Preparation of polyamide-PVDF composite hollow fiber membranes with well-developed interconnected bicontinuous structure using high-temperature rapid NIPS for forward osmosis

Poly(vinylidene difluoride) (PVDF) hollow fiber membranes were prepared by using high-temperature rapid nonsolvent-induced phase separation (HTR-NIPS). The conditions for fabricating interconnected bicontinuous structures via HTR-NIPS were investigated. We found that (1) the stock solution must not have a gelation temperature and (2) the membrane must be produced at an outer coagulation solution temperature higher than the upper critical solution temperature of the stock solution. A stock solution of PVDF/gamma-butyrolactone/poly (vinylpyrrolidone) that satisfied these conditions was used for successful fabrication of PVDF membranes with a well-developed interconnected bicontinuous structure. The pure water permeances and strengths of these PVDF membranes were higher than those of previously reported PVDF membranes. A polyamide (PA) active layer was prepared on the PVDF hollow fiber membranes via interfacial polymerization to obtain PA-PVDF. Investigation of forward osmosis (FO) in the active layer facing the feed solution showed that the water flux J(w) increased with decreasing thickness of the support layer. Higher water transport was achieved when the tortuosity tau, estimated from the S value, was lower. This suggests that the interconnected bicontinuous structure of the PVDF membrane is a key factor for FO membranes.