Fouling-resistant ultrafiltration membranes prepared via co-deposition of dopamine/zwitterion composite coatings

Poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), a polymer zwitterion, is known to reduce biofouling and impart low friction characteristics to materials surfaces. Using a one-step solution coating process, we report the incorporation of PMPC into polydopamine (PD) coatings on ultrafiltration (UF) membranes for oil-water separations. Polysulfone UF membranes were surface-modified with either pure PD or a mixture of PD and PMPC (PD-PMPC). Unmodified and surface-modified membranes were characterized with respect to pure water permeance, contact angle, coating thickness, threshold flux and molecular weight cutoff (MWCO). Both types of modified membranes showed a significant decrease in contact angle compared to unmodified membranes, indicating an increase in hydrophilicity. PD-PMPC-modified membranes had a slightly lower underwater contact angle than PD-modified membranes, as well as a higher threshold flux. Constant flux crossflow fouling experiments were conducted on unmodified, PD-modified, and PD-PMPC-modified membranes using a soybean oil emulsion as a model foulant at fluxes near and below the measured threshold flux. The fouling profiles of membranes with similar pure water permeance values were compared to assess the effect of different coating materials and coating conditions on fouling. PD-PMPC-modified membranes exhibited the greatest fouling resistance. Zeta potential measurements showed only small differences in surface charge between the membranes. MWCO experiments showed no difference in nominal pore size or pore size distribution for the modified membranes, indicating that the difference in fouling performance is likely due to the strongly hydrophilic surface properties contributed by PMPC. PD-PMPC-modified membranes exhibited stable, low transmembrane pressure operation at fluxes where PD-modified and unmodified membranes suffered from rapid fouling and an unstable transmembrane pressure profile.