Surface zwitterionization of PVDF VIPS membranes for oil and water separation

This work aims at applying a combined polymerization and membrane surface-modification process, in order to hydrophilize poly(vinylidene fluoride) membranes prepared by vapor-induced phase separation (VIPS), and eventually make them suitable for low transmembrane pressure (Delta P = 0.5 bar) membrane separation of various oil-in-water (O/W) emulsions. Styrene and sulfobetaine monomers were mixed and allowed to react while the PVDF membrane was in contact with the reactive mixture, enabling self-assembling of the random polymer on the membrane as it was formed. Reaction parameters were optimized, and it was found that a solid content of 5 wt%, a styrene/SBMA ratio of 40/60 and a reaction time of 5 h led to very hydrophilic membrane (water contact angle: 12 degrees). The combination of chemical analyses evidenced the successful and controlled surface modification process. Physical analyses showed that deviating from the optimized conditions of styrene/SBMA ratio led to the formation of agglomerates (styrene-rich or SBMA-rich), associated to low porosity, and high coating density. The membranes were used to separate emulsions of toluene/W, hexane/W, hexadecane/W, diesel/W and soybean oil/W, leading to separation efficiency of 99.0%, 99.2%, 99.1%, 99.0% and 99.0%, respectively. This work thus presents a new avenue for surface modification of membrane with extremely efficient copolymers for which there is no common solvent, and brings several evidences of the suitability of VIPS membrane for cost-effective membrane separation of various emulsions.