Size-Selective Nanofiltration Membranes with Amphiphilic Zwitterionic Polymer Selective Layers

Membranes with permeate size cut-offs in the nanometer range have numerous applications in the biochemical, pharmaceutical and food industries, as well as in specialized wastewater treatment processes. To be commercially viable, membranes for molecular-size-based separations need to exhibit high flux and fouling resistance, and be readily synthesized and fabricated into large-area membranes. Zwitterionic groups strongly resist biomacromolecular fouling due to their high degree of hydration. Copolymers of zwitterionic and hydrophobic monomers microphase separate to form clusters of the zwitterions 0.6-2 nm in size. We hypothesize that within certain composition ranges, these clusters connect with each other to form bicontinuous networks of nanochannels, and these copolymers can serve as selective layers of membranes with ~1 nm size cut-off, as well as high flux and fouling resistance. We have synthesized such copolymers and formed thin film composite membranes by coating them onto commercial ultrafiltration membrane supports. These membranes exhibit fluxes as high as 21 L/m².h.bar, which can be further be improved by better coating methods. Based on the rejection of anionic dyes of varying sizes, they show size-based selectivity with a cut-off around 1 nm. These are the first examples of membranes that gain their selectivity from the microphase separation of zwitterionic groups, in addition to exploiting this functionality for fouling resistance. We expect these membranes to be promising candidates for various applications including the purification of pharmaceuticals and antioxidants, and textile wastewater treatment.