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Crosslinked polybenzimidazole (PBI) was synthesised via free radical polymerisation between N‐vinylimidazole and vinylbenzyl substituted PBI. The degree of crosslinking increases with increasing content of the crosslinker. The phosphoric acid doping behaviour, mechanical properties, proton conductivity and acid migration stability of crosslinked PBI and linear PBI are discussed. The results show that the acid doping ability decreases with increasing degree of crosslinking of PBI. The introduction of N‐vinylimidazole in PBI is beneficial to its oxidation stability. The mechanical stability of crosslinked PBI/H3PO4 membrane is better than that of linear PBI/H3PO4 membrane. The proton conductivity of the acid doped membranes can reach ∼10–4 S cm–1 for crosslinked PBI/H3PO4 composite membranes at 150 °C. The temperature dependence of proton conductivity of the acid doped membranes can be modelled by an Arrhenius relation. The proton conductivity of crosslinked PBI/H3PO4 composite membranes is a little lower than that of linear PBI/H3PO4 membranes with the same acid content. However, the migration stability of H3PO4 in crosslinked PBI/H3PO4 membranes is improved compared with that of linear PBI/H3PO4 membranes.

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