Ultrafiltration membranes with markedly different pH- and ion-responsivity by photografted zwitterionic polysulfobetain or polycarbobetain

Stimuli-responsive ultrafiltration (UF) membranes were synthesized via photo-initiated grafting from of the zwitterionic monomers N-(3-sulfopropyl)-N-methacroyl-oxyethyl-N,N-dimethyl-ammonium betaine (sulfobetaine methacrylate, SPE) and 2-carboxy-N,N-dimethyl-N-(2'-(methacryl-oyloxy)ethyl) ethan-aminium inner salt (carbobetaine methacrylate, CBMA) onto commercial polyethersulfone UF membranes. The effects of kosmotropic and chaotropic potassium salts and pH on flux were determined in order to investigate the stimuli-responsive behavior of the polyzwitterion-grafted membranes. PolySPE showed the expected stimuli-responsiveness based on the anti-polyelectrolyte effect. The degree of swelling of grafted sulfobetaine polymer increased and water permeability decreased with increasing salt concentration; and the results can be related to the Hofmeister series, i.e. the magnitude of the effect increased with increasing chaotropic character of the anion, in the order: SO42- < H2PO4- < CI- < Cl-4(-). However, the carbobetaine polymer (polyCBMA) did not show such antipolyelectrolyte behavior at pH values in neutral and basic range. In contrast to the strong-strong type zwitterionic material polySPE, the swelling behavior of the strong-weak polyCBMA was significantly influenced by pH value because protonation of carboxylic acid side groups changes the grafted chains from net neutral zwitterionic into polycationic with ordinary polyelectrolyte properties. Hence, permeability of polyCBMA-grafted membranes increased with increasing salt concentration at pH=3. Furthermore, the rejection of dextrans as a function of electrolyte concentration and pH was determined and all results were in agreement with changes of water fluxes. For specific salt conditions, large and reversible changes of dextran rejection were found, confirming the possibilities of adjusting the sieving properties of the responsive zwitterionic membranes. Detailed studies by zetapotential measurements were also performed to investigate the specific effects of ion type, electrolyte concentration and pH onto the stimuli responsive changes of surface charge of grafted membranes. All results support that the effects of the grafted zwitterionic polymers on the barrier properties of the UF membranes can be described with a pore-opening/-closing mechanism based on reversible deswelling/swelling of grafted chains on the pore walls in response to composition changes with respect to sail (for polySPE) or salt and pH value (polyCBMA). (C) 2015 Elsevier B.V. All rights reserved