Preparation and properties of covalently cross-linked sulfonated copolyimide membranes containing benzimidazole groups

A series of sulfonated copolyimides containing benzimidazole groups (SPIs) were synthesized by random copolymerization of 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), 2-(4-aminophenyl)-5-aminobenzimidazole (APABI), 4,4′-diaminodiphenyl ether-2,2′-disulfonic acid (ODADS) and 9,9-bis(4-aminophenyl)fluorene (BAPF) in m-cresol in the presence of benzoic acid and triethylamine at 180 °C for 20 h. Membranes with good mechanical properties were prepared by solution cast method. Proton exchange treatment resulted in ionic cross-linking and the membranes were further covalently cross-linked by treating them in polyphosphoric acid (PPA) at 180 °C for 6 h. The covalently cross-linked membranes displayed slightly lower ion exchange capacities (IECs) and proton conductivities than the corresponding covalently uncross-linked ones because small part of the sulfonic acid groups had been consumed during the cross-linking process. Fenton's test (3% H₂O₂ + 3 ppm FeSO₄, 80 °C) revealed that benzimidazole groups played an important role in the radical oxidative stability of the membranes, while the cooperative effect of benzimidazole groups and covalent cross-linking led to much more significant enhancements in the radical oxidative stability of the membranes than each alone. The membrane 4 (ODADS/APABI/BAPF = 2/1/1, by mol), for example, after covalent cross-linking could maintain membrane form within 8 h measurement, which was much longer than that (3 h) before covalent cross-linking under the same conditions. The membrane 5 (ODADS/BAPF = 3/1, by mol) without benzimidazole groups, however, after covalent cross-linking started to break into pieces after 85 min measurement, which was only slightly longer than that (60 min) before cross-linking under the same conditions.