Zwitterionic Copolymers for Next-Generation Membranes

Zwitterionic groups, 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 grafting from and surface functionalization of existing porous membranes. In addition, however, zwitterionic groups 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. One of the objectives of our group is to better understand how zwitterion-containing amphiphilic copolymers self-assemble, and utilize their behavior to develop membranes with improved capabilities: controlled, monodisperse pore size, high flux, fouling resistance, and scalable manufacture.

We have prepared high flux, fouling resistant, size-selective membranes whose selective layers are made of random and comb-shaped copolymers of zwitterionic and hydrophobic monomers. We have shown that within certain composition ranges, these copolymers self-assemble to form bicontinuous networks of nanochannels that allow water passage, and filter out solutes larger than the channel size. Membranes made using random copolymers exhibit fluxes as high as 50 L/m².h.bar and size-based selectivity with a cut-off around 1 nm. This pore size closely matches the size of the zwitterionic nanochannels. Depending on the selected zwitterion chemistry, these membranes can be exceptionally fouling resistant, showing little to no flux decline and complete flux recovery with a water rinse upon the filtration of foulants such as protein solutions and oil suspensions. Well-designed membranes show no flux decline even in week-long fouling experiments with oil suspensions. We are currently considering these membranes for treating challenging wastewaters from oil production.

Membranes prepared from comb-shaped copolymers are capable of forming a variety of selective layer morphologies that can be tuned through the use of additives. The ionic strength response of the membranes depends strongly on copolymer morphology. Random copolymer membranes exhibit little to no change in pore size and flux with changing ionic strength, whereas membranes prepared from comb-shaped copolymers exhibit hysteretic changes in effective pore size. The broad range of self-assembled morphologies accessible with these systems demonstrate that zwitterionic materials constitute a powerful family of membrane materials that can access many important applications.