Effects of adjacent groups of benzimidazole on antioxidation of polybenzimidazoles

The chemical oxidative stabilities of poly(2,2-(m-phenylene)-5,5-bibenzimidazole) (PBI-ph), poly(2,5-benzimidazole) (ABPBI), poly(2,2′-hexyl-5,5′-bibenzimidazole) (PBI-hex), and poly(2,2′-imidazole-5,5′-bibenzimidazole) (PBI-imi) are studied. By means of FTIR and 1HNMR analysis, more information about the degradation process of PBI-ph is found as: CH2 groups are left in the residual polymers; after the N-H bond and the trisubstituted benzene ring are oxidized by oxidative free radicals, the meta-phenylene is relatively stable. Through Fenton tests, the chemical oxidative stabilities of these PBIs are compared and results show that PBI-ph is the stablest material while PBI-imi is the unstablest one. Through FTIR analysis, the structure changes to those degraded PBIs are compared. The conjugated structure formed between meta-phenyl and benzimidazole can protect the main chain of PBI-ph from the attack of oxidative free radicals. Additionally, effects of acid on PBI-ph degradation rate are evaluated and the results show that phosphoric acid can slow down the chemical oxidative degradation.