(728e) Zwitterionic Copolymer Self-Assembly for Extremely Fouling Resistant, High Flux Membranes with ~1 Nm Pore Size: Understanding Zwitterion Chemistry and Increasing Permeance

Zwitterions, defined as functional groups with equal numbers of positive and negative electrostatic charges, strongly resist biomacromolecular adsorption due to their high degree of hydration. This has led to their incorporation into membranes to prevent fouling by various methods, especially by surface functionalization of existing membranes. Zwitterions also have interesting self-assembly capabilities due to their high dipole moments and strong intra- and inter-molecular interactions. They also interact with small ions and exhibit ionic strength response. Random copolymers of zwitterionic and hydrophobic monomers self-assemble into ~1 nm interconnected domains. This makes them excellent candidates for membrane selective layers with high flux, fouling resistance, size selectivity, and chemical resistance. Composite membranes made by coating these copolymers onto commercial ultrafiltration membrane supports exhibit fluxes comparable with or higher than commercial membranes of similar pore size (~1000 Da MWCO). Based on the rejection of anionic and neutral dyes of varying sizes, they show size-based selectivity with a cut-off around 1 nm, independent of zwitterion chemistry among four zwitterionic monomers surveyed. This pore size closely matches the size of the zwitterionic nanochannels. Zwitterion chemistry, however, has a significant impact on selective layer permeability and fouling resistance. Membranes prepared from copolymers with sulfobetaine methacrylate exhibit excellent fouling resistance, showing little to no flux decline and essentially complete flux recovery with a water rinse upon the filtration of foulants such as protein solutions and oil suspensions. Those prepared with phosphorylcholine methacrylate are even more fesistant to fouling, exhibiting no flux decline even in week-long fouling experiments with oil suspensions. Furthermore, the permeance of these membranes can be boosted 10 times without any changes to the selectivity by using an ionic liquid co-solvent, to about 50 L/m².h.bar. This arises from the formation of worm-like supramolecular assemblies in solution that are deposited on the support to form an ultra-thin selective layer. These membranes are highly valuable for bioseparations and wastewater treatment applications.