Regulation of sub-Tg solid-state surface crosslinking of reactive polyarylene ether substrate into loose nanofiltration membranes for dyes desalination.

The scalable fabrication of high performance dyes desalination loose nanofiltration (LNF) membrane through facile thermal annealing remains challenging due to the susceptible pore collapse. Herein, we have developed a metal ion mediated sub-Tg thermal crosslinking protocol, which can convert the phase inverted reactive polymeric ultrafiltration substrate into LNF membrane showing high permselectivity as well as resistance to both extremely acid and alkaline solution. The original ultrafiltration substrate was composed of scalable-produced reactive polyarylene ether amidoxime (PEA) that was pre-crosslinked with ferric ions. After heat annealing at a temperature much lower than the bulk Tg temperature of PEA, the nascent Fe pre-crosslinking PEA ultrafiltration substrate was converted into LNF membrane with an ultrathin, highly cross-linked surface selective layer and stable finger-like porous sub-layer. Consequently, the optimal LNF membrane achieved 81.6 L m⁻² h⁻¹ bar⁻¹ permeability with 97.24 % rejection for 50 ppm RhB solution, and exhibited a stable dyes desalination for repeated filtration under aggressive HCl (5 wt%)/NaOH (10 wt%) solution. Moreover, scalable membranes were fabricated via a roll-to-roll process and demonstrated excellent long-term stability after 24 h of tangential flow operation at both pH 5 and pH 9. Various characterizations and molecular dynamics simulation revealed that the competitive dyes desalination was ascribed to the asymmetric enhancement of crosslinking between Fe and surface-enriched lower molecular weight PEA fractions during the lower temperature annealing. Given the facile synthesis of reactive polymer and scalable membrane fabrication, the current work opens a new way to develop highly permeable LNF membranes for purification of aggressive industrial dye wastewater.