Tuning the Pore Structure of Poly(vinylidene fluoride) Membrane for Efficient Oil/Water Separation: A Novel Vapor-Induced Phase Separation Method Based on a Lower Critical Solution Temperature System

The conventional vapor-induced phase separation (VIPS) method for membrane fabrication faces practical challenges related to the long contact time to gaseous nonsolvent and complexity in pore structure control. This study reported a novel strategy to flexibly control the pore structure of poly(vinylidene fluoride) (PVDF) microfiltration membranes in VIPS utilizing a novel lower critical solution temperature (LCST) system, which was reported for the first time. The results showed that increasing the exposure temperature to humid air and exposure time or decreasing PVDF concentration and membrane thickness were favorable for forming a cellular-like pore structure with graded density while eliminating undesirable fingerlike pores. In particular, by tuning the vapor exposure temperature to above the LCST (i.e., 63 degrees C) of the selected dope composition under extremely short exposure time (40-80 s), the phase separation for membrane formation followed a combined VIPS and TIPS mechanism, namely, V-TIPS, resulting in a desirable cellular-like porous structure with high porosity (similar to 85%). The V-TIPS membrane t-80 (14 wt % PVDF and 80 s exposure to 80 degrees C humid air) exhibited superior mechanical robustness, e.g., tensile strength of 3 MPa. The filtration performance of t-80 with surfactant-stabilized oil-in-water emulsion under an ultralow pressure of 20 kPa showed an ultrahigh flux of 3028 L m(-2) h(-1) bar(-1) and greater than 99% rejection of oil contaminants. The membrane permeability was 3-fold higher than that of the membrane produced below LCST, significantly improving the purification efficiency. Preliminary results showed that the t-80 exhibited resilience to flux decline in oily water separation with good flux recovery. This simple but novel and effective V-TIPS approach shows promise for large-scale continuous production of high-performance membranes for ultralow pressure or gravity-driven water purification.