Polyelectrolyte complex composite nanofiltration membranes fabricated in two steps with hydrocarbon-based sulfonated ionomer and polyethyleneimines

This study presents a novel approach to fabricating chlorine-stable nanofiltration (NF) membranes using a scalable two-step dip-coating process using the hydrocarbon-based ionomer Nexar and branched polyethylenimine (PEI) as polyelectrolyte. A thin-film composite (TFC) membrane was first formed by casting Nexar onto an aminated polyacrylonitrile (aPAN) support, followed by complexation with PEI to create a polyelectrolyte complex (PEC) film, to yield PEC-NF membranes. The influence of solvent systems, drying conditions and cast film thickness on the performance of the Nexar TFC membrane has been studied initially. With the best performing Nexar TFC prototype, it has been found that molecular weight (MW) of PEI has a profound influence on PEC-NF membrane structure and performance. A comprehensive characterization of the membrane structures has been performed by scanning electron microscopy, atomic force microscopy, TOF-SIMS analyses, as well as zeta potential and contact angle measurements. Compared to the TFC precursor, the PEC-NF membranes exhibited improved multivalent salt rejection as well as higher rejection of sucrose and organic micropollutants, particularly for neutral and hydrophobic compounds, at either reduced (low MW PEI) or slightly increased (high MW PEI) permeance. All membrane variants demonstrated excellent stability under prolonged exposure to active chlorine (500 ppm NaOCl, pH 9). Increasing feed salt concentration yielded markedly decreasing salt rejection and increasing solution permeance. Remarkably, the rejection of neutral solutes such as sucrose (∼90 % for PEC-NF with low MW PEI) was not influenced by feed salt concentration in the range of 0.2–10 g/L. This work highlights the potential of ionomer-based PEC membranes as a robust and tunable platform for advanced NF applications.