Facilely prepare high-performance loose nanofiltration membranes through regulation of polymer chain entanglements

The membrane forming process was fundamentally affected by the thermodynamics of polymer solution, the core of which involved the governance of polymer chain entanglements in the solution. In this work, a high-performance loose nanofiltration (LNF) membrane with high water permeance and high separation accuracy was facilely fabricated by the regulation of polymer chain entanglements through only changing CaCO3 nanoparticle (nano-CaCO3) size in the casting solution. The degree of polymer chain entanglements was characterized by the critical entanglement viscosity of the casting solution. The solution viscosity was markedly increased with the decrease of nano-CaCO3 size and then significantly reduced when nano-CaCO3 disappeared. While the mutual diffusion rate between the non-solvent (acidic water) and the solvent was increased to bring about rapid phase inversion with increasing the viscosity of the casting solution. The high solution viscosity and rapid phase inversion, which was a unconventional phenomenon, leaded to the formation of LNF membranes with outstanding performance. The LNF membranes obtained had a pore diameter of 1.4 nm (MWCO = 1625 Da) and a high pure water permeance of 98 L·m−2·h−1·bar−1, accompanying an excellent separation accuracy of small molecule dyes and NaCl mixtures (the rejection is 99.9% for eriochrome black T (EBT, Mw = 461 Da) and 4.5% for NaCl). This work provides general guidance for the structural regulation of high-performance separation membranes.