Interplay between pH-responsive hydrolyzed PAN and layer-by-layer assembly: Implications for membrane performance

• Hydrolysis of PAN membranes introduces pH-responsiveness via carboxylate groups • PAN hydrolysis enhances hydrophilicity, while resulted swelling reduces permeability • LbL coating at acidic pH mitigates swelling via carboxylate group protonation • LbL formation at acidic pH yields denser multilayers with higher MgSO 4 rejection • LbL assembly at acidic pH enhances MgSO4 rejection and water flux of hydrolyzed PAN Layer-by-layer (LbL) is a versatile method to tailor membrane selectivity, yet the role of pH-responsive membrane supports during LbL deposition has not been systematically investigated. In this work, we investigate the relation between the hydrolysis-induced pH-responsiveness of polyacrylonitrile (PAN) membrane supports and poly(diallyldimethylammonium chloride) (PDADMAC)/poly(sodium styrene sulfonate) (PSS) multilayers. PAN hydrolysis introduces negative carboxylate groups, imparting pH-responsiveness via reversible protonation-deprotonation and enabling LbL modification to improve selectivity. Increasing hydrolysis time enhances carboxylate density while reducing pure water permeability (PWP) due to gradual swelling-induced pore constriction. This challenge was addressed by exposing membranes to an acidic solution, where protonation of carboxylates induced deswelling, thereby restoring pores and increasing PWP. In contrast, exposure to an alkaline solution maintained the membranes’ swollen state, where pore constriction resulted in decreased PWP. The membrane pore size differences determined PDADMAC/PSS deposition regimes, with multilayers formed at pH 4, yielded denser but thinner structures than those applied at pH 10. Accordingly, LbL-coated membranes at pH 4 demonstrated higher MgSO₄ rejection than those prepared at pH 10. To distinguish between the effect of LbL multilayer and membrane support on the rejection, all membranes were tested in forward osmosis (FO) using Pluronic® L-35 as the draw solution. The FO performance was mainly affected by the PDADMAC/PSS deposition regime rather than by the membrane swelling-deswelling behavior. This study highlights the importance of support responsiveness during LbL and its potential to use it as a tool to tune membrane performance.