible to use anthraquinone dyes absorbing in the near-infrared to obtain non-coloured laser-addressable displays of exceptionally high contrast and spatial resolution. Similarly, it is possible to use fluorescent or non-linear dyes6 to produce a wide range of laser-induced phenomena, in which the orientation of the optically ‘active’ side group may be controlled and stored in an electric or magnetic field.It is suggested that this will provide a major source of new electro-optic effects and displays in the near future. CONCLUSIONS It has been shown that, despite their high intrinsic viscosity, side-chain poly- siloxane liquid crystals may be used to demonstrate a variety of electro- or opto-optic switching effects. By carefully optimising the operating parameters (Le. voltage, frequency, temperature and cell thickness), electro-optic switching phenomena on the timescale of ca. 200 ms or less have been demonstrated. The textures induced by such electric fields have been shown to be stored above Tg in the smectic phase. This has allowed the operating temperatures of the devices to be lowered to an acceptable range. It has been shown that high optical contrast may be achieved in both undyed and dyed systems and that the guest dyes have no apparent influence on the electro-optic switching times.Using dyed textures of the same polymer system, high-resolution laser writing has also been demonstrated at room tem- perature. This high-contrast writing was also stored in the smectic phase. A further use of the polymers has been shown to be as additives to modify the elastic and viscotic constants of monomeric liquid crystals. Finally, the key polymer properties likely to lead to further improvements in the above electro- or opto-optic effects have been discussed. I thank Dr Richard Simon, Mark Sefton and Tony Hopwood, with whom much of this research was carried out. I also thank Professor George Gray F.R.S.and colleagues at Hull University for kind provision of the polymer samples, and the S.E.R.C. for support of this research through the electro-active polymer scheme. ’ C. Robinson, Trans. Faraday SOC., 1956, 52, 571; Discuss. Faraday SOL, 1957, 25, 29. W. R. Krigbaum and H. J. Lader, Mof. Cryst. Liq. Cryst., 1980, 62, 87. A. Blumstein, S. Vilasagar, S. Ponrathnam, S. B. Clough and R. B. Blumstein, J. Polym. Sci, PoZym. Phys. Ed., 1982; 20, 877. H. Finkelmann and H. Ringsdorf, Makromol. Chem., 1978, 179, 273. H. Ringsdorf and A. Schneller, Br. Pofym. J., 1981, 13, 43. H. J. Coles and R. Simon, Br. Patent. 8422705, 1983. H. Finkelmann, V. Kiechle and G. Rehage, Mof. Cryst. Liq. Cryst., 1983, 94, 343. V. P. Shibaev and N. A. PlatC, Adv. Polym. Sci., 1984, 60/61, 173. ’ H. Ringsdorf and R. Zentel, Makromol. Chem., 1979, 180, 803. l o H. Finkelmann, Phifos. Trans. R. SOC. London, 1983, 309, 105. ” V. P. Shibaev, S. G. Kostromin, N. A. Plat&, S. A. Ivanov, V. Ju. Vetrov and I. A. Yakovlev, l 2 H. Ringsdorf and H-W. Schmidt, Makromol. Chem., 1984, 185, 1327. Polym. Commun., 1983, 24, 364. G. H. Heilmeier, J. A. Castellano and L. A. Zanoni, MoZ. Cryst. Liq. Cryst., 1969, 8, 293. 13214 ELECTRO-OPTIC EFFECTS IN POLYMER LIQUID CRYSTALS P. A. Gemmell, G. W. Gray and D. Lacey, Mol. Cryst. Liq. Cryst., in press. H. J. Coles and R. Simon, in Recent Advances in Liquid Crystalline Polymers, ed. L. L. Chapoy (Plenum Press, London, 1984), chap. 22, p. 323. H. J. Coles and R. Simon, Mol. Cryst. Liq. Cryst. Lett., 1984, 102, 75. 14 l 5 R. Simon and H. J. Coles, Mol. Cryst. Liq. Cryst. Lett., 1984, 102, 43. 16 17 l 8 H. J. Coles and R. Simon, Mol. Cryst. Liq. Cryst. Lett., in press. l9 A. I. Hopwood and H. J. Coles, Polymer, to appear in the Proceedings of the September 1984 2o M. Sefton, A. R. Bowdler and H. J. Coles, Mol. Cryst. Liq. Crys?., in press. conference on Speciality Polymers.

ISSN:0301-7249    

DOI:10.1039/DC9857900201

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1985 79, 215-224 Defects and their Relationship to Molecular Configurations in Nematic Polymers BY MAURICE KLEMAN Laboratoire de Physique des Solides (associ6 au C.N.R.S.), Universitk de Paris-Sud, 91405 Orsay Cedex, France Received 15th January, 1985 Some recent observations of defects in main-chain nematic and chiral polymers are reviewed in order to obtain a better understanding of the nature of molecular configurations. Important parameters are (a) the chain length, the density of free ends and the length of flexible spacers and (b) the orientational correlations between chains. The first topic, (a), is discussed with respect to observations of defects in uniaxial nematics; the second, (b), is discussed in the light of observations of biaxial nematics and of cholesteric textures of rigid polymers of biological interest. The elements of a geometrical (structural) model which includes local competitions between coiling and orientational correlations and which uses the methods developed for the representation of frustration in curved spaces are given.Defects in small-molecule liquid crystals (SMLC; typical molecular length 30 A) have been studied actively for more than ten years, and had already attracted the attention of physicists by the beginning of this century.' It was the observation of defects with the help of the polarizing microscope'.* which started a long era of fruitful research that has since benefited the whole field of condensed-matter physics. Liquid-crystal polymers (LCP) are vastly different from SMLC and also very different from one another.If they do present any analogies with SMLC and among them- selves, it is at the structural level (the same symmetries and the same type of order parameters). This implies the same topological properties for the defects but not the same energetical properties. Defects are indeed characteristic breaks in local symmetry3 of the order parameter and can be distinguished one from another by (a) their dimensionality (point defects, line defects or surface defects) and (6) the particular type of symmetry they break. The same structure in nematic SMLC and nematic LCP therefore imply equal topological stabilities for disclination lines of the same strength, S, or point defects of the same topological charge.Similarly, dislocation lines of the same Burgers vector are equally topologically stable in SMLC smectics and LCP smectics etc. However, things may be very different with regard to energetical stability; i.e. (a) the relative occurrence of defects of a given type (each defect, which is a metastable configuration of the order parameter, carries a positive energy which depends on the stiffness constants of the material); (6) the nature of the core of the defect, i.e. of this region, whose size is generally a coherence length 5, and where the order of the liquid crystal is replaced by the (dis)order of the higher-temperature phase, but which can suffer various types of arrangements; (c) the mobility of the defect, which always involves molecular processes etc. Also, mutual organization of defects (textures) depends on energetical considerations and physicochemical conditions (boundaries).The same ingredients enter the observa- tional study of instabilities, flow properties etc4 While the observations of defects in SMLC has quickly proved rewarding' and although many observations have already been made on LCP, very few conclusions 215216 DEFECTS AND MOLECULAR PROPERTIES CONFIGURATIONS of a general nature have been reached, except the discovery that the observed defects pertain to the expected topological classes. The experiments are difficult because of the high temperature range in which many of the compounds exist and because of their high viscosity: equilibrium textures are obtained long after the sample has been brought to the temperature of observation.Also, most of the compounds are polydisperse in a way which is not usually known, so that the transition temperatures between the various phases are badly defined and two-phase regions often occur. Hence we are often led to make investigations on ill-defined products. This situation makes all the more interesting future studies on defects in mixtures of mesomorphic polymers in their parent monomer which are dilute enough for us to consider that the polymers behave as independent macromolecules. This paper will mainly discuss observations which have been made on defects and textures in main-chain polymers (either thermotropic or lyotropic) and will leave aside important questions relating to side-chain polymers and block copoly- mers, where the problems of molecular structure are of a different nature.In block copolymers the preferred affinity of each segment along a chain for a segment of the same nature, or for some specific solvent, makes this class of chemicals more akin to lyotropic In side-chain polymers the mesomorphic properties are carried by the small side-chains: the question of the nature of the order parameter is then similar to that in SMLC (observations’ show that, at first sight, most of the properties of defects met in SMLC are found in side-chain LCP), while the viscoelas- tic properties’ are ruled by the conformational properties of the polymeric backbone, which are still quite obscure, as well as the nature of the coupling between the backbone and the side-chain.This paper is divided as follows: in the first section we review recent observations onbdefects in nematics and interpret them, as far as possible, in terms of classical geometries for the director (splay, bend and twist). Frank’ has recently insisted on the fact that the experimentally defined optical axis n ( r ) is not necessarily the best physical molecular axis; this is true in SMLC, and even truer in LCP, where the essential new question is the nature of the orientational coupling between semi- flexible chains. The recent discovery of a biaxial LCP nematic illustrates Frank’s remark, and we stress the interest of a detailed study of defects in these new media. In the second section we discuss observations in cholesterics made of rigid chains in solutions and interpret them in terms of a competition between two incompatible tendencies: local two-dimensional ordering and cholesteric order.This brings us in the third section to a more general discussion of the concept of ‘frustration’ in systems of flexible chains; in particular we propose a geometrical representation in a curved space of the competition between local coiling and two-dimensional (or nematic) ordering, which might be of interest as a basic tool in studying molecular correlations not only in nematic LCP but also in polymer melts. DISCLINATIONS AND WALLS SUMMARY OF THE THEORY OF DISCLINATIONS AND WALLS IN NEMATICS The free energy of a disclination line of strength S, S being the number of times the director rotates by an angle 2.n about the line, is typically of the form R rc W = vKiS2 In -+ W,M. KLEMAN 217 S = * ' / 2 S=+l S=+l s=-1 s= +3/2 Fig.1. Wedge disclination lines: two-dimensional representations in planes perpendicular to the lines. c * t - - _= 1 .... t ......... --I 4 4 4 4 Fig. 2. Twist disclination lines (use is made of the nail convention to represent directors at an angle to the plane of the cut3). Fig. 3. Cut in a plane containing the line S = + 1 without a singularity. (a) Singularity present; (b) no singularity after escape in the third dimension. where Ki is the Frank constant involved in the deformation due to the line ( i = 1,2,3 or Ki is some function of K , , K2 and K 3 ) , R is a typical distance between defects, r, is the core radius and W , is the core energy per unit length of line defect.Fig. 1 shows, after Frank," typical arrangements of wedge disclinations (here wedge qualifies the fact that the axis of rotation of the molecules is about the line itself) and fig. 2 gives a typical arrangement of a twist disclination (twist for a director rotating about an axis perpendicular to the line). The free energy varies as the square of the strength, which favours lines of half-integral strength S = *$. In fact in SMLC the most frequent disclinations have S = *l. This happens because of a possible 'escape in the third dimension' (R. B. Meyer) of the director, in such a way that the singularity of the order parameter in the core vanishes. This218 DEFECTS AND MOLECULAR PROPERTIES CONFIGURATIONS phenomenon is topologically possible for S integral, but forbidden for S half- integral.3 Fig.3 represents part of a S=+l wedge line where the deformation involves splay ( Kl), bend ( K 3 ) but no twist ( K 2 ) . Other geometries exist.3 The free energy per unit length of an integral line reads W=27i-KlSl (2) and can be definitely smaller than eqn (1) if K1, K2 and K3 are of the same order of magnitude ( K is some function of K1, K2 and K3). This is what happens in SMLC, but lines of half-integral strength can be stable against three-dimensional perturbations if some inequalities are reached. More precisely," if K2 > ;( K1 + K3), wedge lines S = *; (as in fig. 1) are stable; if K,<$(K,+ K 3 ) , twist lines IS(=; (as in fig. 2) are stable. The core of a half-integral line is a region where the order parameter is broken, and is described as a true (isotropic) liquid in SMLC.Its radius is a few molecular lengths far from the clearing point T, but increases without limit at T'. When K1 and/or K3 are much larger than KZ, a singular core can be favoured in an integral line. r,, by definition, is always of the order of the coherence length 6, which is ultimately the only characteristic length for an isolated disclination line with a singular core. For a non-singular disclination the energy per unit length does not depend on R [eqn(2)]. Application of the theory to the special case of LCP has been carried out by Meyer;'* in particular, he discusses how the chain length, the density of free. ends and the flexibility affect the magnitude of the coefficients K I , K2 and K3.Walls are not topologically stable defects. They generally occur when surface anchoring is in competition with the bulk tendency to homogeneity. A typical wall width is therefore a penetration length 6 = K / W s (3) where W, is a surface anchoring energy. When the sample thickness is larger than 6 the wall generally splits into surface disclinations. 0 BS E RVATIO NS The first systematic observations were made in sulphuric acid solutions of various aromatic polyamids by Skoulios and They observed a large number of disclinations in the form of very mobile thin threads, displaying frequent sharp- cornered points. This is certainly related to the large anisotropy of the elastic constants. The authors interpret their observations as demonstrating a large value of K3 (as expected for rigid chains with a large persistence length), a reasonably small value of K1 (splay deformations being easily achieved by the diffusion of chain ends) and claim most of the defects to be of integral strength with a radial singular core structure (fig.1). KlCman et dL5 have investigated thermotropic nematic polymers belonging to a series of polyesters differing in the length of the flexible alkyl group -[CH21n- inserted in the monomer. For large n ( n = 14 say) the viscosity is extremely high and the sample does not anneal at all during the observations; the local orientation does not vary even after long reheating in the isotropic phase. However, X-ray diffraction gives evidence of the existence of the nematic phase; the molecules are probably tightly correlated by coiling around one another (hence there is no anneal- ing), and these coilings subsist in the isotropic phase while the extent of the correlations decreases.For n = 5 the texture varies with the degree of polymerization.M. KLEMAN 219 For shorter chains ( M == 1000) one observes a typical SLMC thread texture, with thin lines of strength IS1 = $ and thick lines of strength IS( = 1. In the course of time the threads have a tendency to disappear and give way to well resolved Friedel's nuclei (pluge 6 noyuux, with only integral lines). However, for longer chains ( M = 10000 and more) there are very few integral lines or nuclei, and most of the defects are half-integral lines, either loops floating in the bulk and tending to collapse in a few minutes after their appearance (in which case they have mostly a twist character, as in fig.2) or lines attached to either the glass covers or (in free droplets) the free boundary. In this last case one can observe directly that the size of the cores of S = +$ is much larger than the size of the cores of S = -$ lines.7 The interpretation is qualitatively as follows: Kl is large in this product, and the core of S = +$ lines are clusters of chain ends (all the necessary splay deformation tends to be concentrated here); chain ends accumulate differently near a S = -$ line, where the chains align parallel to the core on a three-branched star (plate 1). Electron-microscopy observations would be useful in understanding this question better. Surface lines are also observed, and in their vicinity are seen clear phenomena of crystallization induced by the boundary conditions.Fayolle et all6 have studied a slightly different polyester and corroborate some of these observations. The same polyester^'^ also display 90" Bloch walls (to use terminology borrowed from the study of ferromagnetic walls) in which the molecules rotate about an axis perpendicular to the wall (plate 2). In these walls the gradient of the molecular directions is practically pure twist, which indicates that K2 is small (relative to K 1 and K3). Finally, walls can be observed in the geometries of the Freederickx tran~iti0n.I~ They have been studied in the C5 polyester recently;'* in the K , geometry they bind domains of elliptic shape; the ratio of the major to the minor axes of the ellipse gives K , / K2 = 10.This ratio is confirmed by direct Freederickx measurements ( K1 == 3.10-6 dyn; K2 == 3 x lov7 dyn). K1 is an order of magnitude larger than K2 in this compound. K3 is more difficult to measure (because of chemical degradation) but is larger than K2 and very probably smaller than K1 [see ref. (18)]. BIAXIAL NEMATICS Investigations of a random copolyester (B-ET) have seemingly shown the existence of two nematic above T,, = 340 "C and below Tc2 =r 350 "C it is an uniaxial nematic, with numerous IS1 = --; and less numerous IS] = 1, as in the above polyesters; below 340 "C only IS1 = 1 defects and walls are present. Much evidence indicates that this second phase is biaxial, i. e. that orientational correlations between molecules exist not only between their long axes but also between their short axes; hence benzene rings in adjacent molecules tend to stack parallel to each other.It is reasonable to assume that, above some temperature Tcl, freedom of rotation along the major axis is recovered, and that the existence of longer flexible spacers along the chain lowers Tc1. Note that B-ET has very small spacers (C, compared with the C5 polyester mentioned above). The question of defects is very different in uniaxial and biaxial nematics;2' each of the three axes of the molecule plays the role of a director and has associated defects, but only defects of even integral strength can 'escape in the third dimension'; defects of odd integral strength are all topologically equivalent, which means that any defect of this type associated with a given 'axis' can turn continuously towards the more favourable 'axis' configuration.This is important essentially for the core region. Such a possibility does not exist for the half-integral disclinations, which220 DEFECTS AND MOLECULAR PROPERTIES CONFIGURATIONS are of three different types. This might be related to the fact that half-integral disclinations have not yet been observed. (Conversely, the absence of integral defects in the C5 polyester^'^ points towards the existence of a true uniaxial nematic.) Also, defects in a biaxial nematic are topologically isomorphic to the elements of the quaternion group ; without entering into details, this property implies that two defects of half-integral strength belonging to two different classes cannot cross without the appearance of a third defect joining them.This obstruction to crossing should play an important role in the rheology of these phases, which must appear effectively more viscous when half-integral defects are present. DEFECTS AND TEXTURES IN CHOLESTERIC RIGID POLYMERS In solution DNA, PBLG, xanthan, collagen and other polymers of biological interest display characteristic cholesteric phases whose defects have been extensively studied recently. Most of these defects are similar to those observed in usual thermotropic SMLC cholesterics. However, there are a number of situations where particular defects or textures are observed; this is the case for Dinoflagellate chromosomes, decondensed chromatin in water, precholesteric phases of sonicated DNA and some large-scale arrangements (self-assemblies) of these molec~les.~~-*~ This point is worth considering in more detail, since it relates directly to how the local correlation of molecular conformations extends at large distances ; in particular, all these molecules, which can be very long (indeed ‘infinite’ for DNA in chromatin), display local two-dimensional order (seen by X-ray diffraction), which is crystal- lographically incompatible on a large scale with cholesteric order.This ‘frustration’ is relieved for distances of the order of the pitch of the defects and textures described below. COLLAGENz4 AND DECONDENSED DNA2372S The long molecules are arranged in bundles of lines twisted along their length (fig.4). The central molecule is straight, while the others rotate helically about it with a constant pitch equal to the pitch of the cholesteric phase. There is also helical rotation of the molecules along any radius of the bundle. Therefore the configuration is doubly twisted and, as can easily be shown, splayless. It is therefore favoured where K , is very large, as one might expect for molecules which are extremely long (having no free ends available in large numbers) and rigid (hairpins have a very high energy); it is also favoured (because of double twist) when KZ4, the saddle-splay constant, is large and positive. (It can be shown that a large K24 favours the nucleation of double-twist configurations.) The analogy with the blue phases (local configurations of SMLC) is striking:26 blue phases of SMLC are stable when the ratio t / p is large, 5 being some correlation length which scales necessarily with the length of the molecules (for SMLC) or the persistence length Zp (for LCP). The question of the presence of two-dimensional ordering is more novel and would require a larger development than allowed here.Let us just indicate that it is possible to show that the molecules are at the intersection of two sets of orthogonal surfaces, so that there is locally two-dimensional ordering; these surfaces are not equidistant (except in the vicinity of the central molecule) and the two-dimensional ordering is strained. It can be any two-dimensional local order, on short distances (see previous section), or nematic order (which is a particular case of two- dimensional ordering).In this respect the blue phases of SMLC appear as resulting from competition between nematic and cholesteric ordering.M. KLEMAN 22 1 f ' 1 IZ Fig. 4. The cylindrical geometry in decondensed DNA, collagen, or in a blue phase. 'i I I Fig. 5. Geometrical model for I I I I Z I the Dinoflagellate chromosome. Molecular configuration drawn in a straight section. THE DINOFLAGELLATE23*27 CHROMOSOME This is also a twisted cylindrical configuration, but orthogonal to the former one (fig. 5). On the axis the molecular directions are horizontal (perpendicular to the axis); they rotate about the axis with a pitch p equal to the cholesteric pitch and generate a helicoid; the other directions of helicity are along the normal to this helicoid.The total configuration is therefore doubly twisted, as in the previous example, but presents two helical defects on the periphery of the cylinder which are two S = ++ disclinations (along C, and C,, fig. 5 ) , corresponding to cuspidal lines on the focal surface of the helicoid. The two sets of orthogonal surfaces along which the molecular directions lie are (a) the helicoid and the surfaces parallel to it and (6) a set of hyperbolic paraboloids. The geometry is limited to the cylinder by the addition of other peripheral disclinations (fig. 6). Although experimentally the chromosome of Dinoflagellate (Prorocentrum micans) has an aspect ratio (pitch over radius) which differs from the mathematical model described here, this model probably offers a good basis for understanding the chromosome configuration (fig 6), which has also been proposed independently by F~-iedel*~ and which fits with the crude model first proposed by Bo~ligand.~~.~' In particular, the mathematical model contains splay, in the form of hairpins, for an infinitely long molecule and implies that K3 and K1 are large compared with222 DEFECTS AND MOLECULAR PROPERTIES CONFIGURATIONS L- J Fig.6. Two-dimensional vertical cut. The disclinations D, and D2 have been added in order to obtain a bounded geometry. KZ. Also it can be shown that the nucleation of the configuration is favoured if there is a strong local tendency to true two-dimensional ordering, with an elastic shear modulus p large compared with K2/ bp, where b is the mean distance between molecules.DNA in chromatin is a complex chemical species whose interactions with the proteins of the matrix can satisfy the above requirements. Moreover, local helix-coil transitions can explain the ease of formation of hairpins. Finally, a description of the chromosome in terms of defects might be relevant to the still unknown processes which, on a semi-macroscopic level, occur during cell division. SELF-ASSEMBLIES The structural elements described above can pack together and form ordered or disordered arrangements on higher scales.22 For example, the cylinders of the blue phase assemble into either cubic crystals or amorphous systems, the regions between the cylinders being filled with disclinations. There are many reasons to believe that similar processes exist for polymeric structural elements.What has been described in the previous sections is a cholesteric self-assembly of local hexagonal packings. Self-assemblies with very large pitch of large hexagonal packings of PBLG have also been observed. One can expect, when the self-assembly is disordered, that the process is hierarchical ; at each scale a geometrically well characterized assembly of elements of the lower scale is built. Such concepts of hierarchical ‘frustration’ appear naturally in a theory which introduces a curved-space des~ription:~~ the local competitions (two-dimensional versus cholesteric) which are incompatible with three-dimensional homogeneous euclidean space-filling are reconciled in a three-dimensional curved space, where they build a ‘crystalline’ arrangement. The local arrangement is a projection in flat space of a small piece of the crystal in curved space ; the disclinations which separate the structural elements assembling at a higher scale are the projections of the disclinations in the curved-space crystal.Such a theory is well established for amorphous metals;30 we will now summarize some recent attempts in the same direction made for long flexible molecules. FRUSTRATION IN ASSEMBLIES OF LONG FLEXIBLE MOLECULES3’ In the frame of the theory, the blue phase is an euclidean projection of a sphere S3 (the three-dimensional sphere in four-dimensional euclidean space, i. e. a space of positive curvature R ) endowed with a regular (crystalline) arrangement of directors possessing intrinsically the property of double twist.This same arrange- ment can be given, with a trivial addition, the property of global or local two- dimensional ordering, in many different ways, because the lines of force of theM. KLEMAN 223 directors in S3 are equidistant lines. Now, place long flexible molecules along these lines of force; in S3 they form a set of equidistant and mutually twisted great circles of pitch p = * 2 r R (right or left great circles, according to choice; in spherical geometry these equidistant great circles are called Clifford parallels or paratactic lines). Locally the arrangement is similar to the local arrangement in decondensed DNA or in a Dinoflagellate chromosome. However, a detailed study of all possible local arrangements in S3 enables us to enlarge our point of view. We give only the results.S3 can be described as a fibrous bundle of equidistant right or left great circles, each attached at a different point of an ordinary sphere S2. This is the celebrated Hopf theorem. The structural arrangement of the flexible molecules can therefore be depicted by the mutual arrangement of a set of points on S2. The mapping is such that all the molecules located at a distance d = R8 (0 8 d r / 2 ) of a given molecule map onto a small circle at a distance 28 from the pole of S2, this pole representing the central molecule. Therefore (a) the densest regular packings of molecules in S3 are represented by regular deltahedra on S2, Le. the vertices of an equilateral triangle on a great circle of S2 of a tetrahedron, an octahedron or an icosahedron, (6) other regular packings, represented by the vertices of a cube or a dodecahedron, are not dense packings and ( c ) all other packings of equidistant molecules have no long-range order, since they are not represented by the vertices of platonician solids.Note that the hexagonal packing appears as a special case of regular deltahedra, for six neighbours, i.e. R infinite. The number of neighbours is always (6 for all the cases (a) and (6). It can be any number in case ( c ) , which also includes anisotropic packings with orientational correlations on any given length. Note that our description of frustration in assemblies of long flexible molecules does not have to be specialized to chiral molecules.In fact it also applies to molecules whose flexion has an entropic origin and is locally either left- or right- handed. In particular it leads to a new way of thinking of local arrangements in molten polymers, at a scale 5 larger than the persistence length I,, but smaller than the radius of gyration. Assume for example that each flexible molecule has n = 5 neighbouring molecules and that d is their diameter. In S3 the condition of dense packing with n = 5 leads to p = 11.234. . . d (these figures would be slightly different when projecting in this ‘spherical’ set of strands in flat space); p is therefore this scale 6, which defines the length along which the molecules assemble in a string whose lateral size is of the same order of magnitude. This is not in contradiction to Flory’s conception of molten polymers, but rather superimposed on its model at a scale 6, by achieving local density requirements and using entropic effects which appear at all scales.The same concepts may apply to the isotropic phase of semi-flexible mesogenic polymers as long as the persistence length lp is smaller than the length of the stretched molecules. An isotropic phase would then be described by a local order of one of the types discussed, and locally ordered domains separated by disclinations or walls (for domains of opposite chirality) ; the mobility of the liquid phase would therefore involve the mobility of these defects. Note that in all cases considered (except the true uniaxial nematic case with no positional correlations) there is always obstruction to the crossing of line defects.Finally the biaxial nematic might also be locally described in the framework of our case (c), since a finite coherence length of biaxial correlations has been found. I thank Dr A. M. Donald, Prof. J. Friedel, Dr M. R. Mackley, Dr A. Skoulios, Prof. M. Veyssik and Dr A. H. Windle for discussions.224 DEFECTS AND MOLECULAR PROPERTIES CONFIGURATIONS ' G. Friedel, Ann. Phys. (Paris), 1922, 18, 273. 0. Lehmann, Hussige Kristalle (W. Engelmann, Leipzig, 1904). M. KlCman, Points, Lines and Walls (Wiley, Chichester, 1983). M. KlCman, in Dislocations 1984, ed. P. Veyssibre, L. Kubin and J. Castaing (Editions du C.N.R.S., Paris, 1984). B. Gallot and A. Douy, in Quelques Aspecrs de I'Etat Solide Organique, ed. J. P. Suchet (Masson, Paris, 1972).F. Candau, F. Ballet, F. Debeauvais and J. C. Wittmann, J. Colloid Interface Sci., 1982, 87, 356. ' F. Lequeux, unpublished work. G. Mazelet, work in preparation. a P. Fabre, C. Casagrande, M. VeyssiC and H. Finkelmann, Phys. Rev. Lett., 1984, 53, 993. F. C. Frank, Philos. Trans. R. SOC. London, Ser. A, 1983,309, 71. lo F. C. Frank, Discuss. Faraday SOC., 1958, 25, 1. l 1 S. I. Anisimov and I. E. Dzyaloshinskii, Sou. Phys. JEW, 1972, 36, 774. l2 R. B. Meyer, in Polymer Liquid Crystals, ed. A. Ciferri, W. R. Krigbaum and R. B. Meyer (Academic l 3 M. Arpin, C. Strazielle and A. Skoulios, J. Phys., 1977,38, 307. l4 B. Millaud, A. Thierry and A. Skoulios, J. Phys., 1978, 39, 1109. 15 M. KlCman, L. Liebert and L. Strzelecki, Polymer, 1983, 24, 295. l6 B. Fayolle, C. Noel and J. Billard, J. Phys., 1979, 40, C3-485. l7 F. Brochard, J. Phys. (Paris), 1972, 33, 607. l9 M. R. Mackley, F. Pinaud and G. Sickmann, Polymer, 1981, 22, 437. 2o C. Viney and A. H. Windle, J. Muter. Sci., 1982, 17, 2661. 22 F. Livolant Thesis (University of Paris, 1984). 23 F. Livolant and Y. Bouligand, Chromosome, 1980, 80, 97. 24 Y. Bouligand and M. M. Giraud, in Symp. Collagen Invertebrates (C6me, 1984). 25 M. KlCman, J. Phys. (Paris), submitted for publication. 26 S. Meiboom, M. Sammon and W. F. Brinkman, Phys. Rev, A, 1983, 27, 438. 27 J. Friedel, in Roc. EPS 6th General Con$ (Prague, 1984). 2a Y. Bouligand, J. Phys., 1969, 30, C4-90. 29 M. Kltman and J. F. Sadoc, J. Phys. Lett., 1979, 40, L-569. 30 R. Mosseri and J. F. Sadoc, in Structure of Non-Crystalline materials 82, ed. P. H. Gaskell, E. A. Davis and J. M. Parker (Taylor and Francis, London, 1983); D. R. Nelson, Phys. Rev. B, 1983, 28, 5515. Press, New York, 1982). Sun Zheng-min and M. Kliman Mol. Cryst. Liq. Cryst., 1984, 111, 321. G. Toulouse, J. Phys. (Paris) Lett., 1977, 38, L-67. 21 31 M. KlCman, work in preparation. 32 J. P. Sethna, D. C. Wright and N. D. Mermin, Phys. Rev. Lett., 1983, 51, 467.Plate 1. Half-integral lines in a free droplet of a C5 polyester (courtesy of G. Mazelet). The two terminating configurations of each line on the free surface are, respectively, +; and -;. There is a clear three-fold starred contrast at each --+ (circularly polarized light). Plate 2. A 90" wall in the C5 polyester separating homeotropic region from a planar one; ( b ) schematic drawing of the configuration in the wall. [facing page 224

ISSN:0301-7249    

DOI:10.1039/DC9857900215

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

GENERAL DISCUSSIONt Dr R. Zentel (University of Mainz, West Germany) said: I would like to make two comments on the paper presented by Dr Noel. First, we have carried out dielectric relaxation measurements on liquid-crystalline side-group polymers’ and were able to determine up to five different relaxation processes, depending on the molecular structure. One of these is the so-called ?‘-relaxation, a relaxation of the alkyl-group spacer between the polymer chain and the mesogenic groups. For spacers of six methylene groups it is active and comparable to your y-relaxation. However, if one shortens the spacers to only two methylene groups, it is no longer possible. Secondly I would like to comment OII the fact that Dr Noel was able to supercool the isotropic-liquid-crystalline transition for her polymers.This is very unusual in low-molar-mass liquid crystals, because just below the observed first-order transition, isotropic-nematic or isotropic-smectic A or C, there is a hypothetical second-order transition to the same phases. Thus at the phase transition temperatures, there are still long-range correlations and the fluctuations are strong.2 Therefore one does not have problems with nucleation. Now if one can supercool the transition for liquid-crystal main-chain polymers, there may be two reasons. First the viscosity is higher for polymers. However, more importantly, Maret and Blumstein found3 that for main-chain polymers the hypothetical second-order phase transition temperature is far below the observed first-order transition.Therefore the fluctuations are still small at the transition temperature and one has similar problems with nucleation as for a crystallization. Does Dr Noel think that this may be a correct explanation? ’ R. Zentel, G. Strobl and H. Ringsdorf, Macromolecules, 1985, 18, 960; H. Ringsdorf, G. Strobl and R. Zentel, 29th Symp. on Macromolecules, Bucharest-Romania, September, 1983, Abstracts, section IV, p. 27. ’ See for example: P. G. de Gennes, The Physics ofLiquid Crystals (Clarendon Press, Oxford, 1975). A. Blumstein, Polym. J., 1985, 17, 277. Dr C . Noel (ESPCI, Paris, France) replied: Dr Zentel has made an interesting comment on the intramolecular motional processes occurring in thermotropic liquid- crystal polymers in the glassy state. Dr Lauprctre and I have also investigated molecular motions in side-chain liquid-crystal polymers by an e.s.r.spin-probe technique. ly2 Different relaxation processes have been observed depending on the molecular structure. In the glassy state, one relaxation process is found in all samples investigated and can be assigned to internal motions of mesogenic groups. Another depends on,the length of the flexible spacers and must arise from a local motion of thefCH,), units. Support for these assignments has been obtained for the polymer f CH- CH, t 0’ ‘O+CH,)t,--CO-O 0 0 CN using 13C solid-state n.m.r. techniques.2 From proton-decoupled 13C n.m.r. lineshape analysis of non-spinning samples, it appears that the spectrum of the carboxy carbon I C w t Plates 1-3 face p. 228. 225226 GENERAL DISCUSSION adjacent to the main chain is that of the rigid limit in the glassy state.In contrast, the reduction of the chemical-shift anisotropy of the protonated aromatic carbons indicates fast oscillations of the phenyl rings about their symmetry axis. The increase in magnetization as a function of the contact duration in a spin-lock cross-polariz- ation experiment using magic-angle spinning shows that the main-chain carbons are frozen within the timescale of the experiments ( lo5 Hz). In contrast, oscillations on the valence cone of ca. 20" about one equilibrium conformation are observed for the three central methylene carbons of the flexible spacers. This suggests that jumps between two equilibrium conformations as observed for the y-relaxation of the main-chain polyester TO1 1 require flexible spacers containing more than five CH2 groups.' P. Le Barny, J. C. Dubois, C. Friedrich, F. Lauprgtre, C. Noel and L. Monnerie, IUPAC Int. * G. Decobert, J. C. Dubois, S. Lukovic, C. Noel and L. Monnerie, IUPAC Int. Symp. Non-crystalline Symp. Non-crystalline Order in Polymers, Naples, May, 1985. Order in Polymers, Naples, May, 1985. Prof. G. Williams (University College of Wales, Aberystwyth) said: Dr Zentel has indicated that multiple dielectric loss processes are observed for liquid-crystalline side-chain polymers. In his assignment of the mechanisms for the individual process he has suggested that the process observed at the highest temperatures (say in a plot of loss factor against temperature at a fixed frequency) is due to the end-over-end motion of the side chain with respect to the main chain.It seems likely that the topological constraints of having the flexible alkyl spacer attached to the main chain and having local angular correlations between side-chain mesogenic groups would make it difficult for such a 'flip-flop' motion to occur. Indeed, it is possible that in small-molecule nematics (e.g. alkylcyanobiphenyls) that the well defined principal dielectric relaxation process in the MHz range, usually assigned as a flip-flop motion in the P2 nematic potential, is not of that origin. Several alternative mechanisms may be proposed which would accommodate the main dielectric process in both monomeric and polymeric (side-chain) nematics. One possibility is that in both systems the dipolar mesogenic group is able to move in an effective 'cone' prescribed by the neighbouring mesogenic groups. Such a motion may be modelled by the Warchol-Vaughan- Wang-Pecora model of rotational diffusion, as we have applied it to lyotropic-nematic rigid-rod polymer systems,' but would give a reduced relaxa- tion magnitude compared with that for isotropic motion.Experimentally it is found for alkylcyanobiphenyls2 that the apparent Kirkwood g-factor is ca. 0.66, which might be interpreted as being due to motions of mesogenic groups limited to an effective cone. The total relaxation magnitude would then result from a combination of limited one-body motions and a Kirkwood factor representing the preferred angular correlations of the dipole vectors in the nematic phase.J. K. Moscicki and G. Williams, J. Polym Sci., Polym Phys. Ed., 1983, 21, 197, 213. 2, 1976, 72, 1447. * M. Davies, R. Moutron, A. H. Price, M. S. Beevers and G . Williams, J. Chem. SOC., Furuday Trans. Prof. A. Blumstein (University of LoweEZ, U.S.A.) said: By analogy with the observation by Noel et al. of two distinct glass-transition temperatures ( Tg) for the smectic and isotropic domains of the polyester TO1 1, I would like to mention that the polyester DDA-9 (see our paper) also displays two distinct 7'' values. Both can be observed on samples of DDA-9 only after quenching the isotropic phase in liquidGENERAL DISCUSSION 227 nitrogen. The Tg values observed are at -3 to -7 "C and 13-15 "C and are attributed to the isotropic and nematic domains, respectively.They are the result of quenching of the isotropic and nematic phases in DDA-9. The simultaneous quenching of an isotropic and a nematic phase is a rather rare occurrence, and may be due to the cybotactic nature of the nematic phase of DDA-9. Indeed, we have not been able, under similar conditions, to quench the isotropic phase of MA-9 (again see our paper), which displays a normal nematic mesophase. This difference in the super- cooling behaviour in the homologous series of polyesters between the 'even' and 'odd' specimens can be understood from table 1 of our paper. In that table, high values of the order parameter S are related to high values of supercooling (samples with n even) and vice versa. One can also remark that for DDA-9 ( n = 10) the N/I transition is more first order than for MA-9 ( n = 7 ) , with values of T,- T* = 27 "C for DDA-9 as compared with 14.5 "C for MA-9 and only 5°C for p-azoxyanisole.' ( T , is the clearing temperature and T* is the virtual second-order transition temperature.) These results are in agreement with our proposed model of two distinct molecular nematic organizations for the even and the odd polyesters containing the 4,4'-dioxy- 2,2'-dimethylazoxy mesogen.2 ' G.Maret, Am. Chem. SOC., PoZym. Prepr., 1983, 24, 2, 249. ' A. Blumstein, R. B. Blumstein, M. M. Gauthier, 0. Thomas and J. Asrar, Mol. Cryst. Liq. Cryst. Lett., 1983, 92, 87. Dr C . Noel (ESPCI, Paris, France) said: I would like to make a further comment. We completely agree with the points raised by Prof.Blumstein and Dr Zentel: the simultaneous quenching of an isotropic and a liquid-crystalline phase is a rather rare occurrence. First, we would like to recall that Frosini et al.' were the first to detect two glass transitions in some thermotropic polyesters based on a,o-bis(4- hydroxybenzoy1oxy)alkanes and terephthalic acid. In agreement with our data, the thermal and dynamic viscoelastic behaviour of these polymers suggests that the lower of the two glass transitions is associated with the amorphous phase while the upper glass transition 'is connected with the unfreezing of the super-cooled mesophase and is observed in samples quenched from the liquid-crystalline state. Secondly, we would like to point out that although polyester TO1 1 has a relatively high molecular weight and hence a viscosity which is much higher than that of a conventional liquid crystal, we have not been able to quench the isotropic phase of polyester TO1 1 in liquid nitrogen under the usual conditions.Only ultraquenching in isopentane cooled in liquid nitrogen has resulted in the preparation of sample with a marked TgL. Thus, the thermal behaviour of polyester Toll is not very different from that of a conventional liquid crystal. The remarks that the difference between the N/I transition temperature and the virtual second-order transition temperature may explain the simultaneous quenching of an isotropic and a nematic phase in DDA-9 are correct. This suggestion, however, has not been substantiated for other liquid-crystal polymers, and much study is needed in this area.Systematic research directed toward characterization of the structure and the properties of thermotropic liquid-crystal polymers as a function of quenching technique, quench temperature, melt temperature from which quenched and subsequent annealing occurs and a careful analysis in certain homologous series should suffice to establish certain laws. V. Frosini, S. de Petris, E. Chiellini, G. Galli and R. W. Lenz, MoZ. Cryst. Liq. Cryst., 1983,98,223.228 GENERAL DISCUSSION Dr C. Viney (University of Cambridge) said: Dr Coles' paper refers to molecular orientation effects induced by electric and (occasionally) magnetic fields in ther- motropic side-chain polymers. Such an effect can also be identified when a magnetic field acts on a thermotropic main-chain polymer.Relevant experimental conditions, and some implications of thus being able to modify molecular correlations by means of a magnetic field, are given below. The particular polymer investigated was that designated as B-ET in our paper at this Discussion. Specimens having a reproducible starting texture were prepared by shearing onto an aluminium substrate at 300°C and quenching to room tem- perature. Subsequently, specimens were annealed at 280 "C for 2 h, in a magnetic field of approximate strength 0.5 T; they were then quenched to room temperature so that their textures could be observed microscopically between crossed polars. The magnetic field was oriented either perpendicular or parallel to the specimen surface. [Specimens annealed on aluminium must of course be removed from the substrate before they can be viewed in transmitted light.The aluminium was dissolved in a 1 mol dmP3 solution of NaOH in water, to which a few drops of HgC12 solution (0.01 mol dm-3) had been added; specimens were then washed twice in distilled water and allowed to dry.] It is expected that the magnetic field will encourage particular rotational correla- tions of molecules about their chain axes. The planes of phenyl groups will tend to lie parallel to flux lines, since this gives the least distortion of the field and therefore the lowest energy.' The molecular arrangement actually obtained may also reflect surface interactions between polymer and substrate. The initial texture resulting from shear is typified by plate 1.It is shown together with the corresponding conoscopic image, obtained by using a Bertrand lens. The effect of annealing in the magnetic field is shown in plate 2 (field normal to specimen surface) and plate 3 (field parallel to specimen surface). Clearly, the texture obtained depends markedly on the orientation of the magnetic field relative to the specimen. This effect is less evident if glass substrates, rather than aluminium ones, are used; this may be because glass-polymer surface interactions are stronger. The above observations have some significant implications. ( 1 ) Long-range molecular correlations in thermotropic main-chain liquid-crystal polymers can be influenced by magnetic fields. The necessary field strength is relatively small. (2) The possibility exists for producing 'single crystal' domains of uniform molecular correlation, of lateral extent large enough for structural hnalysis based on techniques such as X-ray diffraction and optical conoscopic imaging.This would allow direct identification of uniaxial and biaxial ordering in polymers such as those referred to in our paper. (3) If fibres are spun from these materials, a magnetic field could be used to control the molecular correlations which involve rotation about the chain axes. The fibres could thus be made to have anisotropic transverse properties. P. G. de Gennes, in 7'he Physics ofLiquid Crystals (Oxford University Press, Oxford, 1979), p. 80.Plate 1. Texture typical of B-ET sheared on an aluminium substrate at 300 "C and quenched to room temperature. The corresponding conoscopic image is also shown. The specimen was photographed between crossed polars, with the polarizer parallel to the shear direction (horizontal). [facing page 228Plate 2. Sheared B-ET specimen annealed for 2 h at 280 "C in a magnetic field applied normal to the specimen surface. The specimen was photographed between crossed polars with the polarizer parallel to the original shear direction (horizontal). The corresponding conoscopic image is also shown.Plate 3. Sheared B-ET specimen annealed for 2 h at 280 "C in a magnetic field applied parallel to the specimen surface and parallel to the original shear direction (horizontal). The cono- scopic image corresponding to the upper micrograph is also shown. (p=polariser; a = analyser. )

ISSN:0301-7249    

DOI:10.1039/DC9857900225

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

Faraday Discuss. Chem. Soc., 1985,79, 229-239 Mesophase Texture and Defects in Thermotropic Liquid-crystalline Polymers BY EDWIN L. THOMAS* AND BARBARA A. WOOD Department of Polymer Science and Engineering, University of Massachusetts - Amherst, Amherst, Massachusetts 01003, U.S.A. Received 18th December, 1984 Two rigid/ flexible thermotropic polyesters based on 1,lO-decane bisterephthaloyl chloride with hydroquinone or methyl hydroquinone have been investigated by transmission electron microscopy. Oriented thin films were prepared by melt spreading on phosphoric acid. After thermal quenching to freeze-in the liquid-crystalline state, films were examined at room temperature using electron microscopy and electron diffraction. High local orientation within 10 pm diameter areas was observed by electron diffraction.Layer-line spacings showed that the molecules adopted a highly aligned conformation. Multiple equatorial reflections in the unsubstituted polymer and the formation of a single-crystal-like texture on annealing the frozen liquid-crystalline state in the methyl-substituted polymer suggest biaxial chain packing in the liquid-crystalline state. Bright-field images of the quenched films were essentially featureless, but equatorial dark field and polarized-light optical images revealed an alternating band structure. Annealing below the crystal-melting transition results in the growth of lamellae which decorate the overall pattern of molecular order in the precursor frozen liquid-crystalline state. The 200A thick lamellae provide high contrast and resolution for observation of characteristic mesophase texture and defects.The molecular trajectory across the transverse bands is found to be sawtoothed rather than serpentine. Two types of disclination lines have been observed: non-singular S = *l disclination loops lying in the plane of the films with their loop axes oriented along the overall chain-axis direction and S = *+ disclination lines lying normal to the film surface. The semicrystalline microstructure consists of thin lamellae separated by frozen liquid- crystal regions. The single-crystal-like texture of the oriented lamellae, together with the nature of the meridional scattering, suggests a biaxial smectic liquid-crystalline phase for these polymers. The liquid-crystalline state is not believed to consist of an intrinsic polydomain structure but rather a continuous liquid-crystalline medium containing various types of disclinations.Texture and defect structures of liquid-crystalline polymers are receiving increas- ing attention not only as indicators of mesophase identity but also as key components in rheological models.'32 Unlike small-molecule liquid-crystal systems, where optical microscopy in particular has been quite successful in elucidating characteristic defects and various textures, the organization in macromolecular liquid-crystal systems is still ambiguous. For example, the structural basis of the observed yield stress in the low-shear-rate limit is still mostly conjecture. In addition, since many liquid-crystalline polymers (LCP) can crystallize, their semicrystalline solid-state morphology and its dependence on the precursor mesophase microstructure is highly relevant but hardly yet explored for establishing processing-structure-property relations.Morphological evidence on LCP shows many complex microstructures arising from variously processed materials. Examinations have primarily involved optical microscopy of thick sections. The scale of the texture is often at or below the 229230 MESOPHASE TEXTURE IN THERMOTROPIC POLYMERS resolution limit of the optical microscope. Moreover, information is also obscured by overlap of features in the projected images. X-ray-scattering methods are extensively used for mesophase identification and measurement of molecular order, including recent work utilizing crystalline textures to infer precursor liquid-crystal- line state symmetry.' However, because the X-ray technique samples macroscopic dimensions, only statistical information is available and the specifics of texture and defects are lost. Electron-microscopy investigations have provided much of the information on general polymer m~rphology;~ indeed the most detailed picture of the microstructure of stiff-chain thermotropic LCP (TLCP) stems from the bright- and dark-field microscopy and electron-diffraction work of Donald et aZ.5-8 We have also been using transmission electron microscopy (TEM) to examine lower-melting flexible- spacer TLCP material using surface-tension-driven melt spreading to form a thin film, followed by thermal quenching to permit room-temperature observation of the metastable frozen liquid-crystal state.Subsequent annealing below the crystal-melt- ing transition results in the growth of lamellae which follow the overall pattern of molecular order in the precursor-frozen TLCP. This 'lamellar decoration' provides an extremely useful contrast mechanism for direct visualization of the molecular director distribution over the field of view, permitting detailed assessment of charac- teristic mesophase texture and defects on a much finer scale than previously possible with other TLCP materials. BACKGROUND AND EXPERIMENTAL METHODS The polymers investigated are thermotropic polyesters from 1,lO-decane bisterephthaloyl chloride and hydroquinones with structure (I), where X represents an alkyl substituent.The synthesis and preliminary characterization of these materials has been described by Lenz et aL' The work in this paper is concerned with the unsubstituted polymer (X = H), designated 'H', and the methyl-substituted polymer (X = CH3), designated 'Me'. Such polymers with a rigid main-chain mesogenic unit alternating with a flexible spacer are representative of a large group of TLCP materials. [For an extensive review of rigid/flexible TLCP see ref. (lo).] The now well known odd-even effect of the degree of polymerization of the spacer on the mesophase-transition temperatures illustrates the influence of the flexible spacer on molecular ordering and physical properties. Attachment of pendant groups to the mesogenic unit can further dramatically decrease the transition temperatures.With the present samples, the effect of substituting methyl in place of hydrogen is to lower the crystal-liquid-crystal (thought to be nematic') transition from 234 to 154 "C and the liquid-crystalline-isotropic transition from 263 to 190 "C as determined by differential scanning calorimetry and hot-stage optical micros~opy.~ X For the particular case of rigid/flexible TLCP with various rigid mesogens but all containing a decamethylene spacer, the characteristic mesophase( s) have been identified as nematic in ten cases," smectic in three"-13 and as simply liquid- crystalline in another three," while one polymer displayed a smectic-nematic transi-E. L. THOMAS AND B. A. WOOD 23 1 tion.I4 Blumstein et al. l 5 have observed Schlieren textures with half-integer disclina- tions in a nematic polyester (with a decamethylene spacer).A nematic polyester with a ( CH2)7 spacer developed Williams domains when subjected to alternating electric fields. l 6 Disclination lines of half-integer strength in another nematic poly- ester with (CH2)5 spacer have been studied by Kleman et all7 Recent optical- microscopy work by Xu et all8 on the bromine-substituted member of the bistereph- thaloyl chloride - hydroquinone series showed the appearance of transverse bands after shear deformation. Such banding is now nearly ubiquitous in sheared lyotropic'' and thermotropic me so phase^.^*^^-^^ Donald et aL20 used electron micro- scopy to show that the molecules in the banded structure follow a serpentine path along the direction of shear in stiff-chain TLCP. Well oriented thin films of the H and Me TLCP samples were prepared by dropping crumbs of the polymer onto hot phosphoric acid.For acid temperatures above the polymer-crystal to liquid-crystal transition the polymer melts and spreads rapidly to form a thin film owing to the high surface tension of the acid. The thin, oriented, molten film could then be either quenched by immersing the polymer film and acid into cold water or cooled gradually by shutting off the heater, eventually followed by a water wash at room temperature. TEM specimens were retrieved on copper grids and viewed substrate-free at l00kV in a JEOL lOOCX instrument. Some specimens were annealed in ovens or in situ using the JEOL heating stage.For the annealing experiments, a thin layer (G50 A) of carbon was evaporated onto the specimens to prevent flow in the melt state. While use of thin films simplifies the interpretation of the morphology and electron microscopy provides high-resolution imaging capabilities as well as local electron-diff raction probes, caveats must be posted at the outset concerning the strong influence of the film thickness (ca. 500-1000 A), which may force approxi- mately homogeneous boundary conditions for the molecules as well as possible specific interactions of the molecules with the phosphoric acid substrate. RESULTS AND DISCUSSION IDENTIFICATION OF BIAXIAL ORDER Bright-field images of quenched films of both the H and Me polymers appear essentially featureless [ e.g.see plate 1 (a)]. Selected area diffraction (SAD) patterns from small (ca. 10 pm diameter) regions demonstrate that locally both H and Me samples possess a highly aligned liquid-crystalline state with the chain axis oriented in the plane of the film and along the direction of spreading [ e g . see plates 2(a) and 3(a)]. The metastability of the 'frozen LC' state is a consequence of the long relaxation times of the TLC macromolecules and their slow crystallization rates as well as the freezing-in of molecular motion below the glass-transition temperature (estimated as 67 and 44 "C for the H and Me polymer, respectively).' For the Me polymer the broad equatorial reflection indicates liquid-like side-to- side packing between the oriented chains. The sharper reflections on the meridian arise from intrachain scattering of the aligned periodic macromolecules. The fall-off of intensity and broadening with increasing scattering angle of these meridional maxima can be attributed to the variation of the projected molecular repeat distance along the chain; the situation is comparable to the case of a one-dimensional paracrystal.Since the length of the stiff phenylene bisterephthalate mesogenic unit is fixed, the breadth of a given meridional peak in the 28 direction is inversely proportional to the width of the decamethylene spacer-length distribution in these232 MESOPHASE TEXTURE IN THERMOTROPIC POLYMERS Fig. 1. Schematic diagram of idealized biaxial arrangement of plate-like molecules. strictly alternating rigid/ flexible molecules. The extent of the meridional reflections normal to the chain axis is a function of chain orientation and the degree and lateral extent of the axial registry of adjacent chains (later we discuss the nematic versus smectic nature of the diffraction patterns).Diffraction patterns of the Me polymer (spread on 180 "C acid) are indistinguish- able for slowly cooled or quenched samples. However, annealing (24 h at 144 "C) produced crystal textures of different symmetries. Quenched samples crystallized to a fibrous texture whereas slowly cooled samples crystallized with a single-crystal- like texture [see plate 3(6)]. Only (h01) reflections are present in the crystalline pattern, indicating in-plane orientation of both the a and c axes. The c parameter of the Me unit cell is 30.8 A, ca.1 8 , less than the calculated projected monomer repeat based on a molecule with an all- trans decamethylene spacer conformation. In addition to the sharp crystalline reflections in plate 3(6), there is still significant scattering from the residual untransformed liquid-crystalline phase. How can the two crystalline textures which develop on annealing from an apparently common liquid-crystalline state be explained? If the TLCP molecules are plate-like rather than cylindrical, they may pack with specific side-to-side as well as axial orientation, as indicated schematically in fig. 1. This corresponds to a biaxial liquid-crystalline phase. One could thus envisage the liquid-crystal state to consist of a high-temperature uniaxial phase and a low-temperature biaxial phase.Slow cooling from an initial temperature in the uniaxial regime would produce a frozen biaxial TLCP which would then lead to the biaxial crystal texture of plate 3( b ) , whereas quenching from the same temperature, owing to insufficient time in the biaxial regime to achieve the favoured biaxial packing over distances large enough in extent for detection by electron diffraction, would lead to the uniaxial (fibrous) textures observed. The diffraction pattern of an aligned biaxial liquid-crystal phase should display equatorial maxima corresponding to both face-to-face and edge-to-edge packing of molecules, but if these distances are nearly equal only a single intense intermolecular scattering peak will be seen on the equator. The quenched H polymer (spread on 235 "C acid) does indeed exhibit multiple equatorial reflections [see plate 2(a)]. The two intense equatorial maxima at S = 0.22 and 0.25 8,-' and the weak broaderE.L. THOMAS AND B. A. WOOD s 233 Fig. 2. Scattered intensity distribution along the equator for quenched H film for 0 and 30" tilt about the chain-axis direction. I I I I Fig. 3. Schematics diagrams of diffraction patterns: ( a ) nematic, ( b ) smectic and (c) hybrid model of smectic ordering with axial-shift disorder within the layers. maximum at 0.32 A-' are consistent with two distinct average intermolecular dis- tances with a relative orientation of ca. 90 O (ie. orthorhombic symmetry). Upon tilting about the chain-axis direction the relative intensities of the equatorial reflec- tions change markedly, also indicating a non-uniaxial texture for the H polymer (see fig.2 ) . Annealing of the H polymer results in a non-uniaxial crystalline texture [see plate 2 ( b ) ] . The projected repeat distance of the H polymer calculated from the layer-line spacing is essentially the same as that of the Me polymer for both liquid-crystalline and crystalline states. Biaxiality, as evidenced by the X-ray cylin- drical distribution functions of fibres and optical properties of sheared melts, has also been recently reported by Windle et uL23,24 for a stiff chain TLC polyester. BIAXIAL SMECTIC MESOPHASE Identification of the type of liquid-crystalline state by diffraction is problematical. The diffraction patterns of nematics and smectics differ mainly in their appearance at small scattering angles (see fig.3 ) . The distribution of the scattered intensity on the meridian for a well aligned region of a polymeric liquid crystal depends essen- tially on three factors: (i) the extent of chain misorientation (causing curvature of the layer lines), (ii) the average lateral size of the axial coherently scattering regions (determining spread of intensity along a direction normal to the meridian) and (iii) the degree of relative axial shift of the monomer repeat units along the orientation234 MESOPHASE TEXTURE IN THERMOTROPIC POLYMERS axis (also determining spread of intensity along a direction normal to the meridian). Provided the orientation is sufficient so that curvature does not smear out the patterns, then if factor three is large and two is small, the liquid-crystal phase is nematic [manifested as diffuse, streak-like layer lines, see fig.3 ( a ) ] , while if the reverse is true, smectic order prevails [evident as short, sharp layer-line reflections, see fig. 3 ( b ) ] . The intermediate case of small laterally ordered regions with some axial disorder produces a hybrid pattern [see fig. 3( c ) ] with a characteristic fanning out of the reflections on successive layer lines. (The first layer line appears smectic- like; higher-order layer lines appear increasingly nematic-like.) Unfortunately, owing to the large c axis repeat distance the lowest-order layer lines for the H and Me polymers are difficult to observe because of the strong inelastic scatter.The layer lines (the fourth layer line is the first one visible in the plates) show increasing lateral spread with increasing order, consistent with the hybrid model of lateral (smectic) ordering with axial-shift disorder within the layers. IMAGING MESOPHASE TEXTURE AND DEFECTS By using the strong equatorial scattering to form a dark-field image, distinct bands of alternating contrast appear in the image of the frozen liquid-crystalline state [ e.g. see plate 1 ( b ) ] . The regions appearing bright are in favourable orientation to scatter into the portion of reciprocal space sampled by the objective aperture. The regions are typically long (ca. 10 pm) and narrow (ca. 0.5 pm). Movement of the objective aperture azimuthally causes abrupt extinction of all dark-field image contrast until after a ca.90" rotation the second set of alternating regions becomes bright, with the original set of bright bands now dark. This texture is reminescent of the 'banding' observed in light optical images of hand-sheared TLCP films25 and to the texture developed after cessation of flow in both thermotropic melts20-22 and lyotropic solution^.'^ In order to compare optical-microscopy observations with those of electron microscopy, thicker portions (unsuitable for electron microscopy) of the same specimens were viewed under crossed polars. Plate 4(a) shows such an image of the frozen H specimen. Sets of fine parallel bands with characteristic dimensions similar to the bands observed by electron microscopy are readily apparent. They are seen to terminate or initiate abruptly at a set of lines roughly perpendicular to the long dimension of the fine bands.Lack of sufficient resolution in the optical- microscope images and lack of adequate contrast in the electron-microscope images prevents further structural characterization of the frozen liquid-crystalline state. Fortunately, the ability of these flexible/rigid TLCP to crystallize on annealing below their respective melting points can be exploited to provide considerably improved electron image contrast for more detailed examination of the texture. Crystallization of the materials results in striking and beautiful image contrast due to the appearance of oriented arrays of lamellae throughout the film [e.g. see plate 4( b ) ] . There is a good correspondence of the nature of the lamellar organization and the precursor liquid-crystal state: the uniaxial Me polymer regime gives rise to short, meandering lamellae whereas the biaxial smectic regimes in the Me and H polymers yield better oriented lamellae of micrometre length.From SAD patterns within local regions of given orientation it was established that the chain axis (in both the crystalline and frozen liquid-crystalline phases) is approximately perpen- dicular to the long dimension of the lamellae. Image contrast in bright field arises from diffraction and mass thickness contrast and can be further enhanced with phase contrast by underfocus of the objective lens. The crystalline nature of theE. L. THOMAS AND B. A. WOOD 23 5 dark lamellae is established by dark-field imaging utilizing an equatorial crystalline reflection (see plate 5 ) .Close inspection of the dark-field image reveals three levels of intensity: long, thin bright Bragg diffracting lamellae, an intermediate intensity from the equatorial scattering of the interlamellar frozen liquid-crystalline phase and long, thin dark non-diffracting lamellae. That the banded texture present in the annealed liquid-crystal film is representative of the original texture in the frozen liquid-crystal film is suggested by the overall similarity of the image texture in the dark-field and optical micrographs of the unannealed specimens [such as plates 1 ( b ) and 4( a ) ] with that of the annealed film texture. Later we show that long-time and higher-temperature annealing does indeed alter the sample texture.A wealth of microstructural detail is observable in the annealed films owing to the high-contrast 'lamellar decoration' which follows (at 90') the molecular director trajectories throughout the film. Three characteristic features appear in both the H and Me specimens: (i) the alternating transverse band structure (the region labelled B in plate 6 ) , (ii) an axial line structure (labelled D) and (iii) isolated and variously clustered arrays of disclinations. The molecular director trajectory across the transverse bands is indicated by the lines in plate 6. The chain-axis direction can change from 40 to 90" between successive boundaries. The band boundaries are simple tilt boundaries since the molecular reorientation is due to pure rotation of the chains about an axis contained in the boundary.These organized, pseudo-periodic bands of alternating molecular orientation may be due to an elastic buckling which occurs to relieve compressive strain after the polymer spreads and then relaxes on the hot acid. The mean spacing, relative widths and molecular orientation within the bands may be related to the strength of the mechanical forces. Such narrow, rather well defined boundaries suggest cooperative bending of adjacent molecules at the flexible spacer. The molecular director trajectory across bands in semiflexible TLCP is sawtoothed rather than serpentine, as is the case for stiff-chain TLCP.*' The bands tend to initiate at the axial lines and end smoothly by a continuous change of the molecular director inside the band towards that of the surroundings.In the upper centre of plate 6, a lateral offset of a pair of axial lines was apparently caused by the intersection with a transverse band. This supports the hypothesis that compressive buckling of the oriented liquid-crystalline medium results in the formation of the transverse bands. The axial lines are oriented parallel to the overall molecular director axis in a given area. The lamellae curve to form a row of 'cusps' along the lines. The lines tend to come in pairs, and from the shape of the lamellar cusps the molecules have an opposite sense of curvature at each line. This particular molecular arrangement is very much like that of the non-singular (coreless) S = f 1 disclination loops observed by Graziano and Mackley in sheared liquid-crystalline melts [see fig.9 of ref. (26)]. Such loops appeared during shear deformation and became elongated and oriented in the flow. Graziano and Mackley found only closed loops or lines ending at S = *$ disclinations. We have found both isolated closed loops [long axis parallel to overall orientation direction in plate 4( a ) ] as well as many pairs of lines of opposite sense which end at S = *; disclinations [see plate 7 ( a ) ] . Alternatively these line features may be interpreted as loci of planes of shear between regions of the liquid-crystalline melt. However, it is not clear why such planes should always occur as relatively closely spaced pairs of opposite sense of deformation. We interpret the singularities with characteristically curved lamellae about them as half-integer disclination lines viewed end-on ( i e .the viewing situation actually sketched in Frank's two-dimensional schematic diagrams*'). Plate 8 is a bright-field236 MESOPHASE TEXTURE IN THERMOTROPIC POLYMERS image of a disclination dipole viewed end-on with the corresponding perspective schematic diagram (the actual molecular director field is 90" to the field of lamellae, which rotates the schematic by T ) . In these thin-film specimens the half-integer disclination lines are perpendicular to the surface, permitting easy identification of their strength and sign from the lamellar decoration about the singularity. The disclination density of the annealed films is of the order of los disclinations per cm2, which is typical of dislocation densities in well annealed metals.Annealing at higher temperatures and longer times appreciably reduces the disclination density and also alters the transverse bands. It appears as though the (proposed) non- singular S = +1 disclination loops can relax during annealing into shorter line segments which terminate with S = *+ disclinations [see plate 7( b)]. Annealing also removes the excess energy associated with the transverse band boundaries and half-integer disclinations and improves overall molecular orientation. Certain stable configurations of disclinations persist, similar to polygonalized dislocation arrays in metals. A relevant question is whether to classify the mesophase texture we have observed as an intrinsic domain structure or an apparent domain structure resulting from the microstructuring of a liquid-crystalline medium with continuous molecular director distribution by mechanical deformation and interaction of defects.On the whole, our evidence would favour the latter point of view. THE SEMICRYSTALLINE STATE The morphology of the crystalline-frozen-liquid-crystalline solid state raises interesting questions. The two popular models of lamellar morphology for semi- crystalline polymer molecules are the folded chain and fringed micelle. Avoiding the question of the precise type of folding and nature of the crystal-liquid-crystal interface, two possible schematic diagrams reflecting the key features of the respec- tive models are shown in fig. 4( a) and (b).The main difference between the models is the presence of folds which have been segregated to the crystal boundaries in fig. 4(a). At present we have no definitive evidence which favours either model. The interlamellar regions are depicted as a residual metastable liquid-crystalline (disor- dered smectic) phase, unable to crystallize owing to the aggregation of entanglements and chain ends rejected from crystallization of the adjacent regions. Chain folding is common in linear polyesters [ poly( ethylene terephthalate) and poly(buty1ene terephthalate) to name but two], so polyesters with decamethylene units would be expected to accommodate folds readily. As drawn, the mesogenic units form layers to permit favourable polar interactions. The lamellae are shown as 5 monomer units thick.The H and Me polymers exhibit lamellar thicknesses from 100-450 A depending on annealing temperature, time and film thickness. Presumably the lamellar thickness and long period increase with annealing tem- perature, but we have no systematic data as yet. Perhaps the most remarkable feature of the semicrystalline solid state is the extremely long lengths of the lamellae (up to 10 pm). Since the films were crystallized at significant undercooling from an oriented biaxial liquid-crystal state, well aligned lamellae are not unexpected. The lengths of the lamellae (interrupted only by the transverse bands or the disclinations) strongly suggests long-range lateral ( i. e. smectic) ordering of the molecules prior to crystallization. A biaxial smectic phase would quite naturally lead to the observed single-crystal-like texture developed on crystallization of both the H and Me polymers.Alternatively, one could argue for texturing by crystallization itself with the fast crystal-growth direction being restricted to the plane of the film.E. L. THOMAS AND B. A. WOOD 8' 237 Id Fig. 4. ( a ) Schematic diagram of chain-folded lamellar crystals and intervening frozen smectic liquid-crystalline phase. ( b ) Schematic diagram of fringed micelle lamellar crystals and intervening frozen smectic liquid-crystalline phase. Finally, dark-field images such as plate 5 demonstrate the rather perfect structure of the crystalline lamellae. Bragg diffracting regions extend up to 1 p m in length, indicating that crystal defects such as faults, twins, dislocations and mosaic boundaries are relatively rare, perhaps also owing to the lateral preordering of the molecules in the smectic liquid-crystalline state before final ordering to the crystalline solid. SUMMARY Thin-film samples of thermotropic liquid-crystalline polymers can be quenched to freeze-in non-equilibrium states for observation at room temperature by electron microscopy.Furthermore, if the liquid-crystalline polymer is capable of crystalliz- ation, the semicrystalline morphology can be related to the precursor liquid-crystal- line textures and defects. Two rigid/flexible polymers based on an ester mesogen23 8 MESOPHASE TEXTURE IN THERMOTROPIC POLYMERS with a decamethylene flexible spacer were investigated. Surface-tension-induced spreading on hot phosophoric acid was used to form highly oriented thin films which were quenched for study of the frozen liquid-crystalline state and then subsequently annealed for study of the semicrystalline solid state. Electron-diffraction patterns showed high local orientation within ca.10 p m diameter areas in the as-quenched and slowly cooled films. The nature of the meridional intensity distributions points to the existence of a high-temperature nematic liquid-crystalline state and a lower-temperature smectic state having axial- shift disorder within the layers. Multiple diffuse equatorial scattering peaks for the unsubstituted polymer in the frozen liquid-crystalline state indicates biaxial chain orientation (Le. chain axis aligned and in the sample plane and a second axis at right angles to the chain axis also aligned in the film plane).Biaxiality of both the methyl-substituted and unsubstituted polymer is evidenced by the single-crystal-like texture of the semicrystalline state formed by annealing the slowly cooled frozen liquid-crystalline state. The layer-line spacings show both polymers to have a highly extended monomer conformation in the frozen liquid-crystalline and crystalline states. Dark-field images of the frozen liquid-crystalline films show distinct banding. These bands correlate well with optical microscope images of thicker specimens. Both the bands and disclination defects could be examined with good contrast and resolution by utilizing ‘lamellar decoration’ of the mesophase texture and defects from the annealing-induced growth of long, thin bmellae which follow the overall pattern of molecular order frozen in the precursor liquid crystal.The well defined tilt boundaries of the transverse bands are suggested to result from the cooperative bending of the molecules at several flexible spacer units during relaxation after melt spreading. The molecular trajectory across the bands is sawtooth shaped. Axial lines which occur in pairs parallel to the overall flow direction and form elongated loops or end at S = *$ disclinations may represent non-singular S = f 1 disclination loops. Half-integer disclinations with the line direction normal to the film surface occur at a density of the order of 10’ disclinations per cm2. Further annealing reduces the disclination density and removes the transverse bands, improving overall molecular orientation. The semicrystalline morphology consists of very long (1 pm), thin lamellae separated by narrow frozen liquid-crystal regions.The single-crystal-like texture of the oriented lamellae, together with the electron-diffraction evidence of the frozen liquid-crystalline phase, implies the existence of a biaxial smectic liquid-crystalline phase for these materials. At higher temperatures the Me polymer exhibits a uniaxial nematic liquid-crystalline phase. An intrinsic domain structure is not believed to exist for these rigid/ flexible thermotropic liquid-crystalline polyesters; rather, micro- structuring of a continuous liquid-crystalline medium occurs by the interaction of transverse bands and various types of disclinations.The support of the Materials Research Laboratory of the University of Massachusetts for financial aid and research facilities is much appreciated. Thanks are due to Drs A. Donald and A. Windle of Cambridge University for hospitality and fruitful discussions during the writing of this manuscript. ‘ T. Asada and S. Onogi, Polym. Eng. Rev., 1983, 3, 323. ’ K. Wissbrun, Faraday Discuss. Chem. SOC., 1985, 79, 161. P. Zugenmaier and J. Mugge, Makromol. Chem. Rapid Commun., 1984, 5, 11. E. L. Thomas, The Structure of CrysfaIIine Polymers, ed. 1. Hall (Applied Science Publishers, Essex, 1984).E. L. THOMAS AND B. A. WOOD 239 A. M. Donald and A. H. Windle, J. Muter. Sci., 1983, 18, 1143. A. M. Donald and A.H. Windle, Colloid Polym. Sci., 1983, 261, 793. A. M. Donald and A. H. Windle, J. Muter. Sci., 1984, 19, 2085. * A. M. Donald and A. H. Windle, Polymer, 1984,25, 1235. Q. Zhou and R. W. Lenz, J. Polym. Sci. Polym. Chem. Ed., 1983, 21, 3313. lo C. K. Ober, J. Jin, Q. Zhou and R. W. Lenz, Adu. Polym. Sci., 1984, 59, 103. " C. K. Ober, J. Jin and R. W. Lenz, Makromol. Chem. Rapid Commun., 1983,4, 49. A. H. Al-Dujaili, A. D. Jenkins and D. R. M. Walton, J. Polym. Sci., Polym. Chem. Ed., 1984, 22, 3129. D. Van Luyen and L. Strzelecki, Eur. Polym. J., 1980, 16, 299. A. Blumstein, S. Vilasagar, S. Ponrathnam, S. B., Clough, R. B. Blumstein and G. Maret, J. Polym. Sci., Polym. Phys. Ed., 1982, 20, 877. W. R. Krigbaum, T. Ishikawa, J. Watanabe, H. Toriumi and K. Kubota, J. Polym. Sci., Polym. Phys. Ed., 1983, 21, 1851. l7 M. Klkrnan, L. Liebert and L. Strzelecki, Polymer, 1984,24, 295. M . Xu, personal communication to R. W. Lenz. G. Kiss and R. S. Porter, Mol. Cryst. Liq. Cryst., 1980, 60, 267. 2o A. M. Donald, C. Viney and A. H. Windle, Polymer, 1983, 24, 155. K. Shimamura, Makromol. Chem. Rapid Commun., 1983,4, 107. 22 D. J. Graziano and M. R. Mackley, Mol. Cryst. Liq. Cryst., 1984, 106, 73. 23 G. R. Mitchell and A. H. Windle, Polymer, 1982, 23, 1269. 24 C. Viney and A. H. Windle, J. Muter. Sci., 1982, 17, 2661. 25 C. Viney, A. M. Donald and A. H. Windle, J. Muter. Sci., 1983, 18, 1137. 27 F. C. Frank, Discuss. Faraday SOC., 1958, 25, 19. 12 l 3 W. R. Krigbaum, J. Watanabe and T. Ishikawa, Macromolecules, 1983, 16, 1271. 14 15 16 18 19 21 D. J. Graziano and M. R. Mackley, Mol. Cryst. Liq. Cryst., 1984, 106, 103. 26Plate 1. ( a ) Bright-field image of quenched H film. ( b ) Equatorial dark-field image of quenched H film. [facing page 240Plate 2. ( a ) Electron-diffraction pattern of quenched H film (chain-axis direction vertical). ( b ) Electron-diff raction pattern of annealed H film. Plate 3. ( a ) Electron-’diffraction of slowly cooled Me film. ( b ) Electron-diffraction pattern of annealed Me film.Plate 4. ( a ) Optical micrograph of quenched H film on a TEM grid (transmitted light, crossed polars). ( b ) Bright-field image of annealed H film.Plate 5. High-magnification equatorial dark-field image of annealed Me film. Plate 6. Bright-field image of a portion of plate 4(b) showing transverse bands (B), axial line structure (D) and disclinations (arrows).Plate 7. (a) Bright-field image of non-singular S = *1 disclination lines ending at S = *; disclination lines. (b) Schematic diagram of the (in-plane) molecular director arrangement about a non-singular disclination line which terminates at two half-integer disclination lines. Plate 8. Bright-field image of annealed H film showing a disclination dipole.

ISSN:0301-7249    

DOI:10.1039/DC9857900229

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1985, 79, 241-256 Chiral Liquid-crystalline Polyesters Structural Effects on Mesomorphic Behaviour BY EMO CHIELLINI" Istituto di Chimica Generale (Facolti di Ingegneria), Universiti di Pisa, 56100 Pisa, Italy AND GIANCARLO GALLI Istituto di Chimica Organica Industriale, Universith di Pisa, 56100 Pisa, Italy Received 10th December, 1984 Thermotropic liquid-crystalline polyesters based on mesogenic aromatic residues and intrinsically non-mesogenic chiral segments are described, and an analysis of the thermal and optical behaviour in the bulk and of the chiroptical properties in dilute solutions is presented. The polymers of interest comprise two major classes according to the nature of the diacid and dihydroxy component. Bipolyesters and cobipolyesters based on the (R)-3-methyladipyl residue and several dihydroxy aromatic mesogens belong to the first class, and in the second class are included polyesters derived from mesogenic bis(4-carboxypheny1)terephthalate itself or mixtures with isophthalate or phthalate isomers and various symmetric or non-symmetric chiral diols.Structural effects on the properties in the bulk and in solutions are discussed. In particular, the nature of the different mesogenic groups and the structure and optical purity of the chiral component (diol or diacid), as well as the chemical composition and monomeric unit distribution, are considered. Convenient routes to the synthesis of mesomorphic polyesters with modulated thermotropic properties relevant to the onset, nature and breadth of the mesophase are indicated.In a previous paper' we have reviewed the work done in the field of chiral thermotropic liquid-crystalline polymers of synthetic and semisynthetic origin. The contributions covering most of the activities reported by the end of 1983 were conveniently grouped into two classes based on polymeric products with optically active groups in either the side chain or the main chain. Within the latter class, including polycondensates such as polypeptides, cellulose derivatives and polyesters, particular attention has been given to the last system because of the potential of achieving a variety of new materials with predetermined structures and properties by means of well established synthetic procedures. This paper is focused solely on chiral liquid-crystalline polyesters, with the aim of offering an up to date survey, including the most recent results from our laboratories, to highlight the relevance of structural features to their thermotropic behaviour.Nevertheless, we cannot disregard polypeptides and cellulose deriva- tives, as their basic and applied characteristics place them at the borderline between thermotropic and lyotropic systems. Particular attention is paid to how rationalisation of the structural requisites at the molecular and sub-molecular levels affects the morphological properties in the bulk. Complementary to this last point is an investigation of the properties of the same systems and low-molecular-weight analogues in dilute solution. These studies, other than providing information on primary structure, can give useful insight into 24 1242 CHIRAL LIQUID-CRYSTALLINE POLYESTERS the secondary structure (conformational properties).This should be retained in the bulk not only at room temperature but also in the melt, provided that an appreciable degree of orientation of macromolecular segments is maintained, especially when preferential twisting of the supermolecular array is induced by the presence of chiral components.2 The chiral liquid-crystalline polyesters known at present are bipolymers derived from an intrinsically non-mesogenic optically active aliphatic diol or diacid and a mesogenic aromatic diacid or dihydroxy compound. Cobipolyesters derived from different aliphatic or aromatic diacids have also been described. For the sake of homogeneity, we will present the reported systems as two separate classes according to the nature of the diacid component: (i) hard aromatic dihydroxy compound/soft aliphatic diacid and (ii) hard aromatic diacid/soft aliphatic diol.A particular position is held by chiral composite materials, which are generally obtained by addition of a compatible optically active low-molecular-weight com- pound to liquid-crystalline polyesters to produce cholesterics and twisted smectics. These system will be described briefly in a separate section. LIQUID-CRYSTALLINE BIP-OLYESTERS AND COBIPOLYESTERS BASED ON (R)-3-METHYLADIPIC ACID The first examples of chiral liquid-crystalline polyesters date back to 1980 and appeared almost simultaneously in the contributions of Strzelecki and coworkers3 and Vilasagar and Bl~mstein.~ Systems containing the same commercially available (R)-3-methyladipic acid, as the chiral component, have been reported by other research g r o ~ p s .~ - ~ A large variety of dihydroxylated benzenoid mesogens with an unsaturated or an oxycarbonyl group bridging two para-substituted aromatic nuclei were used in both bipolyesters and c~bipolyesters.~-~~~~ An aromatic triad mesogen consisting of a terephthaloyl residue bridging two hydroquinone units has also been used for the preparation of bipolyesters and cobipolye~ters.~ Scheme 1 shows the general structures of the prepared liquid-crystalline bipolyesters. X-@ (CH2),0-C-CH2-CH2-EH-CH2-CS,oH II I II 0 0 CH3 R R =O, R=H, X = -CH=CH-COO-, -CH=C(CH3)-, -N=N(O)-, -N=N-, n - o o c ~ c o o - , --coo- n=O,R=CH,,X= -N=N(O)- n = 2 , R = H , X = -N=N(O)- Scheme 1.Cumulative representation of bipolyesters based on (R)-3-methyladipic acid and different dihydroxylated mesogens. Series of cobipolyesters containing the same mesogenic cores and variable mixtures of (R)-3-methyladipic acid and C6-CI2 unbranched aliphatic diacids have also been inve~tigated.~- lo In neither class of macromolecular system is information provided about the relative position of the asymmetric diacid residue or of the non-symmetric dioxyE. CHIELLINI AND G. GALL1 243 mesogenic core." However, owing to the expected identical reactivity of the two carboxy groups in the chiral component and of the hydroxy groups in the mesogen, it is reasonable to assume random sequencing of the orientations of the repeating units in bipolyesters, and even of the different diacid residues in cobipolyester samples.Depending upon the nature of the mesogenic component, the clearing tem- peratures of the investigated bipolycondensates varied from 150 to 300 "C. Samples incorporating either substituted or less rigid mesogens were characterized by lower values of the isotropization temperature. The stability ranges were rather broad (50-140 "C), the greatest breadth being reached for samples containing less flexible mesogens. All the bipolyesters displayed in the melt phase cholesteric Grandjean textures accompanied in some cases by reflection of visible light. However, no information is available on either the twisting power of the chiral structures or the sensitivity of the cholesteric pitch to varying temperature.The cobipolyesters showed typical copolymer effects in improving the solubility properties and in extending the range of mesophase existence. Probably the random- ness of different residues in the macromolecular backbone helps to disrupt the three-dimensional order of polymer segments, which results in a marked depression of the melting temperature with respect to the corresponding bipolyesters. By contrast, the isotropization temperature appears to be affected to a much less extent. Therefore, the breadth of the mesophase is enlarged, the highest value usually occurring at the equimolar composition of chiral and achiral diacid residues in the copolymer, Le. corresponding to a maximum of structural randomness.As with bipolyesters, the copolymer samples assumed cholesteric structures characterized by brilliant reflections of visible light. Sometimes the planar, reflecting texture could be frozen in the solid state by q~enching.~ Samples based on different amounts of the same achiral component showed gradual changes in the mesophase structure consistent with an extension of the helical pitch as the content of the chiral component progressively decrea~ed.~?~'~ Anomalous unwinding effects of the cholesteric helical array were produced by the increasing temperature? As a con- sequence appropriate combinations of the effects caused by an external parameter (temperature) and by an intrinsic factor (chemical composition) may offer a simple route to achieve a wide variety of polymeric materials with suitably differentiated physical properties in the thermotropic melt.No indication is given of the chiroptical properties in dilute solutions of the bipolyesters and cobipolyesters, nor are data available on the effect of the enan- tiomeric purity of the chiral component on the bulk properties. We end the first part by stressing the structural effects of the various mesogens in terms of the following. (i) Extension of the mesogen. Mesogens containing three aromatic rings exhibit comparatively high melting points and clearing temperatures, while the interval of mesomorphic behaviour is restricted with respect to the highest values in polymers containing binuclear aromatic me~ogens.~.'~ (ii) Conformational rigidity of binuclear aromatic mesogens.In systems based on mesogens with the same bridging group, the increase in flexibility or steric crowding produces a narrowing of the mesophase. (iii) Bridging group in binuclear aromatic mesogens. In mesogens with structurally identical aromatic rings interconnected by different polarizable substituents, the effectiveness in extending the mesomorphic behaviour follows the order: -CH=C(CH3)- > -N=N(O)- > -N=N- > -CH=CH-COO-, while as far as the stability of the mesophase is concerned the order is: -N=N(O)- > -CH=C(CH3)- > -N=N- > -CH=CH-COO-. These trends are basically in agreement with those reported for low-molecular-weight244 CHI RAL LIQUID-C RYSTALLI NE POLYESTERS me~ogens'~ and for achiral polymers based on the same mesogenic cores.I4 (iv) Chemical constitution of cobipolyesters. Mesophase persistence is increased accord- ing to a copolymer effect. The maximum extension is reached for equimolar amounts of chiral and achiral diacids, even though this particular feature has yet to be better established. The selective reflection of polarized light by cholesteric phases can be shifted to longer wavelengths by dilution of the prevailing chirality and by increasing the temperature.LIQUID-CRYSTALLINE BIPOLYESTERS AND COBIPOLYESTERS BASED ON DIFFERENT CHIRAL DIOLS Our involvement in the field of liquid-crystalline main-chain polymers stems from an interest in the preparation of segmented polycondensates consisting of flexible segments of varying hydrophilic character and rigid segments of either linear or non-linear s t r u c t ~ r e .' ~ - ' ~ Among these, polyesters based on two p-hydroxybenzoic acid residues (H) built in benzenoid triads with a terephthaloyl (T), isophthaloyl (I) or phthaloyl (P) diacid part interconnected through chiral diol spacers appeared worthy of interest. In particular, their liquid-crystalline properties offered the opportunity of obtaining basic materials for the fabrication of anisotropic mem- branes of suitable mechanical strength and permeability. The synthetic strategy for this was oriented towards several routes for the production of material with modified solubility and thermal properties in the bulk. Accordingly, while keeping the mesogen structure constant, variations were allowed within the dihydroxylated spacer.Three series of bipolyesters based on HTH diacid and optically active propene glycol (PG) or its head-to-tail oligomers [ (PG),], glycerol ethers (GE) and butane- 1,3-diol (BD) were prepared. Their general struc- ture is shown in scheme 2. R m n series CH3 1 1,2,3,7,20 HTH- (PG) CH3 2 1 HTH-BD CH,OR' 1 1 HTH-GE Scheme 2. Cumulative representation of bipolyesters based on HTH diacid and different chiral diols. Two series of chiral cobipolyesters were obtained from various mixtures of isomeric aromatic triads HTH/HPH or HTH/HIH and the same optically active (S)-propane- 1,2-diol, as shown in scheme 3. All the polymer samples were synthesized by polycondensation in solution starting from stoichiometric amounts of the diol of choice and aromatic diacid chloride or mixtures of isomeric diacid chlorides.Details relevant to a typical run of polymerization, purification and characterization of the polymeric product are given in the Experimental section. The polymers were characterized by averageE. CHIELLINI AND G. GALL1 245 4&HTH-O-CH2-eH-Oj&HPH-O-CH2-eH-O$, I I CH3 CH3 I I CH3 CH3 4&HTH-O-CH2-EH-O+&HIH-O-CH2-~H-O~y O s x c l Scheme 3. Representation of cobipolyesters based on (S)-propane- 1,2-diol and different mixtures of HTH/HPH or HTH/HIH diacids. molecular weights {[TI = (0.1-0.4) x lo2 cm3 g-'}, which were relatively high" (d.p., = 8-20) and unlikely to affect the thermotropic properties of the polymer to a significant extent? EXPERIMENTAL The syntheses of optically active diols were performed as previously described"-22 starting with (S)-ethyl lactate or D-mannitol as the chiral precursors.Bis(carboxypheny1)terephthalate diacid and its non-linear isomers were prepared according to the same general starting from benzyl hydroxybenzoate and terephthaloyl, phthaloyl or isophthaloyl chloride, respectively, and converted into the corresponding diacid chlorides by the action of thionyl chloride. Polymerization, or copolymerization, runs were carried out by reacting stoichiometric amounts of the diol and diacid chloride, or an appropriate mixture of diacid chlorides, in a 2: 1 (by volume) mixture of 1,2-dichloroethane and pyridine at 70-80 "C. The polymeric products were purified by pouring the reaction mixture into a large excess of acetone and washing thoroughly with 5% HCl, 5% NaHCO,, water and methanol.Chloroform-soluble samples were additionally precipitated several times from chloroform solution into methanol. Optical-rotatory power measurements were performed using a Perkin-Elmer 141 spec- tropolarimeter (path length 1 dm) using polymer solutions [ c = (0.1-0.3) x 10' g cm-,] in various solvents, as specified. Molar optical-rotation values were calculated from the relation [+]=[a] M/100, where M is the molecular weight of the repeating unit. Ultraviolet and circular dichroism spectra were recorded using polymer solutions ( c = 1 O+- l 0-4 mol repeating unit dm-3) with a Varian DMS-80 and a Jasco J500C spectrophotometer (path length 0.1 cm), respectively. Differential scanning calorimetry analyses were performed using a Perkin-Elmer DSC-2 calorimeter with a heating/cooling rate of 10 "C min-I. The transition temperatures were taken, whenever possible, as corresponding to the maximum of the enthalpic peaks.Texture observations were carried out using a Reichert Polyvar microscope equipped with a Mettler FP52 hot stage at a heating rate of 10"Cmin-I. No particular care was taken to ensure mesophase orientation.246 CHIRAL LIQUID-CRYSTALLINE POLYESTERS Table 1. Physicochemical properties of thermotropic bipolyesters based on HTH diacid and propene glycol oligomers (PG) propene glycol oligomer bipolyester run n configuration [+I: MI: T,/"C TJ"C PG 1 1 (S) +22 -I- 42 334 362 PG2 2 (S, s> +84 +87" 130 290'3d PG3 3 (S, s, S) + 132 + I 12 275 321' PG? 6.6 racemic - - 181f >300 g PGm 20.3 racemic - - 1 88f - (I In chloroform.In sulphuric acid, unless indicated otherwise. Beginning of isotropiz- Smectic-cholesteric transition at 305 "C. ation. Smectic-cholesteric transition at 236 "C. Flow temperature. Not liquid-crystalline. RESULTS AND DISCUSSION HTH-( PG), BIPOLYESTERS The physicochemical properties of a series of bipolyesters based on HTH diacid and (S)-propane-1,2-diol (PG), its head-to-tail dimer and trimer, all having 95% enantiomeric purity (runs PGl-PG3), are reported in table 1. In the same series are also included two bipolyesters derived from commercially available racemic mixtures of poly( propene glycol) characterized by average degrees of polymerization of 6.6 and 20.3 (runs PG? and PGB).In these samples the sequencing of the PG units in structurally complex racemic diastereomers was not defined. Therefore, any speculation on the properties of the corresponding polymers has to be inferred on the basis of their average length and racemate character. Details of the preparation of the optically active diols and polymer samples have been reported Polymers PG1 -PG3 were optically active and their molar optical rotation was of the same sign and same order of magnitude as that of the parent diols. Samples PG2 and PG3 showed in dilute solutions induced circular dichroism absorption in the spectral region from 300 to 210 nm, consisting of a positive and a negative peak symmetrically placed with respect to a zero crossover point coinciding with the maximum in the U.V.absorption band connected with the 7r + 7r* electronic transi- tion of the aromatic This observation demonstrates the persistence, even in dilute solutions, of an intramolecular order of repeating units embedded in a conformationally homogeneous This conclusion is in agreement with theoretical predictions2* and distinguishes the present bipolyesters from those pre- pared from (R)-3-methyladipic acid and the less extended azoxybenzene com- ponent, for which the existence of a random-coil conformation was found in both solutions and the isotropic melt by measurements of the Cotton-Mouton Distinct melting and clearing endotherms were recorded only for PGl and PG3. In the case of PG2, which was reconsidered because of its thermal beha~iour,"~~ no isotropization peak could be detected below the decomposition temperature, nor was complete isotropization visually observed up to 300 "C, where cholesteric and isotropic phases coexist.Samples PG? and PGm were amorphous and the flow temperatures were determined by microscopy. No clearing was detected in the case of PG?, whilst PGm did not display any liquid-crystaIline behaviour. Note that lengthening of the glycol ether spacer led to a depression of the melting temperature that, at least on going from PG to its structurally defined trimer, showedE. CHIELLINI AND G. GALL1 247 Table 2. Physicochemical properties of thermotropic bipolyesters based on HTH diacid and different structural isomers of dipropene glycol (DPG) dipropene glycol bipol yester run isomer ME MI: a T,/"C TJOC DPGl head-to-tail +84b +143' 130 290 DPG2 head-to-head +49 +360 100 210 DPG3 tail-to-tail +2 +7 190 280 DPG4 commercial dimer' - - 110 240 a In trifluoroacetic acid, unless indicated otherwise.' In chloroform. ' Racemic mixture of head-to-tail (7 1 "/o), head-to-head (21 '/o ) and tail-to-tail (8%) diastereomers. a marked odd-even alternation, while the stability of the mesophase decreased gradually and vanished at a certain critical length (d.p., == Smectic and cholesteric mesophases were formed in polymers incorporating optically active diols of intermediate length, whereas a purely nematic structure appeared in PG7, which contains the racemic polydisperse oligomer (d.p., = 6.6). The thermal and optical-rotation characteristics of four samples of HTH poly- esters based on optically active dipropene glycol isomers (runs DPG 1 -DPG3) or on a racemic mixture of diastereomers from a commercial source are collected in table 2.The synthesis of the head-to-head and tail-to-tail dimers will be reported in a forthcoming paper: *' HO-6H-CH2-O-CH2-eH-OH I I CH3 CH3 head-to-head HO-CH2-CH-O-eH-CH2-OH I I tail-to-tail HO-eH-CH2-O-eH-CH2-OH I I CH3 CH3 CH3 CH3 head-to-tail The enantiomeric purity of chemically symmetric isomers is still unknown, although we stress that all the polymers showed molar optical rotation in excess of that of the parent optically active diol. This suggests the existence of a definite contribution to the overall chirality of the polymer from the aromatic segments assembled in a preferentially dissymmetric environment.The polymeric products exhibited melting transitions varying from 100 to 190 "C, depending upon the structure. Clearing temperatures were in all cases >200 "C, thus indicating the formation of fairly stable mesophases, which were consistently maintained over very wide ranges of temperatures (90- 160 "C). The mesophase behaviour was found to be complex, including smectic polymorphism, and still needs further structural investigation before it can be defined in an unambiguous way.248 CHIRAL LIQUID-CRYSTALLINE POLYESTERS Interestingly, a decrease in the inner mobilit.1 of the diol spacer affects the melting temperature, a maximum value being observed for sample DPG3, in which the two methyl groups are located in the 1,3 relative positions, with consequent hindering of the free rotation around the C-0-C ether bond. The symmetrical placement of two methyl groups close to the aromatic diacid core (run DPG2) destabilizes the mesophase and disrupts the order of the macromolecular packing in the solid state.In this respect it is worth noting that ordered mesophases were established even in sample DPG4, despite the structural irregularities of the dipropene glycol spacer. This observation further substantiates the high mesogenic effectiveness of the aromatic triad used. However, in order to obtain a full assessment of the structure-property relationship in the considered systems, it is necessary to take into account the orientational order of the non-symmetric diol residue, as in sample DPG1.In fact, a non-random distribution of the placement order has been shown to hold by high-resolution n.m.r. spectra,30 in agreement with theoretical expectations.' We anticipate that a strategy of preparation of bipolycondensates that is based on subtle variations of the feed composition in terms of isomers or stereoisomers of either one or both components will substantially affect the ultimate thermotropic liquid-crystalline properties of chiral polymeric materials. HTH - B D BIPOLY ESTERS A series of bipolyesters based on the HTH mesogenic component and samples of (R)-butane- 1,3-diol (BD) of different enantiomeric excess (0-80% ) was prepared in order to evaluate the effect, if any, of the optical purity on the thermotropic properties of the polymer.The optical rotation of the polymers had the same sign as the starting diol but, in contrast to what was observed for the polymer based on the lower homologue (S)-propane- 1 ,2-dio17 was one order of magnitude higher. Therefore, extension of the aliphatic chain by one methylene group appears to favour conformations charac- terized by high optical-rotatory power. The trend of the molar optical rotation of the polymer as against the enantiomeric excess in the diol, and consistently in the polymer, was found to be linear, thus indicating that no specific cooperative effect, dependent upon the optical purity, is operating. The increased flexibility of the diol residue with respect to the inferior homologue depressed the melting and isotropization temperatures by as much as 100 "C.This made it easier to investigate the thermotropic behaviour by standard optical micro- scopy and differential scanning calorimetry. Below the onset of a cholesteric mesophase, smectic structures were established with polymorphism extending over narrow ranges of temperatures. As expected, in the case of the racemic sample a nematic phase occurred above the smectic phase. Note that the closely related polyester based on the same HTH mesogenic core and linear propane-l,3-diol spacer had a very high melting point and one purely nematic phase.31 The trends of the phase-transition temperatures against the enantiomeric excess are reported in fig. 1. They do not appear to be influenced by the optical purity of the diol spacer, even though a nearly straight line with a modest positive slope could be drawn for the profiles of the melting, smectic-cholesteric (or nematic) and isotropization transitions.HTH-GE B I POLYESTERS The structural features and chiroptical properties together with melting and isotropization temperatures of HTH bipolyesters with chiral3-0-alkylated glycerolsE. CHIELLINI AND G. GALL1 300 250 Y h 200 249 - 1 /-- - '\/ A d - - - - Ch N Sm - I I I 1 1 0 20 40 60 80 1' e.e. ( o/o ) 0 Fig. 1. Plot of phase-transition temperatures against enantiomeric excess (e.e.) for HTH-BD bipolyesters. Table 3. Physicochemical properties of thermotropic bipolyesters based on HTH diacid and different glycerol ethers glycerol ether" bipolyester run R' MI: [41g T,/"C TJOC GEl CH3 -6.0d +174 135 287 +8.2 +128 145 202 - 151 270 GE2 CH2C6HS GE3 CH2CH20CH3 - +0.6 + 196 103 205 - 115 160 GE5 (CH2CH20)3CH3 - e e GE4 (CH2CH20)2CH3 * ( R ) absolute configuration, unless indicated otherwise.Neat, unless indicated otherwise. In dioxane. In methanol. Racemic. (GE) are reported in table 3. Details of the preparation of the alkylated glycerol derivatives and polymerization conditions have been reported elsewhere. *' Among the five glycerols, distinguishable in terms of the nature and length of the side substituents, three samples were optically active. Bipolyesters GE1, GE2 and GE4 showed a molar optical rotation ca. 2 orders of magnitude greater than that of the starting diols, and in the case of the methyl derivative (run GEl) a change in sign was also observed. Intense induced dichroic absorption bands of comparable rota- tory strength and opposite sign were centred at ca.260 and 230 nm and two typical examples are illustrated in fig. 2. Such profiles resemble the exciton splitting3* of interacting chromophores embedded in a highly dissymmetric conformational en~ironment.~~ The melting temperatures were rather low ( 100- 150 "C) and, when compared with those of the HTH-(PG), series, show the influence of the modifica- tion of the side chain on the thermal properties of a polycondensate with a definite backbone. For instance, simple replacement of a hydrogen atom in the pendant methyl group of propane-1,2-diol by a methoxy group lowered the melting tem- perature bv ca. 200 'C- whereas the isotropization point was altered by ca 70 "C250 CHI JXAL LIQUI D-CRYSTALLI N E POLYESTERS 4 Fig.2. Ultraviolet (u.v.) and circular dichroism (c.d.) spectra in dioxane solution for bipolyesters HTH-GEl (. * .) and HTH-GE2 (-). Within the limitations of the small number of runs and the dishomogeneity of the stereochemical properties of the polymers prepared, an odd-even trend of T, and with increasing length of the side-chain substituent was established. The adoption of local zig-zag planar conformations with different symmetry parameters can affect the bulk properties of the p ~ l y m e r , ~ ' apparently analogous to the effect exerted by flexible spacers inserted in the main Note that the anisotropic melts were obtained over a broad range of temperature (45- I50 "C), in spite of the size and complexity of the pendant group in the repeating unit.With the sole exception of sample GE1, which also possessed a smectic phase in a narrow temperature interval (135- 145 "C), all the other polymers were monomor- phic, namely cholesteric or nematic. Polymers based on methylated and benzylated glycerol (runs GE 1 and GE2) assumed planar textures that selectively reflected visible polarized light. The variation of reflection with increasing temperature was consistent with a compression of the helical pitch, in strict analogy with low- molecular-weight chole~terogens.~~ A similar thermochromic trend has also been reported for cholesteric polysiloxanes containing mesogenic side groups.37 Sample GE4, however, showed iridescent planar textures only on cooling, whereas on heating it exhibited a focal-conic texture.HTH/ HPH-PG AND HTH/ HIH-PG COBIPOLYESTERS The random introduction of kinks in a polymer backbone results in lessening of the tendency of macromolecular chains to close pack, with consequent gains in a lower melting temperature and increased solubility. These concepts have been considered in the preparation of two series of thermotropic liquid-crystalline copoly- mers with improved characteristics of tractability in both the bulk and solutions.22 In table 4 data are reported for the chemical composition, molar optical rotation and U.V. and circular dichroism absorption for two series of cobipolyesters based on the same optically active (S)-propane- 1,2-diol and mixtures of linear mesogenicE. CHIELLINI AND G.GALL1 25 1 Table 4. Properties in dilute solutions of cobipolyesters based on (S) -propane- 1,2-diol (PG) and mixtures of HTH and HPH diacids (runs TP) or HTH and HIH diacids (runs TI) U.V. absorption composition, run HTH ( O/o ) [41g a h,,,/nm &,Jdrn3 mol-' cm-' A(ALE) T TP 1 TP2 TP3 P T TI 1 TI2 TI3 TI4 I 100 90 70 50 0 100 90 70 50 30 0 +42' +337 +283 +259 + 123 +409 +364 +367 +297 +297 +42' 250 246.5 245 243.5 236 250 246 244 241.5 239.5 23 7 53 000 34 000 33 500 32 500 34 000 53 000 40 500 38 500 40 500 41 500 43 000 0 +7.3 +5.6 +5.5 +2.6 0 + 10.2 +9.1 +7.4 +9.4 +7.6 In trifluoroacetic acid, unless indicated otherwise. Difference between molar dichroic In absorption coefficients (A&) of signals centred at cu. 260 and 230 nm, respectively. sulphuric acid.HTH and the corresponding non-linear, non-mesogenic isomers HPH or HIH. The bipolyesters of the three aromatic diacids are also included as extremes of the two systems. A random distribution of the aromatic diacid residues holds in all cases, while non-random sequencing of the relative orientational placements of the non- symmetric glycol residue has to be taken into account." All the polymer samples were optically active and the molar optical rotation was of the same sign as that of the starting glycol and low-molecular-weight model compounds,22 but one or two orders of magnitude greater. As discussed for the above systems, this result, in combination with the circular dichroism responses relevant to the aromatic chromophores,22 supports the existence of a conformation favourable to electronic interactions between even structurally different aromatic units. Exciton splitting phenomena may be present with rotatory strength compar- able to that of the amide band in polypeptides, in spite of the absence of cooperative effects caused by hydrogen-bonding interactions.In particular, a maximum dissym- metric effect was established for low contents ( 10-20°/0 ) of non-linear diacid residues. The U.V. absorption measurements were consistent with a monotonous hypsochromic effect with increasing content of distorted units, while the accompanying hypochrom- ism was characterized by a maximum influence at ca. 40-50%0 incorporation of non-linear aromatic triads. In this respect note the role played by structural para- meters in eliciting suitably modulated interactions between polymer segments.The phase diagrams for the two series of copolymers are represented in fig. 3. In both cases the melting, or softening, temperature and the clearing temperature were strongly dependent upon chemical composition. On increasing the content of non-linear units, T, was depressed much more significantly than Ti. A wide range (100- 140 "C) of existence of mesomorphic properties occurred at intermediate compositions and extended down to ca. 75% of distorted units. This demonstrates the possibility of producing stable and persistent mesophases even in copolymers derived from rigid non-mesogenic precursors. In both series typical cholesteric252 CHLRAL LIQUID-CRYSTALLINE POLYESTERS 3 50 300 9 250 \ h 200 150 1 1 1 I 0 20 40 60 80 100 HPH or HIH (%) Fig.3. Phase diagrams of HTH/HPH-PG (-El-, 4-) and HTH/HIH-PG (-0-, -0-) cobipolyesters. Open symbols refer to melting transition and filled symbols refer to isotropiz- ation transition. textures with oily streaks were observed in copolymers containing up to 70% of HTH units. Cobipolyesters of 50% compositions exhibited unusual textures with very broad extinction bands and a marked tendency to homeotropism. X-ray measurement^^^ confirmed the observed thermotropic behaviour. Copolymers TP1 and TI 1 developed planar textures that selectively reflected the visible light in appropriate ranges of temperature, accompanied by an expansion of the cholesteric helical pitch with increasing temperature. Such anomalous behaviour with respect to low-molecular-weight systems was previously observed for thermotropic c~polypeptides~~ and hydroxypropyl cellulose acetate4' and lyotropic poly( y-benzyl-L-glutamate) ;41 it can be imputed to the population of conformers characterized by reduced anisotropy in the intermolecular potential.42 In the present case the peculiar feature observed was caused by the introduction of small amounts of distorted HPH or HIH units.Hysteresis phenomena occurred on cooling, and quenching of the anisotropic melt gave coloured solid films that retained their own reflecting characteristics with ageing. CHIRAL POLYMER COMPOSITES It is well known that nematic and tilted smectic phases can be converted to mesophases with a preferential chirality by dissolving: in them an oDticallv active component.43 This technique has been successfully extended to liquid-crystalline polymers in an effort to elucidate the mesomorphic structures of both chiral and achiral polycondensates.'8-44945 The polyesters and copolyesters poly[ (ethene glycol-co-oxybenzoate) terephtha- late], poly[(tetraethene glycol)-4,4'-p-terphenyldicarboxylate], poly[(methylhy- droquinone-co-pyrocatechol) terephthalate] and poly{ ethene [bis( p- phenoxy)ethy- lene-4,4'-dicarboxylate]-co-oxybenzoate} have been investigated by the contact method, and the isobaric phase diagrams of their mixtures with the optically active,E.CHIELLINI AND G. GALL1 253 low-molecular-weight mesogens 4’-(2-methylhexyloxy)-biphenyl-4-carboxylic acid and terephthalylidene-bis[4-(4’-methylhexyloxy)aniline] are known.A twisted smectic C phase was induced in the polyester sample containing the p-terphenyl aromatic residue, while in all the other cases cholesteric structures were obtained. Analogous behaviour, with the formation of iridescent cholesteric structures, was reported by us14 for a blend based on a nematic poly(P-aminoester) obtained by Michael-type stepwise addition of 2-methylpiperazine to (4,4’-azoxybenzene)- bisacrylate with cholesteric bis{4-[(S)-2-methylbutoxycarbonylphenyl]} tereph- thalate. The effect of the addition of either an achiral low-molecular-weight or polymer liquid crystal such as p-azoxyanisole or poly[ (4,4’-dihydroxy- a-methylstilbene) adi- pate], respectively, to poly[ (4,4’-dihydroxy-a -methylstilbene) (R)-3-methyladipate] has been in~estigated.~ Extension of the cholesteric helical pitch was achieved, according to the shift to the visible range of the maximum reflection previously centred in the U.V.region for the undoped polymer. Structural effects to be taken into account in these systems are essentially related to the general problem of compatibility of the blend components and to the possible influence exerted by the nature and content of the chiral mesogenic components on the morphological properties of induced polymer cholesterics. Composite systems which are worthy of attention appear to be those comprising a cholesterogenic polymer and a compatible non-mesomorphic conventional polymer, in which special induced ordering in the melt should affect the ultimate properties of the composite material.CONCLUSIONS Chiral liquid-crystalline polyesters known to date comprise non-chiral dioxy- or dicarbonyl-terminated aromatic mesogens and structurally complementary chiral dicarbonyl or dioxy flexible residues. The first class includes bipolyesters of diff erent polarizable binuclear or trinuclear benzenoid mesogens with (R)-3-methyladipic acid or cobipolyesters based on the same mesogens and mixtures of the mentioned chiral diacid and linear aliphatic diacids. Considerations of the structure-property relationship have been confined to the influence of structural features of the mesogens and to copolymer effects connected with the different chemical compositions of the flexible diacid mixture in cobipolyesters. No information is provided on the influence of the enantiomeric composition and on the orientational order of the two possible relative placements of the non-symmetric flexible and rigid segments along the macromolecular back- bone.Very few studies have been performed on the properties in dilute solutions. Major conclusions on the relevance of structural parameters to the thermotropic properties of this class of polymers have been outlined in terms of the effects of the extension and conformational rigidity of the mesogen, the structure of the bridging group in binuclear aromatic mesogens and the chemical composition of copolymers. The second class of chiral liquid-crystalline polyesters includes bipolyesters of the bis(4-carboxypheny1)terephthalate mesogenic component with structurally different chiral aliphatic dihydroxy derivatives.Cobipolyesters based on ( S ) - propane- 1,2-diol and mixtures of the same mesogenic diacid with non-linear isomeric triads, in which the terephthalate residue is replaced by the isophthalate or phthalate isomer, are also included.254 CHIRAL LIQUID-CRYSTALLINE POLYESTERS In bipolyesters the affects of structure on the thermal-optical behaviour in the bulk and chiroptical properties in dilute solutions have been studied on the basis of the chemical and stereochemical features of the chiral diol component. The polymer systems investigated are derived from chiral diols (S)-propane- 172-diol and its three possible isomeric dimers and head-to jail trimer, all characterized by the prevalent chirality, diastereomeric mixtures ‘of oligomers (d.p., = 2, 6.6 and 20.3) of racemic propanediol, optically active or racemic 3-0-substituted glycerol ethers, (R)-butane-1,3-diol of different optical purity and (R)-3-methylhexane- 1 ,tj-dioI.The information gained, within the limitations of the uncertainty in the distribu- tion of orientational placements and microtacticity of the non-symmetric diol residues, can be summarized as follows. The presence of a chiral diol component, even at fairly low levels of enantiomeric excess, induces preferential chirality in the supermolecular array. Cholesteric mesophases are usually obtained and their tem- perature range is little influenced by the enantiomeric purity. With racemic com- ponents the most stable mesophase prior to isotropization is untwisted nematic in nature.Lengthening of the diol spacer inserted in the polymer backbone lowers the melting point according to an odd-even trend, while the clearing temperature is influenced much less. Smectic phases are formed at intermediate lengths of the flexible segment. Structural isomerism of the chiral diol residue plays a role in determining the onset and stability of the mesophase. The tendency to disrupt the mesomorphic order and crystalline packing of macromolecular segments is favoured by the proximity of diol branching to the mesogenic core. Substitution in the side-chain of a glycol component modifies the thermal behaviour with a substantial depression of the melting temperature, while preserving high isotropization temperatures.Systems with extended mesomorphism can thus be obtained. In a homologous series of side-chain substituents odd-even alternation of both melting and clearing temperatures occurs. Also, the solubility properties are improved. This approach appears to be promising for the provision of liquid- crystalline polymers with tailored thermal-optical properties and amphiphilic character. An analogous modulation of the thermotropic responses, not characterized by a pulse-entry profile, can be obtained by the introduction of rigid non-linear, non-mesogenic co-units into the macromolecular backbone. Liquid-crystal behaviour can be retained down to compositions of mesogenic units as low as The selective reflection of visible light from the cholesteric structure is influenced by the chemical composition and is maintained for up to 70% of mesogenic units.Films reflecting the light can be prepared for some samples by quenching from the cholesteric melts. Preferentially chiral bipolyesters and cobipolyesters that contain unsubstituted methyl groups in the side chain of the flexible residue are characterized by the anomalous property of giving rise to an extension of the helical pitch under a positive gradient of temperature. However, in the presence of a modified side chain, shrinkage of the cholesteric helix occurs. The copolymerization procedure, particularly with respect to the huge variety of diol mixtures which can be used in the formulation of cobipolyesters, offers a powerful breakthrough in the design of liquid-crystalline polymeric materials.Fur- thermore, the solubility properties are improved by copolymer effects as in conven- tional copolymers. 25-30°/o.E. CHIELLINI AND G. GALL1 255 Optically active polymers show, in correspondence to the T-* T* electronic transitions of the aromatic chromophores, a marked induced circular dichroism absorption, which is not observed for low-molecular-weight model compounds. The profile and intensity of the dichroic bands closely resemble typical exciton splitting and, in combination with the very high optical rotations observed, substantiate the existence of strong interactions among aromatic chromophores assembled in a highly homogeneous conformational environment. The persistence of these interactions, even in dilute solutions, supports the conclusion of the tendency of the polymer backbone to maintain cooperatively superstructures with a certain local degree of orientational and positional order.Finally, we hope we have provided others with suggestions as to how to pursue a synthetic approach addressed to the preparation of chiral polymers exhibiting predetermined liquid-crystalline properties. We thank the Minister0 hbblica Istruzione of Italy for financial support of this work (Fondi Progetti Nazionali 40%). E. Chiellini and G. Galli, in Recent Advances in Liquid Crystalline Polymers, ed. L. L. Chapoy (Applied Science, London, 1985), p. 15. E. A. 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ISSN:0301-7249    

DOI:10.1039/DC9857900241

出版商:RSC    

年代:1985

数据来源: RSC