High solvent-resistant and integrally crosslinked polyimide-based composite membranes for organic solvent nanofiltration

Abstract This paper reports a new class of integral polyimide (PI)-based thin film composite (TFC) membranes (with improved solvent resistance in both the skin layer and the substrate) for organic solvent nanofiltration (OSN). The OSN membrane was prepared via interfacial polymerization (IP) onto a PI ultrafiltration (UF) substrate, followed by an imidization, a chemical crosslinking, and a solvent activation process. During the IP process, m-phenylenediamine (MPD), dopamine (DA), and 1,2,4,5-benzene tetracarboxylic acyl chloride (BTAC) were respectively used as an aqueous monomer, an aqueous additive, and an organic monomer. We proved that amide bonding formed between MPD and the PI substrate during the immersion of the substrate in the aqueous MPD solution, thus providing a strong binding between the substrate and the subsequently formed skin layer of polyamide acid (PAA), which was formed due to reaction between MPD and BTAC during the IP process. DA contains amine group and could also help build a strong binding between the substrate and the skin layer during the same IP process. The subsequent imidization step converted the PAA molecules of the skin layer into PI polymers, which was quite similar to that of the substrate. The final crosslinking step crosslinked not only the inner molecules of the skin layer, and the inner molecules of the substrate, but also the interface molecules between the skin and substrate so as to form an integral composite membrane with improving solvent resistance. The fabricated OSN membranes under optimal preparation conditions exhibited an ethanol permeance of 2.03 L m−2 h−1 bar−1 with a rejection of 98% for Rhodamine B (479 Da) and exhibited an outstanding organic solvent resistance without compromising separation performance during the persistent immersion in DMF at 80 °C for two weeks, indicating a promising prospective in OSN applications.