Preparation of Polyamide Thin-Film Composite Membranes Using Hydrophilic Hollow Fiber PVDF via the TIPS Process Modified by PVA Diffusion

We developed a new hydrophilization method that we refer to as polyvinylalcohol (PVA) diffusion. This new method modifies the entire surface area of a hollow fiber membrane by diffusing PVA from the bore fluid to the outer surface. This method was used to prepare hydrophilized polyvinylidene difluoride (PVDF) hollow fiber membranes (PVA-PVDF). A hydrophilized polysulfone (PSf) hollow fiber membrane (PVA-PSf) was also prepared for reference. Interfacial polymerization was performed on the outer surface of the hollow fiber substrates during the preparation of polyamide (PA) thin-film composite membranes. This study examined the hydrophilicity effect that these modified support layers exert on the performance of membranes during reverse osmosis (RO) and forward osmosis (FO). Compared with PSf membranes, PVDF membranes had a higher physical strength of over 10 MPa, which was credited to production using a thermally induced phase separation (TIPS) process compared to the non-solvent induced phase separation (NIPS) process that is used to produce PSf membranes. The TIPS process could be useful for developing mass production techniques such as roll-to-roll processing. Comparisons of the RO test performance of hydrophilized PVDF membranes (PA-PVA-PVDF, PA-PVDF) showed that the 1000 ppm of NaCl rejection of hydrophilized support membrane was higher than that of nonmodified support membranes. The FO performances were similar regardless of whether or not the support membranes were hydrophilized when measured under wet conditions. In measurements under dry conditions, however, the performance was significantly lowered in the PA-PVDF membrane that was not hydrophilized. Hydrophilization via PVA diffusion effectively produced equivalent FO membrane performances under either wet conditions or dry conditions.