Nanofiltration (NF) technology has emerged as a promising water treatment technology owing to its low energy consumption and high water flux. However, conventional negatively charged NF membranes are incapable of satisfying the demands of hard water softening, and severe membrane fouling still acts as a critical bottleneck limiting their practical application. Herein, a novel dual-polymer cross-linking strategy is developed to construct positively charged sieving layers on ultrafiltration substrates through a one-step coating and thermal curing approach, in which zwitterionic polyethyleneimine (QSPEI) and polyacrolein (PA) serve as dual building blocks. The interpenetrating covalent cross-linking network between QSPEI and PA macromolecules forms a compact and stable separation layer, enabling the successful fabrication of high-performance composite NF membranes. Under optimal preparation conditions, the obtained membrane exhibits a high MgCl₂ rejection of 95.1% and an excellent pure water permeance of 10.9 L·m⁻²·h⁻¹·bar⁻¹, while maintaining a high flux recovery rate of 91.0% against 100 ppm BSA fouling. The prominent antifouling performance, comparable to that of typical zwitterionic NF membranes, originates from the coexisting positive and negative charges on QSPEI chains induced by quaternization and sulfonation modification. Meanwhile, the dense cross-linked network endows the membrane with outstanding separation efficiency toward both monovalent and divalent cations. This work provides a feasible fabrication strategy for advanced NF membranes with synergistic separation and antifouling performance, possessing great application potential in water softening and ion separation.

Poly(vinylidene fluoride) (PVDF)-based ultrafiltration membranes play key roles in aqueous separation fields. However, the inherent hydrophobicity of PVDF always generates higher water permeation resistance and a greater fouling tendency in the filtration process. Different to the widely reported and widely used blending methods of increasing the hydrophilicity of PVDF membranes, the mass-produced hydrophilic PVDF copolymer is expected to be more efficient in producing high performance membranes. For this purpose, the present research offers a new and scalable approach to improving the hydrophilic properties of PVDF-based membranes through amphiphilic copolymers. Using 2-trifluoromethylacrylic acid (MAF) and hexafluoropropylene (HFP), carboxylated PVDF (PVHM) was synthesized following simple radical suspension copolymerization. Via a non-solvent-induced phase separation (NIPS) method, PVHM membranes were prepared and characterized. It was found that the PVHM membranes had enhanced hydrophilicity, permeability, fouling resistance, and alkali resistance compared with PVDF membranes. For the PVHM containing 8.3 wt% MAF, its membrane demonstrated superior static/dynamic fouling resistance to sodium alginate (FRR up to 99.1% for SA). Therefore, carboxylated PVDF polymers show potential for use in the industrial production of high-performance ultrafiltration membranes.