Nano‐confined Supramolecular Assembly of Ultrathin Crystalline Polymer Membranes for High‐Performance Nanofiltration

Abstract Polymer membranes with high permeability, high salt rejection, and mechanical integrity are desirable in water treatment and purification. However, it remains a daunting challenge to achieve ultrathin yet robust polymer membranes harvesting all the above features for nanofiltration. Here, a new approach of nano‐confined supramolecular assembly to fabricate ultrathin crystalline polymer membranes with a modulus of 1 GPa and a thickness of 6.5 nm is reported. The microdroplet carrying amphiphilic tetra‐oligomers can quickly spread at the air–‐water interface, where the hydrophilic motifs such as carbonyl and hydroxyl groups can reconfigurably anchor down to water molecules via abundant hydrogen bonding interactions, significantly promoting the alignment and orientation of hydrophobic alkyl chains within the nano‐confined space. The resultant nano‐films exhibit mechanical robustness as well as excellent ion sieving with improved NaCl rejection of 81.3% and unprecedented Na 2 SO 4 rejection of 99.9% without compromising water permeation, outperforming the reported and commercial state‐of‐the‐art polymer membranes. This work enables the rapid production of over 100 cm 2 ultrathin crystalline polymer membranes with great nanofiltration potential and highlights the critical role of supramolecular assembly in the chemical and structural configurations in a nano‐confined space.