Effect of Chemical Structure on Permeability and Fouling Resistance of Self-Assembling Zwitterionic Copolymer Membranes

Polymer self-assembly is a promising tool for scalable manufacture of membranes while maintaining high permeability and controlled pore size. Tuning copolymer composition (chemical structure, monomer ratios) and processing methods can change copolymer behavior which can dramatically affect the self-assembly and hence the membrane performance (permeability, selectivity, fouling resistance).  Membranes with ~1 nanometer effective pore size have numerous applications in the biochemical and pharmaceutical industries, as well as wastewater treatment processes.  However, there are very few commercial membranes in this size range, and those that are available tend to show poor selectivity due to broad pore size distributions, and charged surfaces which lead to the rejections being affected by solute size, charge and chemical interactions between the membrane and solute. There is a need for new membrane materials that enable separation of small organic molecules while exhibiting ease of fabrication into large-area membranes. Zwitterionic groups are shown to strongly resist biomacromolecular fouling due to their high degree of hydration, which makes them promising materials for membrane applications. Studies based on zwitterionic surfactants and simulations have reported zwitterionic groups have different degrees of interaction with water. But studies on the influence of the chemical structure of zwitterionic groups on membrane performance, especially fouling resistance are rather limited. Zwitterions are also documented to self-assemble into channel-type clusters 0.6-2 nm in size. We introduce a new class of membranes whose selective layers are made of self-assembling zwitterionic amphiphilic random copolymers. These membranes derive not only fouling resistance but also their permeability and selectivity from this self-assembled nanostructure. We synthesized random copolymers of three hydrophobic monomers (acrylonitrile, methyl methacrylate, trifluoro ethyl methacrylate) and four zwitterionic monomers (sulfobetaine methacrylate, sulfopropylbetaine vinylpyridine, sulfobutylbetaine vinylpyridine and phosphobetaine methacrylate) by free radical polymerization. These copolymers self-assemble into bicontinuous networks of ~1 nm nanochannels, driven by the strong dipoles of the zwitterionic groups, and documented by transmission electron microscopy (TEM) imaging. Membranes prepared by coating these polymers on porous supports exhibit size-based selectivity with a ~1 nm size cut-off demonstrated by filtering negatively charged dyes. Performance of membranes whose selective layers are made of zwitterionic amphiphilic random copolymers depends on the zwitterionic chemical structure. By systematically varying the chemical structure of the zwitterionic monomer (anionic group, spacer length between the charges) and hence the copolymer, we have shown that for similar zwitterionic content in the copolymers, membranes formed from these copolymers have very different permeability, and fouling resistance behavior. This is attributed to the different hydrophobicities, and solubilities of the different zwitterionic groups. These structure-property relationships serve as a guideline to develop new materials for various applications such as textile wastewater treatment, pharmaceutical purification and bioseparation applications.