Membrane Surface Modification Using Thiol-Containing Zwitterionic Polymers via Bioadhesive Polydopamine

Zwitterionic materials have been widely used to modify membrane surfaces to increase surface hydrophilicity and mitigate fouling, which would otherwise decrease water permeance. However, zwitterionic materials are water-soluble, and it is challenging to graft or coat zwitterions for long-term underwater operation. In this work, we demonstrate a facile two-step coating of membrane surface using a novel zwitterionic copolymer that contains thiol groups, which is achieved using bioadhesive polydopamine (PDA) as the intermediate layer. The copolymers were synthesized by reverse addition-fragmentation chain transfer (RAFT) copolymerization of phosphobetaine methacrylate (MPC) and 2-(methacryloyloxy) ethyl lipoate (MAEL), followed by reducing the MAEL units of the resulting copolymers to dithiol-containing units (i.e., DTMAEL). While strongly bound to the membrane surface, PDA reacted with the thiol groups through Michael addition reaction to covalently graft the zwitterionic polymer (p(MPC₁₆₀-co-DTMAEL₄₂) or PMD) onto the membrane surface. The modified membrane surface was characterized using X-ray photoelectron spectroscopy (XPS) and water contact angle measurement. The effect of surface modification on pure water permeance was investigated. The modified membranes were challenged with water containing bovine serum albumin (BSA) using a crossflow filtration system. The modified membrane exhibited less flux decline (38% reduction) than the PDA-coated one (45% reduction) or the unmodified one (53% reduction), but lower water flux than the unmodified one.