Towards organic solvent nanofiltration (OSN) membrane recyclability through Diels-Alder (DA) dynamic covalent bonds

Chemical crosslinking of polymeric membranes has been widely explored in the field of organic solvent nanofiltration (OSN) to induce superior chemical stability in harsh organic solvents, prevent membrane swelling and allow prolonged separation performance. However, covalent crosslinking leads to formation of polymeric networks that cannot be easily recycled or reprocessed at the end-of-life. In this work, dynamic covalent bonds (specifically furan-maleimide Diels-Alder (DA) cycloadducts) are used as a new approach to crosslinking of OSN membranes to obtain membranes which are simultaneously stable at operating conditions but can be recycled after use. A novel furan-modified poly(amide-imide) (PAI-FU) was prepared by reacting commercial polyimide (PI) (Matrimid® 5218) with furfurylamine (FU). Integrally skinned asymmetric PAI-FU membranes were fabricated through non-solvent induced phase separation (NIPS) and crosslinked with bismaleimide (BMI) at varying times to produce PAI-FU-BMI membranes consisting of a covalent adaptable network (CAN). The membrane crosslinked for 0.5 h exhibited favorable performance, with pure solvent permeances of 0.05, 0.52, 1.95 and 3.68 L m^-2 h⁻¹ bar⁻¹ for IPA, ethanol, ethyl acetate and acetone, respectively, and with 35 % and 95 % rejection of dyes Disperse Red 1 and Rose Bengal in acetone, respectively. Thanks to the thermoreversibility of the DA reaction, it was possible to de-crosslink and re-dissolve the OSN membranes and upcycle them to another material morphology − crosslinked PAI-FU-BMI dense films (over three cycles), hence showing the potential of alternative and more sustainable end-of-life management of used membranes.