Surface Nanostructured Reverse Osmosis Membranes

Surface wettability (or surface hydrophilicity) is of considerable importance in a variety of applications, including membrane separations, lubrication, fibers (e.g., textiles), and biomedical applications. Alteration of surface wettability to the desired level can be of significant benefit in the above applications. Accordingly, the present study focused on a systematic investigation of the modification of surface hydrophilicity via the synthesis of hydrophilic surface tethered polymers. This approach to surface nanostructuring (SNS) was achieved by a two-step process, whereby surface activation is achieved using atmospheric pressure plasma (APP) followed by graft polymerization with a suitable vinyl monomer. The resulting polymer layer consists of chains that are terminally and covalently attached to the underlying surface, being polyamide in the present study. Polyamide (PA) was the selected substrate given the importance of this polymer in various applications (e.g., reverse osmosis (RO) membranes, clothing, body armor, etc.). The degree of surface hydrophilicity imparted to the PA surface was evaluated with respect to the conditions of APP surface activation (i.e., hydrogen, oxygen, and helium as plasma source gases, and exposure time) and graft polymerization (i.e., reaction time, 2-hydroxyethyl methacrylate, acrylamide, acrylic acid, n-vinylpyrrolidone, methacrylic acid, and vinylsulfonic acid monomers, and initial monomer concentration). Helium APP was found to be most effective for the synthesis of tethered polymers on the PA surface leading to an increase in hydrophilicity as quantified by a 15 – 51% reduction in the free energy of hydration (ΔGiw) of the underlying PA substrate. In particular, polymer-water affinity of the nanostructured PA surfaces, as quantified by the polar component of the surface energy, was a factor of 2.2 – 6.5 higher than for the native PA surface. Overall, surface hydrophilicity increased with increasing tethered polymer layer surface roughness, volume, and thickness; the above trend is consistent with the expected corresponding increased water sorption capacity by the grafted water soluble polymers.The hydrophilic polymer brush layer effectiveness in reducing biofouling propensity and improving surface cleaning post-biofouling (i.e., decreasing surface-solute affinity) was demonstrated for SNS-PA thin-film composite (TFC) RO membranes. SNS-PA RO membranes with polyacrylamide (PAAm), poly(acrylic acid) (PAA), poly(methacrylic acid) (PMAA), and poly(vinylsulfonic acid) (PVSA) brush layers were synthesized, yielding permeability and salt (NaCl) rejection ranges of 4.8 – 6.7 L/m2∙h∙bar and 95.2 – 96.6%, respectively. Performance testing of the SNS-PA-TFC membranes was carried out using secondary wastewater from a municipal wastewater treatment (MWT) plant. Performance tests with the PMAA-SNS-PA and PAAm-SNS-PA membranes (highest and lowest ranking with regard to hydrophilicity, respectively) demonstrated measurable resistance to biofouling. Biofilm layer thickness was up to 4.7 times lower for the above SNS-PA membranes relative to a commercial TFC membrane of similar salt rejection. Moreover, up to 89% of the SNS-PA membrane permeability was recovered after water cleaning, and complete restoration of membrane permeability was attained after chemical (Na2∙EDTA) cleaning. In summary, the present approach for tailor-designing surfaces is effective for wettability control through adjustment of the brush layer topography and chemistry which affect the polymer-water affinity. Such hydrophilic tethered polymer surface layers can reduce the biofouling propensity of surfaces, and in particular, increase the biofouling resistance and improve cleaning efficiency of RO membranes.