Highly Permeable Nanofibrous Composite Nanofiltration Membranes by Controllable Interfacial Copolymerization

An ultrathin nanofibrous composite nanofiltration (NF) membrane was developed through controlled interfacial copolymerization where an electrospun sulfonated poly(ether sulfone) (SPES) nanofibrous membrane serves as the substrate and 2,5-diaminobenzenesulfonic acid (2,5-DABSA) and piperazine (PIP) serve as aqueous phase monomers. The integration of the electrostatic interaction and hydrogen bonding between SPES nanofibers and PIP/2,5-DABSA triggered the controlled diffusion rate of monomers into the organic phase, resulting in the fabrication of an ultrathin polyamide barrier layer (∼56 nm). Additionally, a polyamide structure was created through the ternary interfacial copolymerization of PIP/2,5-DABSA and trimesoyl chloride (TMC), which offers high permeability to the composite NF membrane. Meanwhile, the −SO₃H groups on 2,5-DABSA issued highly negative charges to the polyamide barrier layer, leading to a significant improvement in the rejection ratio against SO₄^2- and fouling resistance against bovine serum albumin. The impact of 2,5-DABSA monomer on the cross-linking degree and pore size distribution of the polyamide barrier layer was investigated by optimizing the proportion of PIP and 2,5-DABSA monomers in the interfacial polymerization (IP) process. The ion selectivity and robustness of the composite NF membrane was determined and compared with conventional and commercial NF membranes comprehensively. Molecular dynamics simulations were conducted to demonstrate the mechanism of the controlled diffusion of monomers; the cross-linking degree and fractional free volume of the polyamide barrier layer were also evaluated. The NF-M(1:1) composite membrane exhibited a significant enhancement in the permeation flux as 137.4 L/m²·h at 0.5 MPa, which was 4 times higher than that of conventional NF membranes, while maintaining excellent divalent salt rejection against Na₂SO₄ at 99.4%, compared with 98.0% of the conventional NF membrane, effectively breaking through the trade-off effect in the long-term filtration performance.