Enhancing the antifouling properties of a PVDF membrane for protein separation by grafting branch-like zwitterions via a novel amphiphilic SMA-HEA linker

The fouling caused by protein ultrafiltration is a bottleneck during wastewater treatment in the biotechnology industry. In this study, the antifouling and mechanism of superior protein rejection by a branch-like structure of zwitterions grafted onto a PVDF membrane via a novel amphiphilic linker is reported. The SMA-HEA amphiphilic linker, 2-hydroxyethyl acrylate-terminated poly(styrene-alt-maleic anhydride) (SMA-HEA), consisting of a hydrophobic poly(styrene-alt-maleic anhydride) (SMA) alternating copolymer and a hydrophilic 2-hydroxyethyl acrylate (HEA) monomer, was first synthesized in this study by a ring-opening reaction. The hydrophobic styrene unit of SMA-HEA was first adsorbed onto the PVDF surface via hydrophobic-hydrophobic adsorption, and then the C[dbnd]C bond of HEA was anchored to graft sulfobetaine methacrylate (SBMA) zwitterions via thermally induced free-radical polymerization using 2,2′-azoisobutyronitrile (AIBN) as the initiator. The effects of the hydrophobic adsorption and free-radical polymerization time on membrane characteristics, such as the hydrophilicity, pore size distribution, static protein adsorption, antifouling and flux recovery rate were analyzed. The modified membrane possessed a superior water contact angle (WCA), which decreased from 130° to 24.9°, and the protein absorption decreased from 112.5 to 4.2 μg cm−2. The degree of polymerization of SBMA and the mechanism of the SMA-HEA-linked PVDF membranes were determined by Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA), and a branch-like zwitterionic structure of the modified membrane was proposed. It was shown that the best pure water flux recovery rate of the modified PVDF membrane was significantly enhanced from 42.5 to ca. 100% with over 90% protein rejection. This self-designed and synthesized SMA-HEA linker had great potential for the surface modification of hydrophobic membranes.