Sequentially regulated reaction and diffusion kinetics in cross-linking assisted phase inversion for sub-nanometer pore engineering in nanofiltration membranes

Efficient removal of Ca 2+ and Mg 2+ is critical for water softening; however, nanofiltration (NF) membranes remain constrained by the permeability-selectivity trade-off, largely because precise control over the selective-layer structure is difficult to achieve. Herein, we report an in situ cross-linking assisted non-solvent induced phase separation (CL-NIPS) strategy for regulating coupled reaction-transport processes during membrane formation. An amphiphilic polysulfone- block -β-cyclodextrin copolymer enables homogeneous incorporation of polyethyleneimine (PEI) into the casting solution, while glutaraldehyde (GA) introduced in the coagulation bath triggers rapid PEI-GA Schiff-base cross-linking concurrently with phase inversion. The interfacial reaction promotes PEI retention and formation of a PEI-rich, positively charged selective layer, while subsequent phase inversion develops the porous supporting structure. A controlled bulk-release assay using FITC-labeled PEI further demonstrates that GA-containing coagulation conditions substantially delay and suppress the apparent release of PEI relative to the water control, providing comparative evidence for reaction-modulated PEI retention during membrane formation. The resulting membranes exhibit a high water permeance of 38 L m −2 h −1 bar −1 and >90% rejection of Mg 2+ and Ca 2+ , together with excellent operational stability. Overall, this work demonstrates that reaction coupled phase inversion can be used to regulate PEI retention and selective-layer formation, providing a practical route toward high-permeance, charge selective NF membranes.