Towards permanent hydrophilic PVDF membranes. Amphiphilic PVDF-b-PEG-b-PVDF triblock copolymer as membrane additive

Poly(vinylidene fluoride) (PVDF) ultrafiltration membranes were prepared by non-solvent-induced phase separation (NIPS) using a blend of a new amphiphilic PVDF-based triblock copolymer (PVDF-b-PEG-b-PVDF) and high molar mass PVDF. During the phase inversion step, the triblock copolymer acts both as a pore forming and surface modifying agent. Thanks to the presence of the short PVDF blocks in the triblock copolymer, the hydrophilic PEG segment is fixed in the PVDF matrix, thus reducing the gradual loss of hydrophilic additive by leaching of the triblock copolymer out of the membrane matrix. The PVDF-b-PEG-b-PVDF triblock copolymer additive improved the surface hydrophilicity and significantly increased the pure water flux and permeability of the PVDF membrane. The membrane composition was optimized in terms of additive concentration and compared to membranes prepared with an equivalent amount of commercial PEG of similar molar mass. Pure water filtration tests and contact angle measurements suggested that addition of small amounts of the triblock copolymer additive (2-5% w/w) has a strong impact on the performance and hydrophilic characteristic of the PVDF membranes. The control tests showed that less than 30% w/w of the additive is lost after 9 months, while most commercial PEG (59% w/w) leached out of the membrane matrix in only two months.