Polymorphism in porous poly(vinylidene fluoride) membranes formed via immersion precipitation process

Porous poly(vinylidene fluoride) (PVDF) membranes were prepared via immersion precipitation process from PVDF/dimethyl sulfoxide (DMSO)/water system. By manipulating precipitation temperature and PVDF concentration in casting solution, the polymorphism in the formed porous membranes was studied. Membrane morphologies were observed by field emission scanning electron microscopy (FESEM) and thermal behaviors of membranes were examined by differential scanning calorimetry (DSC). Wide angle X-ray diffraction (WAXD) and Fourier transform infrared spectroscopy-attenuated total reflection (FTIR-ATR) were employed to analyze the crystalline structures of the overall membranes and the surface layers. It was found that, for all the investigated cases, PVDF mainly crystallized into the α form crystalline structure in the overall membrane, while the α and β phases coexisted in the surface layers. The total crystallinity of the membrane and the β/α phase ratio in the surface layers increased simultaneously while raising PVDF concentration in casting solution or reducing precipitation temperature.