Thermoreversible gelation of poly(vinylidene fluoride) in propylene carbonate

Thermoreversible gelation of poly(vinylidene fluoride) (PVDF) in propylene carbonate (PC) solutions at relatively low concentrations (0.5-3.5 wt.%) has been studied in terms of the gelation time (or rate) as a function of temperature and polymer concentration. A muster curve could be made for the gelation time-temperature curves using the temperature shift based on the difference in the gel melting temperature (T-m(g)) at different PVDF concentrations. The apparent activation energy of gelation was estimated to be proportional to (T-m(g) - T)(-1.8). The gel melting enthalpy (DeltaH(m)(g)) determined from the modified Eldrige-Ferry equation was 18.67 kJ/mol. In dried gel films many spherulites connected by tie molecules were observed by scanning electron microscopy (SEM). From the wide angle x-ray diffraction (WAXD) measurement, crystallites from PVDF gels were found to consist only of the gamma-type crystal with the TTTGTTT (G) over bar conformation, irrespective of the polymer concentration. As judged from the results obtained, the gelation of PVDF in PC seems to proceed mainly via liquid-liquid phase separation (even if followed by crystallization in the final stage) within the concentration range covered.