Controllable thermal annealing of polyimide membranes for highly-precise organic solvent nanofiltration

In this work, the design of robust polyimide (PI) membranes applicable for precise molecular sieving in organic solvents using an ingenious thermal annealing process is reported. In this method, a porous PI ultrafiltration membrane was used as precursor and the temperature was controlled at around the glass transition temperature (Tg) of PI. The crosslinking and relaxation of the polymer chains occurred simultaneously to facilitate in-situ healing of the pores on the surface to form a solvent-resistance, thin, and defect-free separation layer, without affecting the bulk structure. The resultant PI membrane showed superb sieving performance towards molecules with a narrow gap in the molecular weight around 300 Da (i.e., steep S-shape rejection curve), and acceptable permeabilities for many solvents (e.g., 1.5 L m- 2 h-1 bar- 1 for acetone, and 0.76 L m- 2 h-1 bar- 1 for ethanol). These permselective performances conferred upon the membrane with great potential for precise organic solvent nanofiltration (OSN). Furthermore, a remarkable stability without obvious change in the perm-selective performances during long-term OSN application was displayed due to the crosslinked structure. This work demonstrated that thermal treatment is a useful and promising method to fabricate organic solvent resistant membranes with molecular-level sieving ability for highly-efficient OSN.