Structure and Performance of PVDF Porous Membrane Prepared by Surface Spraying DMAc/H₂O-Immersion Precipitation Phase Inversion Method

Polyvinylidene fluoride (PVDF) porous membrane was prepared by a two-step method of surface gel-immersion precipitation phase inversion. First, N,N-dimethyl diamide/ H₂O (DMAc/H₂O) solutions with different volume ratios were sprayed on the PVDF solution, leading to surface gelation of the solution membrane. Then, the solution films with the surface gel were immersed into the deionized water of the coagulation bath to form the porous membranes by the precipitation phase inversion. The influences of spraying different DMAC volume fractions in DMAc/H₂O solutions（φ（DMAc）） on the structures and performances of PVDF porous membranes were investigated. Results showed that as φ（DMAc） increasing, the β crystal content on the upper surface of the membranes gradually decreased, and the α crystal content gradually increased, but the overall crystallinity of the membranes gradually increased. PVDF membranes average pore size and porosity increased firstly and then reduced with the increasing of φ（DMAc）, and the membrane porosity greatly affected the water flux of the membrane. It was observed by scanning electron microscope (SEM) that the upper surface of the PVDF membranes sprayed with DMAc/H₂O solution became a porous skin layer, and with an increment in the DMAc ratio, the spherical crystal grains on the upper surface gradually increased, and the cross sections of the membranes changed from a finger-like macroporous structure to a sponge-like pore structure. When φ（DMAc） was 0.3 , the water flux and bovine serum albumin (BSA) rejection rate of the membrane reached the maximum.