Facile fabrication of PVDF-CTFE microporous membranes with optimized surface and sublayer structure via non-solvent induced phase separation

Poly (vinylidene fluoride) (PVDF) has been widely implemented in fabricating membranes, but pure PVDF membrane need to be modified by blending or copolymerization to cope with its weak alkali resistance. In this work, the function of chlorotrifluoroethylene (CTFE) chain segment was studied, PVDF/ECTFE blending membranes and poly (vinylidene difluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) membranes were prepared via non-solvent induced phase separation (NIPS) method. The effect of modification methods and solvents on the membrane morphology, micropore and physicochemical structure, permeability, ultrafiltration performance, anti-fouling property and alkali resistance were investigated systematically. More than that, the surface charge and roughness of membrane prepared by different solvents were determined in the varying environment to better understand the anti-fouling property. The result presented that the comprehensive properties of PVDF-CTFE copolymerization membrane were better than that of PVDF/ECTFE blending membrane. The microporous structure of PVDF-CTFE membrane surface and sublayer was optimized by adjusting the solvent. The effect of solvent on the rate of L-L de-mixing during phase inversion was investigated from thermodynamic and kinetic factors. The prepared PVDF-CTFE membrane using N-methylpyrrolidone (NMP) as solvent broke the trade-off between the permeability and selectivity, so it was simultaneously possessed superior pure water flux (306.5 L·m−2·h−1) and higher BSA rejection rate (98.4%). The combination of NMP and PVP significantly decreased the rate of phase inversion without changing the polymer content, and provided a simple and feasible improvement idea for non-solvent-induced phase separation. In addition, the PVDF-CTFE membranes possessed better alkali resistance compared to PVDF membranes, demonstrating its promising potential in harsh environment.