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