Transformation of cross-linked polyimide UF membranes into highly permeable SRNF membranes via solvent annealing

Abstract

A simple method for the preparation of highly permeable solvent-resistant nanofiltration (SRNF) membranes was developed. By applying a solvent treatment to cross-linked polyimide ultrafiltration membranes, polymer chain flexibility increased and the matrix rearranged into a more dense structure, creating highly selective SRNF membranes with exceedingly high ethanol permeance. This densification was driven by the ability of the membrane to lower its free energy while in the solvated state via the establishment of extra favorable interactions, like hydrogen bonds and π interactions. Moreover, further reaction of only partly reacted cross-linker molecules was completed during the treatment, thus enhancing the cross-linking degree. The extent of densification depended on the type of solvent, the immersion time and the initial cross-linking degree of the membrane, all influencing the degree of solvation and chain rearrangement. By altering the synthesis conditions, a membrane with equal selectivity to Duramem 300 but showing a 400% higher ethanol permeance was obtained. This demonstrates the high potential of the technique to be applied as novel method for the preparation of SRNF membranes with exceptionally high solvent permeance.