Surface effect-tailored polymer membranes with an integrated extra-thin separation layer for robust nanofiltration

Abstract

Nanofiltration (NF) technology has been keeping an unassailable lead in molecular and ionic separations. However, current polymeric NF membranes fall short of expectations in selectivity and stability when it comes to some heavy-duty organic, alkaline, and acidic environments. In this work, an asymmetric poly(ether sulfone) (PES) NF membrane with an integrated and extra-thin separation layer was developed by utilizing a thermal treatment method based on the polymer surface effect. A PES ultrafiltration membrane with a porous patterning surface was first fabricated by nonsolvent induced phase separation, and then thermally annealed at 5 degrees lower than its glass transition temperature to fully activate the motion of surface polymer chains to kinetically heal surface pores. The healing mechanism of surface pores was confirmed as the thermodynamic minimization tendency of surface energy driven by the advanced mobility of PES chains on the surface. The formed separation layer was 88 nm in thickness, far less than conventional integrated asymmetric membranes and comparable to current thin-film composite (TFC) membranes. As expected, the membranes had high rejection to organic molecules and inorganic salts with a molecular weight cut-off of around 600 Da, while maintaining a competitive water permeance up to 8 L m−2 h−1 bar−1 to current TFC membranes. Further, they demonstrated high acid tolerance without performance decline during a 30-h-long test after being immersed in 8 wt% H2SO4 aqueous solution for 24 h. This work offers a new strategy to develop high permeability, selectivity, and environmental-resistance NF membranes to achieve efficient molecular and ionic separations in extreme conditions.