Polyvinylidene difluoride membranes with a broad time window for membrane formation via vapor induced phase separation: Fabricating, modifying, and separation properties

Non-solvent-induced phase separation is a significantly important method for the fabrication of polymer membranes for water treatment. However, the rather short time of phase separation causes serious trouble for engineering repeatable and controllable surfaces, which affects the separation performances of the membranes. Here, vapor-induced phase separation (VIPS) with a broad time window for membrane formation was employed for the fabrication of polyvinylidene difluoride-graft-poly(N-isopropyl acrylamide) (PVDF-g-PNIPAM) membranes. Efforts were made to study the effects of exposure time of a liquid film to water vapor (te), relative humidity of water vapor (RH), dissolution temperature of the polymer (Td), and other parameters on the microstructures of the obtained membranes. It can be found that membranes tend to evolve from dense/smooth to porous/rough with te, RH, and Td, which is associated with the development of the crystallinity and the β-crystal of PVDF. Furthermore, the pure water flux of membranes with spherical-like surfaces increases significantly, despite hydrophobicity surfaces, highlighting the efficiency of the pore structure. The rough membranes were further decorated with hydrophilic β-FeOOH nanorods, which endowed the surfaces with underwater superoleophobicity, exhibiting anti-oil fouling and efficient separation for oil-in-water emulsion. Overall, the present work suggests that the VIPS process can be readily employed to construct PVDF membranes with controllable surface microstructures, which provides an excellent platform for further modification.