ABSTRACT:Thermally induced inversion of the cholesteric sense has been found in lyotropic polypeptide liquid crystals. The cholesteric sense of poly(y-benzyl L-glutamate) liquid crystal in mcresol (17 vol%) inverts from right to left at 60°C, and the reciprocal of the cholesteric pitch changes linearly with temperature. The compensated liquid crystalline solution at 60°C shows no helical twist. Similar phenomena are found for poly(y-benzyl L-glutamate) in benzyl alcohol and for poly(y-propyl L-glutamate) in m-cresol. The solvent effect on the helical twisting power is discussed in the light of the fact that the side-chain ester groups of poly(y-benzyl L-glutamate) form hydrogen bonds with the hydroxyl groups of m-cresol.KEY WORDSConcentrated solutions of poly(y-benzyl L-glutamate) (PBLG) and poly(y-alkyl L-glutamate)s (alkyl=methyl, ethyl and propyl) in certain organic solvents form cholesteric liquid crystals.1.2 The cholesteric structure in such solutions is characterized by the microscopically visible striation patterns when viewed along a direction normal to the axis of torsion. The distance between striations corresponds to one-half the pitch of the helical structure. It is known that the cholesteric pitch varies with polymer concentration, temperature and solvent. The temperature dependence of the pitch has been investigated in various solvents, such as dichloromethane, chloroform, dichloroethane, benzene, dioxane, and dimethylformamide.3.4 In all these systems, the pitch increases with temperature, in contrast to the change in ordinary thermotropic liquid crystals, for which the pitch decreases with temperature.5 The positive temperature dependence of the pitch observed for PBLG liquid crystals is usually attributed to the isotropic thermal motion of the PBLG molecule around its long molecular axis, t To whom correspondence should be addressed.which reduces the anisotropy in the intermolecular potential. 6 Robinson et a/. 2 have investigated PBLG liquid crystals in a number of solvents, and.have shown that the sense of the cholesteric twist depends on the nature of solvent. For example, the PBLG liquid crystal in dioxane forms a right-handed cholesteric structure, while in dichloromethane forms a lefthanded one. In an appropriate solvent mixture of dioxane and dichloromethane (at 0.2 volume fraction of dioxane), the solution shows no regular striation, thus indicating that the cholesteric structure is compensated. A similar compensation phenomenon has been found in a mixture of dioxane and nitrobenzene (at 0.4 volume fraction of dioxane) by DuPre et a/., 7 who also reported that the addition of trifluoroacetic acid to PBLG in dioxane (up to 10%) causes a decrease in the pitch of the cholesteric liquid crystal. There are some observations indicating the presence of intermolecular hydrogen bonds between the PBLG side chains and the solvent molecules. Electric dichroism studies show that a small amount of trifluoroacetic acid interacts with the side chain ester group either by hydrogen bonding or pro-863
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