Pergamon Prog. Polym. Sci., Vol. 19, 1083 1087, 1994

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C H O L E S T E R I C L Y O T R O P I C L I Q U I D C R Y S T A L S A N D T H E R M A L L Y R E V E R S I B L E G E L S F R O M

P O L Y I S O C Y A N A T E S *

MARK M. GREEN,• AKIO TERAMOTO~ and TAKAHIRO SATO~

t Department of Chemistry, Herman F. Mark Polymer Research Institute, Polytechnic University. Brooklyn, NY 11201, U.S.A.

~Department of Macromolecular Science, Osaka University, Toyonaka, Osaka 560, Japan

Abstract - The Flory phase diagram for solutions of rod-like polymers reveals two possibilities for the formation of liquid crystal states. One involves high concentrations of polymer in good solvents and the other dilute solutions in poor solvents. We have taken both routes for poly(alkyl isocyanates). In the former route, we have developed a thermodynamic theory to gain insight into the chirality source of the twisting, and have also tested the possibility of a polymer conformational response to the liquid crystal ordering. By the latter route we encounter thermally reversible gels with unusual chiroptical properties.

C O N T E N T S

1. Discussion 1083 2. Thermally reversible gels 1085 Acknowledgements 1086 References 1087

1. D I S C U S S I O N

The general phase diagram expected for solutions of liquid crystal forming polymers is well understood, l and predicts the formation of this ordered state on increasing concentration. In poly(n-hexyl isocyanate) this behavior has been found in various solvents, 2 and the resulting nematic phase characterized and compared with theory) This class of polymer offers an unusual insight into cholesteric properties since, in contrast to most lyotropic polymers studied in this way, e.g., poly(7-benzyl-L-gluta- mate), the polyisocyanates may be prepared with varying chiral properties without change in overall shape. 4

For the situation where the mesogenic entity is poly(n-hexyl isocyanate), a macro- molecule that is a dynamic racemic mixture of helical segments of opposite sense, the cholesteric state can be formed by doping with chiral nonracemic molecules. This latter approach, used once in a nonquantitative example on a polyaramid, 5 is otherwise unknown.

Either via the intrinsically optically active polymer or via doping, the cholesteric state is subject to the same theoretical analysis involving the often used Taylor expan- sion of free energy. 6 We find that this analysis, in combination with experimental evidence and the theoretical expectation that the twist modulus can be ascribed to an entropic term, leads to the expectation that a plot of the cholesteric wavenumber,

*Presented at the symposium entitled Polymer Science and Technology in the 21st Century, New York, November 8-10, 1992.

1083

1084 M.M. GREEN et al.

q(27r/pitch) vs the inverse absolute temperature should be a straight line. In such a plot the slope and intercept of the line at infinite temperature reveal the enthalpic and entopic (hard-core) chiral terms, respectively driving the cholesteric twisting. 7

We have applied these theoretical findings to the cholesteric state formed from poly((R)2,6-dimethylheptylisocyanate), an experiment which represents one of the extremes available in the polyisocyanate system, i.e., a single helical sense. In this same work we carried out a parallel analysis on poly(-),-benzyl-L-glutamate) and on the triple helical polysaccharide, schizophyllan. The slopes of the straight lines in the plot of q vs 1/T for the latter two polymers tended to a left-handed cholesteric pitch at lIT equals zero, while that for the polyisocyanate was opposite. Based on theoretical analysis, 7 this intercept would be a measure of the hard-core or entropic chiral twisting force. Such a hard-core term can be connected to screw sense by extrapolation of an argument made some time ago by Straley. 8 It can be shown that screws of one sense, if the grooves are not exceptionally narrow, can be arranged to take up less space if twisted in an opposite sense, e.g., right-handed screws make a left-handed arrangement among them. 8 In Straley's analysis this translates into a smaller excluded volume for this above relationship and therefore a more favorable hard-core or entropic term over that for screws of one sense fitting into an interscrew relationship of the same sense.

The above analysis 7'8 requires that, since the entropic twisting force obtained by the extrapolation of q to 1/T equals zero is left-handed for schizophyllan and poly(7- benzyl-L-glutamate), these polymers be right-handed screws while poly(R)-2,6- dimethylheptyl isocyanate) which has an opposite intercept, must be left-handed. Assignment of helix sense to a synthetic polymer would require an X-ray analysis with atomic resolution and although a preliminary study is available for schizophyllan, no such study is available for the synthetic poly(7-benzyl-L-glutamate) or the polyiso- cyanate above. For the polypeptide, the helix sense is known to be right-handed only by correlation to an X-ray study on sperm whale myoglobin. 7 For the polyisocyanate, an empirical force field calculation 9 confirms the left-handed assignment. Thus, we see that this method based on cholesteric ordering offers a new approach to solving this kind of problem which otherwise is not possible by a direct method.

In principle, the separation of the enthalpic and entropic cholesteric twisting forces in a lyotropic liquid crystal could allow a quantification of the chirality of small molecules, such molecules acting as chiral dopants to an otherwise lyotropic nematic phase.

Encouraged by these ideas, we have begun to study the temperature-pitch depen- dence, i.e., q vs 1/T, for poly(n-hexyl isocyanate) nematic solutions doped with opti- cally active small molecules. In this work we find, for example, that although the helical twisting power of the phenylcarbamate of L-menthol is nearly identical to that for the methylene-bridged ether of 1,1-binaphthyl,-2,2-diol, the temperature dependencies are entirely different. While the cholesteric wavenumber of the menthyl derivative is strongly dependent on temperature, that for the binaphthyl is not. This is consistent with the nature of their chirality. Binaphthyl is shape dependent while the menthyl has hydrogen bonding and other polar interactions. The latter can be expected to reside in the enthalpic chiral twisting force, while the former are hard- core or entropic interactions.

The experiments reported above 7 also allow the quantitative testing of molecular theories of the cholesteric phase. In particular, if one considers the "Sergeants and

CHOLESTERIC LYOTROPIC LIQUID CRYSTALS 1085

Soldiers" copolymers in which there is an excess of one helical sense compared to the other, l° an interesting question arises. The helical senses exist as stereoblocks separated by helix reversals and these latter conformational defects are known, due to their population and geometry, in poly(n-hexyl isocyanate). Thus, we know they form a kink of about 130 ° in chain direction and in this regard they precisely model a theoretical model proposed in which such mobile kinks should be excluded in a liquid crystal director field.ll In the mesogenic matrix they would interfere and act to reduce the order parameter.

As an experimental test of the above idea we have compared two "Sergeants and Soldiers" copolymers with widely different isotropic dilute solution-phase optical activities. This means the compared copolymers differ significantly in the ratio of the helical sense blocks. Nevertheless, the two copolymers give the same pitch when used as chiral dopants to a nematic lyotropic phase of poly(n-hexyl isocyanate) in toluene. Moreover, the temperature dependence of this pitch is identical, although the temperature dependence of the isotropic optical activities of the two copolymers differ widely.

Such results can be understood by allowing an exclusion of the helix reversals in these copolymers. In this case both copolymers would take on the favored helical sense quantitatively and therefore act identically as chiral dopants. Thus, this offers an unusual experiment to test the theoretical calculation. 11 As noted below, this may be seen again in a different way.

2. T H E R M A L L Y R E V E R S I B L E G E L S

Hydrocarbon solvents such as n-alkanes cannot support the high concentrations necessary to form the lyotropic liquid crystal states of polyisocyanates discussed above, but are good solvents to moderate concentration. Such hydrocarbon solvents show interesting differences from toluene or chloroform, the latter allowing the higher concentrated solutions. In several examples of the "Sergeants and Soldiers" copoly- mers in which, as discussed above, the polyisocyanates have a less than quantitative excess of one helical sense, 1° lowering the temperature in n-hexane or n-octane causes at about - 5 °C and + 15 °C, respectively, a sudden increase in the magnitude of optical activity. This is not seen in other solvents for these polymers.

Such an increase in optical activity must be associated with a reduction in the population of helix reversals which separate the blocks of opposite helical sense. These reversals are known to be responsible for the cooperativity in the polyiso- cyanates and for the limit on the optical activity. 9'12'13 Thus again, as in the doping experiment described above, we see evidence for a reduction of the helix reversal population but, in this case, the solution is apparently isotropic (see below).

Of special interest for the above result is the short temperature interval for the increase in optical activity. 1° The latter fact suggests a cooperative event associated with a phase change which brings us again to the phase diagram for solutions of stiff polymers. 1 In this phase diagram there is a second route, other than increased con- centration, to the liquid crystal state. Reduction in solvent quality in a dilute solution of the polymer, caused by lowering the temperature, can lead to the broad biphasic region of the phase diagram where the liquid crystal and isotropic states are in equilibrium. 1 In this region a liquid-liquid phase separation can be expected which would lead to aggregates of the polymer, i.e., regions of high local concentration. In

1086 M . M . G R E E N e t al.

I,-

O

such an aggregate which can have a liquid crystal organization, we could have the source of the helix reversal exclusion demanded by the experimental optical activity change.

Such aggregates could be linked together, i.e., crosslinked, by portions of the poly- mer chains extending from the aggregate as in a fringed micelle. This would lead to a thermally reversible gel and such gels have in fact been observed in other stiff polymers under poor solvent conditions. In these cases, the broad biphasic region of the Flory phase diagram has been invoked as the source of the observation. 14'15 In poly(7- benzyl-L-glutamate), which also shows these effects, there is no evidence for the aggregation event causing conformational changes. This observation, the exclusion of the helix reversals, is unique to the polyisocyanates.

Indeed, the picture above is supported in poly(n-hexyl isocyanate), where at 5 mg/ml (a concentration far higher than that necessary to observe optical activity changes) in n-hexane and n-octane thermally reversible gels are formed. Moreover, the sol-gel temperatures in these solvents correspond to those for the cooperative optical activity changes seen in more dilute solutions. Viscosity changes and the onset of cloudiness associated with a liquid-liquid phase separation are also in line with the general picture in which liquid crystalline aggregates can be crosslinked, as hypothesized, by dangling chain segments in the manner of fringed micelles. 16

In summary, the optical activity changes are associated with a reduced population of helix reversals. The helix reversals cause a local kink in the chain direction, 9 and liquid crystal theory 11 predicts precisely that such conformationally mobile defects can be expected to be reduced in population in a liquid crystal matrix where they would otherwise interfere with the director field and reduce the order parameter. Thus, the optical activity changes 1°'16 are in line with a liquid crystal organization and consistent with the cholesteric experiment first discussed above.

A C K N O W L E D G E M E N T S

The National Science Foundation, Chemistry and Polymer Divisions and the Petroleum Research Fund administered by the American Chemical Society are thanked with gratitude for financial support for this work, as is Professor Otto Vogl

CHOLESTERIC LYOTROPIC LIQUID CRYSTALS 1087

for helpful discussions which may lead to practical uses for some of these findings. We also wish to congratulate Professor Vogl on his sixty-fifth birthday (Fig. 1) and send him best wishes for many more.

R E F E R E N C E S

1. A .R. KHOKHLOV (Ch. 3), A. ABE and M. BALLAUFE (Ch. 4), Liquid Crystallinity in Polymers, Principles and Fundamental Properties, (A. Ciferri, Ed.), VCH, New York, and references therein (1991).

2. S.M. AHARONI, Macromolecules 12, 94 (1979). 3. T. SATO and A. TERAMOTO, Mol. Cryst. Liq. Cryst. 178, 143 (1990). 4. M.M. GREEN, S. LIFSON and A. TERAMOTO, Chirality (Memorial Volume for Piero Pino) 3, 285, and

references therein (1991). 5. M. PANAR and L. F. BESTE, Macromolecules 10, 1401 (1977). 6. S. CHANDRASEKHAR, Liquid Crystals, 2nd edn, Cambridge University Press, Cambridge (1992). 7. T. SATO, Y. SATO, Y. UMEMURA, A. TERAMOTO, Y. NAGAMURA, J. WAGNER, D. WANG, Y. OKAMOTO,

K. HATADA and M. M. GREEN, Macromolecules 26, 4551, and references therein (1993). 8. J.P. STRALEY, Phys. Rev. A 14, 1835 (1976). 9. S. LIFSON, C. E. FELDER and M. M. GREEN, Macromolecules 25, 4142 (1992).

10. M.M. GREEN, M. P. REIDY, R. D. JOHNSON, G. DARLING, D. O'LEARY and G. WILLSON, J. Am. Chem. Soc. 111, 6452 (1989).

11. A.R. KHOKHLOV and A. N. SEMENOV, Macromolecules 17, 2678 (1984). 12. M.M. GREEN, N. C. PETERSON, S. LWSON, T. SATO and A. TERAMOTO, Makromol. Chem., Macro-

mol. Syrup. 70[71, 23, and references therein (1993). 13. M.M. GREEN, C. KHATR! and N. C. PETERSON, J. Am. Chem. Soc. 115, 4941 (1993). 14. J.M. GUENET, Thermoreversible Gelation of Polymers and Biopolymers, Academic Press, New York

(1992). 15. A.M. DONALD and A. H. WINDLE, Liquid Crystalline Polymers, p. 81if, Cambridge University

Press, Cambridge (1992). 16. M.M. GREEN, C. A. KHATRI, M. P. REIDY and K. LEVON, Macromolecules 26, 4723 (1993).