Thermal-induced surface densification of polyimide asymmetrical membranes for efficient organic solvent nanofiltration

Integrally-skinned asymmetric (ISA) polymer membranes show enormous potential in efficiently energy-saving organic solvent nanofiltration (OSN) due to their excellent structural stability. However, the control of their microstructures at the nanoscale is technically challenging, making it difficult to simultaneously achieve high separation resolution and high solvent permeance. In this work, we report the in-situ evolution of polyimide ISA membranes from ultrafiltration to nanofiltration through thermal-induced densification of the surfaces. This process was attributed to the surface dynamics of polymers — i.e., the polymer segments on surface are more freely movable than that in bulk. The optimum treatment temperature was located between the glass transition temperatures of surface and bulk, which was measured based on porosity changes. The as-prepared ISA membranes with thin and dense skin layers exhibited high solvent permeance (e.g., over 5 L m−2h−1 bar−1 for ethanol) and favorable molecular weight cut-off of around 300 Da. The membranes rivaled the state-of-the-art thin film composite membranes, demonstrating great potential in fast and precise molecular separation in OSN.