Polymer surface nano-structuring of reverse osmosis membranes for fouling resistance and improved flux performance

A new class of surface-structured RO membranes was developed with a hydrophilic brush layer of terminally anchored polymer chains resulting in fouling resistant membranes of low mineral scaling propensity and high permeability. The approach is based on free-radical graft polymerization of a suitable monomer, onto the active polyamide (PA) layer of a thin film composite (PA-TFC) membrane, post-surface activation with an impinging atmospheric pressure plasma source. Two types of nano-structured (SNS) RO membranes (SNS-PA-TFC) were synthesized based on methacrylic acid (MAA-SNS-PA-TFC) and acrylamide (AA-SNS-PA-TFC) graft polymerization. The poly(methacrylic acid) and poly(acrylamide) brush layers, on the PA surface, resulted in RO membranes of significantly lower mineral scaling propensity, evaluated with respect to the mineral scalant calcium sulfate dihydrate, compared with commercial RO membrane (LFC1) of about the same salt rejection (similar to 95%) and surface roughness (similar to 70 nm). Direct membrane surface imaging indicated that the rate of nucleation and thus mineral scaling were reduced owing to the polymer brush layer. Fouling resistance of the SNS-PA-TFC membranes was also demonstrated with respect to model foulants (the protein BSA and alginic acid). The MAA-SNS-PA-TFC and AA-SNS-PA-TFC membranes had a negatively charged and near neutral surfaces, respectively, with water contact angles somewhat lower for the former and higher for the latter membranes relative to the LFC1 membrane. The AA-SNS-PA-TFC membrane displayed lower mineral scaling propensity than the MAA based membrane, although its alginic acid fouling resistance was inferior, despite its lower surface roughness (25-33 nm) relative to the MAA-SNS-PA-RFC (similar to 60-80 nm) and LFC1 (similar to 73 nm) membranes.