High performance membranes using self-assembling amphiphilic polymers
Membranes are an important technology for many separations including wastewater treatment, water desalination, and purification of biologic and pharmaceutical products. While membrane technology has become more wide-spread in recent years, the chemical composition of most commercial membranes have changed little over decades. Our research focuses on developing new membrane materials by designing polymers that will self-assemble to impart desired properties to the membrane, such as fouling resistance and selectivity. This presentation will focus on new membranes that use the self-organization of copolymers to create membranes with permeate size cut-offs around 1 nm, which are not currently commercially available. Such membranes would 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. One approach to making these membranes employs comb-shaped copolymers with a hydrophobic backbone and hydrophilic side-chains. These copolymers microphase-separate into interconnected domains ~1 nm in diameter, which act as nanochannels for the permeation of water, and solutes that are smaller than the nanochannel size. The resultant membranes are also highly fouling resistant. More recently, we are studying new alternatives to this system that employs the self-assembly of zwitterionic groups to create similar structures with smaller pore sizes, eventually aimed at seawater desalination. 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 (higher than that for the comb-shaped copolymer membranes), 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 self-assembly 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.