A study on the mechanism of pore formation through VIPS-NIPS technique for membrane fabrication

In this research, the pore formation mechanism of the membranes through the vapor-induced phase separation (VIPS) technique is studied. Polyvinylidene Fluoride (PVDF) microfiltration membranes are successfully fabricated by a combination of the vapor-induce phase separation (VIPS) and non-solvent-induced phase separation (NIPS), known as VIPS-NIPS method, and the influence of exposure time is investigated on the characteristics of the fabricated membranes. The membranes are characterized by scanning electron microscope (SEM) images, porosity, maximum pore radius, pore size distribution (PSD), liquid entry pressure of water (LEPw), mechanical strength, and flux of ethanol through the membrane. The results demonstrated the pore radius of the membranes increased, then decreased, and then increased again by increasing the exposure time. The narrowest pore size distribution was obtained for the membranes when the polymeric film was exposed to the humid environment for five minutes. The pore size distribution results confirmed the four-step mechanism for pore formation in the VIPS process. The nucleation points are formed on the membrane surface in the VIPS step. At low exposure times, the number of nuclei is low and they grow in the coagulation bath and form large pores. By increasing the exposure time, the number of nuclei is increased and reaches a maximum value. At this point, the growth of nuclei in the coagulation bath is minimum and results in a membrane with a smaller pore radius. Afterward, by increasing the VIPS time, the nuclei become larger and join each other to make larger pores, therefore, a wider pore size distribution with a larger average pore radius of the membrane would be expected.