Polyimide solution with reversible sol-gel transition by construction of dynamic π-π stacking

Constructing fully rigid chain polyimides with reversible sol-gel/gel-sol transition is significant for developing high-performance and functional materials. π-π stacking, a typical weak interaction, has attracted great interest in producing functional gels, but less has been reported in polyimide organogels. This work developed new polyimide organogels using π-π interactions by incorporating bisbenzoxazole moieties with fully coplanar structures in polyimides. The reversible sol-gel transition behavior with temperature and strain response was systematically studied using rheological methods. DFT simulations, temperature-dependent NMR, UV–Vis, and fluorescence spectroscopy results confirmed the existence of the dynamic π-π interactions in polyimides. Based on the success, we further combined sol-gel methods and non-solvent phase separation methods to prepare a new type of nanofiltration (NF) membranes. The resulting NF membranes showed a higher rejection of methylene blue (rejections of 99.6%) and a good permeability of sodium salt (rejections of 5.1–7.7%), showing great promise for selective filtration of industrial dye wastewater.