PHASE-TRANSFORMATIONS IN CONCENTRATED-SOLUTIONS OF POLY(GAMMA-BENZYL L-GLUTAMATE)

The phenomenon of thermoreversible gelation is studied in solutions of poly(gamma-benzyl L-glutamate) (PBLG) in benzyl alcohol at concentrations above the onset of liquid crystallinity. Phase transformations are determined by optical microscopy, differential scanning calorimetry (DSC), and small-angle X-ray scattering (SAXS) measurements. The gels obtained from these solutions exhibit two melting endotherms upon heating in the DSC. Strong diffuse scattering at small angles indicates a heterogeneous microstructure even in gels obtained from a single-phase liquid crystal solution. Analysis of the scattering patterns indicates a random two-phase structure having a characteristic length scale of 5-30 nm depending on sample history and concentration. The state of aggregation of the PBLG helices in the gel, determined from the low-angle diffraction, depends on the kinetic pathway for gelation. Disordered packing of adjacent helices is observed in a quenched gel. Two different crystalline phases are obtained either by slow gelation at a high temperature or by annealing a quenched gel. The observation of three states of aggregation in gels at the same concentration and temperature confirms the idea that concentrated PBLG gels may exist in a nonequilibrium state which depends strongly on sample history. We suggest that the two melting endotherms observed by DSC are also due to the heterogeneous microstructure. At-a lower temperature aggregates melt to a surrounding isotropic solution whereas final disaggregation to a Liquid crystal solution occurs at a somewhat higher temperature.