Zwitterionic copolymer self-assembly for fouling resistant, high flux membranes with size-based small molecule selectivity

Abstract

Membranes that exhibit size-based rejection between small molecules (<1500g/mol) independent of solute charge would be valuable for many separations, but commercial membranes show poor and unpredictable selectivity at this size range. This arises from high pore size polydispersity and charged membrane surfaces that lead to separations affected by solute charge. Furthermore, these membranes are susceptible to fouling. Self-assembly is a promising technique for making polymeric membranes with fouling resistance, high permeability and selectivity along with opportunity for easy fabrication and scalability. We report the first examples of a new class of membrane materials that derive their permeability, size-based selectivity and fouling resistance from the self-assembly of zwitterion-containing amphiphilic random copolymers. Zwitterionic groups strongly resist biomacromolecular fouling due to their high affinity with water, and self-assemble into channel-type clusters of 0.6–2nm in size. We show that these copolymers self-assemble into bicontinuous nanodomains. Thin film composite membranes formed by coating these copolymers onto porous supports exhibit permeances as high as 8.4Lm−2h−1bar−1, over three times that of a commercial membrane with similar MWCO. They show size-based selectivity, with a cut-off around 1nm. This closely matches the size of zwitterionic domains measured by transmission electron microscopy, indicating permeation occurs through these self-assembled “nanochannels.” These membranes show excellent fouling resistance during filtration of protein solutions and oil emulsions, completely resisting irreversible fouling and retaining over 96% of their permeance during filtration. These membranes have potential uses in biomolecule separations, purification of pharmaceuticals, and wastewater treatment.