Zwitterionic poly(sulfobetaine methacrylate-co-acrylic acid) assisted simultaneous anti-wetting and anti-fouling membranes for membrane distillation

Membrane wetting and fouling are the main challenges that restrain membrane distillation (MD) from practical applications. Zwitterionic sulfobetaine methacrylate (SBMA) polymers are known for their extraordinary anti-fouling properties due to their high hydration capacity. In this study, a facile layer-by-layer (LbL) assembly method is utilized to prepare zwitterionic Janus membranes for both wetting and fouling resistant MD applications. To address the difficulties in the immobilization of zwitterionic polymers, a copolymer of sulfobetaine methacrylate and acrylic acid (poly(SBMA-co-AA), or p(ScA)) was synthesized. The zwitterionic Janus membranes were then fabricated on a hydrophobic polyvinylidene fluoride (PVDF) membrane by LbL assembly of polyelectrolyte multilayers (PEM) of polyethyleneimine (PEI) and poly(acrylic acid) (PAA), followed by p(ScA) deposition. The resultant zwitterionic Janus membranes can simultaneously prevent wetting and fouling, evidenced by direct contact membrane distillation (DCMD) tests using different feed solutions containing sodium dodecyl sulfate (SDS) and/or mineral oil. Results show that the synergetic effects of the collapsed polyzwitterionic brushes, the charged PEM layers, and the elevated capillary force are the keys to relieving the membranes from wetting and fouling. In addition, the applicability of the proposed method to modify commercial polypropylene/polytetrafluoroethylene (PP/PTFE) membranes has been demonstrated.