Formation mechanism and crystallization of poly(vinylidene fluoride) membrane via immersion precipitation method

Poly(vinylidene fluoride) (PVDF) membranes were prepared by immersion precipitation method at different coagulation bath temperatures. Effects of coagulation bath temperature on the morphology and crystal structure of PVDF membranes were Studied. The phase diagram of PVDF/DMAc/H2O ternary system was divided into three regions: one phase region, liquid-liquid two phase region and gelation region. Binodal line and spinodal line of phase inversion were calculated based on the thermodynamics equations of membrane formation, and the gel phase boundaries of the systems at 25 and 60 degrees C were determined via cloud point measurement. Based on the phase diagram analysis and experiment results, it Was found that gelation is the dominate membrane formation mechanism at low temperature, but the bicontinuous structure of the formed membrane at 60 degrees C is attribute to spinodal decomposition. Moreover, the thermal behavior and crystal structure of membranes prepared using different coagulation bath temperatures were examined by differential scanning calorimetry (DSC) and wide angle X-ray diffusion (WAXD). The results shows that the PVDF membrane precipitated at 60 degrees C had a typical alpha form crystal structure. While at 15 degrees C, the PVDF crystallites are a mixture of alpha and beta form crystal structure. The crystallization degree of the prepared membrane has not increased with the coagulation bath temperature decreasing.