POLYMER-SOLVENT COCRYSTALLIZATION DURING THERMOREVERSIBLE GELATION INSOLUTIONS OF THE HELICAL POLYPEPTIDE PBLG

Macromolecular aggregation during thermoreversible gelation in solutio ns of the helical polypeptide poly(gamma-benzyl-L-glutamate) [PBLG] in benzyl alcohol [BA] were studied by small angle neutron and small ang le X-ray scattering. Gelation is apparent as a large increase in the i ntensity scattered at low angles, signifying formation of a microfibri llar PBLG network. The aggregated phase in isotropic gels from semidil ute solutions contains about 28% solvent. A periodic structure is obse rved when gelation is induced by rapid cooling to a low temperature, b ut not by slow cooling or gelation at a higher temperature. In gels fr om concentrated liquid crystal solutions, two crystalline structures a re observed, depending on whether the solution is rapidly quenched and then annealed or slowly gelled at an elevated temperature. A phase di agram for the PBLG/BA system is presented and the observed microstruct ural transitions are rationalized in terms of a gelation mechanism inv olving a combination of liquid-liquid phase separation and crystalliza tion in the form of polymer-solvent co-crystals.