Improved permeability and biofouling resistance of microfiltration membranes via quaternary ammonium and zwitterion dual-functionalized diblock copolymers

In order to improve permeability and biofouling resistance of microfiltration (MF) membranes, quaternary ammonium and zwitterion dual-functionalized diblock polyelectrolyte copolymers, poly(dimethylaminoethyl methacrylate-butylammonium bromide)-b-poly(sulfobetaine methacrylate) (PDB-b-PSB) prepared via RAFT polymerization, were immobilized on commercial poly(vinylidene fluoride) (PVDF) MF membrane surface via using a versatile and facile mussel-inspired method to fabricate hierarchical copolymer brushes. Hierarchical PDB-b-PSB brushes contained bottom passive anti-adhesive zwitterionic PSB layer and upper active biocidal quaternary ammonium PDB layer. The surface chemical composition and morphology characterizations of pure and modified membranes demonstrated that PDB-b-PSB copolymers were successfully grafted to the dopamine-modified membrane surface via thiol mediated Michael addition reaction, which did not significantly change the membrane pore structure. However, the initial contact angle of PDB-b-PSB modified membrane (M/PDB-b-PSB) was changed from 128° for pure membrane (M/pure) to 43.2° and became 0° within 10 s, indicating the enhanced hydrophilicity. More importantly, the M/PDB-b-PSB membrane exhibited active biocidal effects against E. coli and S. aureus with inhibition rates over 98% and the E. coli and S. aureus suspension fluxes of M/PDB-b-PSB membranes were 1.61 times and 1.35 times higher than those of M/pure membranes. The M/PDB-b-PSB membrane had excellent anti-biofouling property with a better water flux recovery rate of 72%, which was 30% higher than that of the M/pure membrane. Moreover, PDB-b-PSB brushes showed a good immobilization stability. Therefore, the approach of poly(dopamine)-assisted fabrication of anti-biofouling MF membranes with quaternary ammonium and zwitterion dual-functionalized diblock copolymers had a prospect application in membrane-based separation technology and water purification.