Tuning pore structure and hydrophilicity of high-performance loose nanofiltration membrane via sequential boroxine/boronate crosslinking strategy

Loose nanofiltration (LNF) membranes with precise molecular sieving and good permeability are ideal for dye/salt separation, where tuning the microporous structure is essential for performance enhancement. Herein, a facile “layer-by-layer” approach via cross-linking of polyhydroxy polymer and boronic acid-decorated polymers on PAN substrates was developed to construct LNF membranes. Sequential vacuum-assisted dip-coating enabled dual cross-linking: dehydration-induced condensation of B–OH to form boroxine, and boronate ester formation between residual B–OH and vicinal diols. Precise tuning of selective layer pore size and hydrophilicity was achieved by regulating the dip-coating concentration and cross-linking sequences. The optimized membrane exhibited excellent permeability (36.2 L m⁻² h⁻¹ bar⁻¹) and high selectivity (NaCl/DR23 separation factor up to 1278.3), along with outstanding anti-fouling performance (FRR >94.1 %) against multiple contaminants. Notably, the water-stable boroxine-boronate polymer layer maintained stable separation performance during continuous operation.