Innovative role of polyvinylpyrrolidone in tailoring polyamide layer for high-performance nanofiltration membranes

While being the preferred choice in the desalination industry, polyamide (PA) nanofiltration (NF) membranes still suffer from the permeability-selectivity trade-off challenge. This study successfully fabricated a high-performing PA NF membrane by regulating the role of polyvinylpyrrolidone (PVP) in the PA layer, which is contrary to the traditional view of PVP as a simple pore-forming agent. The study demonstrated that PVP can partially persist in the polyethersulfone (PES) substrate and impact the interfacial polymerization (IP) process, resulting in a thinner PA layer with a higher cross-linking degree and a more uniform pore structure compared to a PA layer formed on a control substrate without PVP. Molecular dynamic simulations revealed that the van der Waals force between PVP and piperazine (PIP) slowed down the diffusion of PIP at the PVP-PES substrate surface, facilitating the formation of a PA layer with a uniform pore size distribution and reduced thickness. By adjusting the concentrations of PVP in the dope solution and PIP during interfacial polymerization, a PA membrane with pure water permeability of 30.2 ± 0.9 L·m−2·h−1·bar−1 and a Na2SO4 rejection rate of 97.7 ± 0.1 % was obtained. This study provides new insights and presents a novel approach to tailoring the PA layer structure of NF membranes.