Poly(vinylidene fluoride) crystallization behavior and membrane structure formation via thermally induced phase separation with benzophenone diluent

Microporous poly(vinylidene fluoride) (PVDF) membranes were prepared by thermally induced phase separation (TIPS) at different quenching temperatures with benzophenone as the diluent. The crystallization behavior and crystal structure of PVDF in PVDF/benzophenone systems were investigated by differential scanning calorimetry (DSC) and wide angle X-ray diffraction (WAXD). The different PVDF concentrations had a remarkable effect on PVDF crystallization behavior and resulted in different membrane structures. Spherulitic structures were vague when the PVDF/benzophenone solution was quenched to -8 degrees C; however, discernable spherulitic structures were obtained when quenched to 34 and 49 degrees C. Additionally, two phase separation mechanisms (solid-solid (S-S) and solid-liquid (S-L) phase separation) were observed during membrane preparation. It was revealed by scanning electron microscopy (SEM) that microporous membranes had more discernable spherulitic structures formed by S-L phase separation than by S-S phase separation, which induced macrovoids and irregular pores on the fracture surfaces of membranes.