Preparation of PVDF membranes via the low-temperature TIPS method with diluent mixtures: The role of coagulation conditions and cooling rate

To obtain a high-performance polyvinylidene fluoride (PVDF) membrane, low-temperature thermally induced phase separation (LT-TIPS) was employed to prepare microporous flat PVDF membranes and hollow-fiber membranes using diluent mixtures of triethylphosphate and diethylene glycol monoethyl ether acetate. The membrane morphology was characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results show that thermally induced phase separation (TIPS) played a more important role than nonsolvent-induced phase separation (NIPS) in determining the cross-sectional morphology of the membranes, and the surface structure was mainly affected by NIPS in a low-temperature (0 °C) coagulation bath. By increasing the temperature of the coagulation bath, different structures of membranes, such as spherulites and cellular and sponge structures, were observed. It is believed that the formation mechanism of these membranes depended on a combined NIPS and TIPS process. Wide-angle X-ray diffraction (XRD) analysis indicated that PVDF crystallized into an α-phase crystal structure for all the membranes tested. In addition, the porosity, tensile strength and filtration performance of the flat membranes were measured. On the basis of the above results, a hollow fiber membrane with highly interconnected pores and high porosity could be prepared employing LT-TIPS.