Liquid-crystalline properties and reentrance phenomena in PBLG solutions

Abstract

In dilute solutions of poly-$\ensuremath{\gamma}$-benzyl-L-glutamate (PBLG), the only observable transition is intramolecular helix-coil transition. At higher concentrations, intermolecular correlations become important under suitable conditions resulting in transitions of the system to liquid-crystalline state. We present here a mean-field analysis of combined helix-coil and isotropic-nematic (cholesteric) transitions, taking into account the effect of "induced rigidity" as proposed by de Gennes and Pincus. Coupled self-consistent equations for the orientation and helix-fraction order parameters are derived and solved, a universal phase diagram as well as temperature and density dependence of order parameters calculated, and results compared to actual experimental data. We find qualitative agreement but a poor quantitative fit for PBLG in pure solvents. In solvents containing denaturing agents, we predict an isotropic-nematic-isotropic reentrance phenomenon, and propose experimental conditions under which it can be observed.