Cholesteric Structure of Lyotropic Poly (γ-benzyl L-glutamate) Liquid Crystals

The cholesteric liquid crystals formed by poly(γ-benzyl L-glutamate) (PBLG) in various solvents exhibit a cholesteric sense inversion when the temperature or the solvent composition is changed. The sense inversion in 1,2,3-trichloropropane and in m-cresol occurs from right to left-handed via an untwisted nematic state with increasing temperature. The helical twisting power (reciprocal of the cholesteric half-pitch) in mixtures of these two solvents shows a quadratic dependence on the solvent composition, accompanied by a two-fold sense inversion (right → left → right) at low temperatures.The measurements of the twisting power are also reported for PBLG in a series of alkylcholorieds in order to establish the role of solvent. In solvents with low dielectric constants PBLG forms the right-handed cholesteric sructure while it changes to the left-handed one when the dielectric constant is increased beyond a critical value of ca. 9. The increase in temperature enhances the left-handed twisting power in all solvents studied here regardless of the cholesteric sense at room temperature. These observations clearly separate the effects of solvent and temperature on the cholesteric structure and are discussed in comparison with theories.