Solvent resistant asymmetrical polyimide nanofiltration membranes prepared via combining surface chemical grafting with thermal-induced pore narrowing

Abstract

Polymeric membranes with precise sieving and robust solvent resistance are desirable for molecular separation via organic solvent nanofiltration (OSN). Polyimide (PI) is a promising membrane-forming material used in OSN owing to its high thermal stability and excellent solvent resistance. However, effective methods constructing integrated ultrathin selective layer for asymmetrical PI membranes to achieve high separation accuracy and enhanced solvent resistance are rarely reported. In this work, we developed an integrally skinned asymmetrical OSN membrane by grafting tris(hydroxymethyl) aminomethane onto the PI ultrafiltration membranes, followed by a thermal annealing step at a temperature that is much lower than the glass transition temperature of PI. A dense skin layer was successfully created on the monoamine-modified PI membranes due to thermal-induced surface pore narrowing, companied by efficient intermolecular imidization. The optimised membrane exhibited acceptable and stable solvent permeance for ethanol (1.6 L m⁻² h⁻¹ bar⁻¹) and acetonitrile (9.8 L m⁻² h⁻¹ bar⁻¹). The typical membrane had a molecular weight cut-off of approximately 350 Da and demonstrated highly efficient separation of methyl orange from isatin in ethanol, indicating excellent molecular sieving properties. Moreover, the chemically crosslinked characteristic endowed the thermal-treated PI with remarkable stability of the permeation and separation performances in long-term OSN application. This work offers a convenient strategy to manufacture solvent-resistant polymeric membranes for highly-efficient molecular separation in organic solvent.