Reversible crosslinking of polybenzimidazole-based organic solvent nanofiltration membranes using difunctional organic acids: Toward sustainable crosslinking approaches

Solvent resistance is a crucial property for achieving organic solvent nanofiltration (OSN) membranes with good separation performance and long lifespan. Polybenzimidazole (PBI) has been widely used for OSN membrane fabrication owing to its excellent mechanical properties, thermal stability, and resistance in various organic solvents. However, pristine PBI membranes show unsatisfactory stability in polar aprotic solvents. To circumvent this issue, extensive research has been conducted on the covalent crosslinking of PBI membranes. Herein, to develop a more sustainable crosslinking strategy, difunctional organic acids — oxalic and squaric acids — were used for the reversible crosslinking of PBI membranes in water at room temperature, affording robust membranes with long-term stability in polar aprotic solvents. An analysis at the molecular level of the difunctional acid crosslinking system based on pK$_a$ values and binding energies was presented. The molecular weight cutoff and solvent flux of the PBI membranes crosslinked with oxalic and squaric acids were 779 and 844 g mol$^{−1}$ and 153.9 and 139.7 L m$^{−2}$ h$^{−1}$ bar$^{−1}$ in N,N-dimethylacetamide at 30 bar, respectively. The crosslinking was successfully reversed under basic conditions, allowing the recovery of the pristine PBI polymer. A sustainability assessment revealed the advantages of the proposed crosslinking system over conventional covalent crosslinking approaches.