(226b) Elucidation of the Effect of Hydrophobic/Hydrophilic Domains in Polyamide Thin-Film Composite Reverse Osmosis Membranes

Thin film composite polyamide (TFC-PA) membranes are employed in desalination industries due to their energy efficiency and ease of operation. In cases where selectivity is not ideal, and for post-treatment to eliminate boron and chloride, up to 1 kWh/m³ is consumed for additional RO passes. Therefore, it is imperative for membrane research to prioritize improvements in selectivity over permeability.

It is predicted that a better understanding of the structure-function relationships for aromatic PA will lead to new polymeric membranes that capitalize on the same transport mechanisms while avoiding PA’s current limitations. In this study, we probed the effect of hydrophobic/hydrophilic domains to understand whether these domains in general or if the specific functional groups found in PA are necessary to achieve its benchmark separation performance. Hydrophobic/hydrophilic domains were synthesized using surface-initiated atom-transfer radical copolymerization of various methacrylate monomers to produce an ultrathin brush active-layer membrane. We observed that in all membranes, the rejection of sodium sulfate is notably greater than calcium and sodium chloride (up to > 95% at a concentration of 2 mM). Membranes synthesized with equimolar hydrophobic and hydrophilic monomers retained the most salt. By investigating if the rejection of neutral solutes is also salt concentration-dependent, we elucidated whether this concentration dependence is attributed to charge-based mechanisms or membrane swelling. The salt rejection analysis of the membranes revealed that incorporating hydrophobic/hydrophilic domains is important but insufficient to replicate the performance characteristics of TFC-PA membranes, and additional intermolecular interactions impact the performance of TFC-PA membranes.