Metal ion cross-linked nanoporous polymeric membranes with improved organic solvent resistance for molecular separation

Abstract

The strategy of chemical cross-linking has been well used in the system of polyimide membranes for organic solvent nanofiltration (OSN). However, this method significantly affects the permeability and mechanical properties of polyimide membranes, as the opening of imide bond during the chemical cross-linking reaction damages the original polymer backbone structure. This work presents the formation of a new generation of OSN membranes through facile metal ion cross-linking of integrally skinned asymmetric polyimide membranes. A type of specially designed polyimide is synthesized with carboxyl functional groups introduced into the polymer backbone as organic ligands, and metal ion coordination cross-linking reaction takes place between the functionalized polyimide membranes and metal ions. Especially, the Cu2+ ion cross-linked polyimide membranes exhibit good stability in a variety of organic solvents including toluene, DMF, acetone, methanol, acetonitrile, etc., with >99% rejection to Coomassie brilliant blue (MW = 854) in acetone. Meanwhile, no significant attenuation of membrane permeability is observed after the cross-linking, as the metal ion does not occupy as much free volume as traditional chemical cross-linkers. Furthermore, the metal ion cross-linking is found to greatly improve the compaction resistance of the membrane due to the enhanced polymer-chain rigidity. This work confirms the feasibility of ion coordination based polymeric membranes for organic solvent separation applications and provides a brand-new route for the fabrication of OSN membranes with unique advantages.