Monomer Concentration Regulation Enables Highly Cross-Linked Polyamide Membranes for Robust High-Temperature Separation

Separation under high temperature is a crucial challenge for polymer nanofiltration (NF) membranes. Polyamide NF membranes with favorable compactness and pore size distribution are promising for boosting their thermal stability and high-temperature separation performance. Despite recent strategies to improve thermal stability, the effect of polyamide membrane compactness on structural changes and separation performance at high temperatures remains insufficiently explored. Herein, we propose a straightforward and effective concentration regulation strategy to address this challenge in the classic piperazine-trimesoyl chloride (PIP-TMC) interfacial polymerization system, focusing on the precise tuning of the polyamide network structure and the high-temperature NF performance through the controlled adjustment of monomer concentration and the relative ratios. Polyamide layer fabricated with a PIP/TMC ratio of 30:1.5 exhibits higher cross-linking degree than that with a PIP/TMC ratio of 2:1.5, and the NF membrane exhibits exceptional thermal stability, with MgSO₄ rejection remaining >98.5% at 85 °C. Molecular dynamics simulations reveal that denser polyamide network possesses significantly weaker segmental motion, along with smaller changes in pore size distribution and fractional free volume at elevated temperatures. This concentration regulation strategy effectively enhances the intrinsic thermal stability without complex modifications and provides critical insights into polyamide segment evolution at high temperature.