Formation of Macrovoid-Free PMDA-MDA Polyimide Membranes Using a Gelation/Non-Solvent-Induced Phase Separation Method for Organic Solvent Nanofiltration

For the first time, we demonstrated that the gelation/nonsolvent induced phase separation (Gel-NIPs) method could be applied to fabricate macrovoid-free flat-sheet nanofiltration membranes based on organo-insoluble polyimide (pyromellitic dianhydride–4,4′-diaminodiphenylmethane; PMDA-MDA). Specifically, the PMDA-MDA polyamic acid (PAA)/ N -methyl pyrrolidone (NMP) dope was cast on a polyester-based nonwoven, gelatinized in a fridge, and immersed in water. Gelation greatly inhibited the growth of macrovoids and resulted in an asymmetric membrane with high permeability but low rejection. After adding 4 wt % TiO 2 sol into the dope, the gelation process was accelerated, and the PMDA-MDA/TiO 2 membrane had a dense skin layer with a macrovoid-free cross-sectional structure. Its permeability and rejection to Rose Bengal (RB, M w = 1017 Da) were 8.3–9.2 L·m –2 ·h –1 ·bar –1 and 89.5–95.7%, respectively, in solvents including DMF, THF, and DMSO. The separation performance was stable at a feed pressure of 25 bar, indicating a great potential for organic solvent nanofiltration (OSN).