Effects of polyvinylidene fluoride substrate characteristics on the selectivity of thin‐film composite nanofiltration membrane

Abstract Thin‐film composite (TFC) nanofiltration (NF) membranes have been fabricated on polyvinylidene fluoride (PVDF) substrates to separate Cl − and SO 4 2− . With increasing polyvinylpyrrolidone (PVP) content (0–1.00 wt%) in PVDF casting solution, the corresponding substrates have similar surface pore size and hydrophilicity, but have enlarged surface porosity and bulk pore size. Obvious defects were found on TFC membrane surface, which were fabricated on the substrate with large and deep finger‐like pores. On such substrates, piperazine solution is prone to sink to the pore bottom and cannot react with trimesoyl chloride to form a defect‐free polyamide layer. Different from previous reports, the bulk pore size and structure, especially for the finger‐like pore, is found to be a key factor influencing the interfacial polymerization (IP) process. The optimal TFC membrane was prepared on the substrate with 0.15 wt% PVP in casting solution, which had crater‐shaped ridge‐valley structures, relatively high permeability and the largest separation factor of 4.66 for Cl − and SO 4 2− . Although the NF performance needs to be further improved, the results shed lights on the important effects of the substrate bulk pore size and structure on the IP process, which will enlarge the knowledge on the impact of substrate on IP process.