Viscoelastic Properties of Mixtures of Optical Isomers of Poly-Benzylglutamate in Liquid Crystal Solution in Tetrahydrofuran

Abstract

Liquid crystal solutions of the Polypeptide enantiomers poly-benzyl -L-glutamate (PBLG) and polybenzyl-D-glutamate (PBDG), prepared so as to keep the volume fraction of polymer constant, provide a unique opportunity to examine separately the effect of intermolecular helical twisting power on rheological properties with other molecular factors held constant. The viscoelastic behavior of solutions of the individual polymers^1-4 and the racemic mixture^2,4 have been studied at concentrations both below and above the critical point for liquid crystal formation. One might expect that the viscous component of the liquid crystal is in some way related to the degree of cholesteric twist superposed on the nematic ordering of the essentially rod shaped^5,6 macromolecules. This twist accessory must to some extent be disrupted by the flow field and the impedence may be overcome by either a tilting of the spiral axis or a partial unwinding of the molecular superstructure. The cumulative pitch, P, of the cholesteric liquid crystal is given by⁷ P = 2πd/<θ> where <θ> is the average angular displacement between adjacent molecular planes separated by a distance d. The reciprocal of P is consequently a measure of the twisting power⁸ of the medium, <θ> may be either positive or negative depending on the chirality of the polypeptide and the nature of the solvent.⁹