Relationship between absorbed water and proton conductivity in sulfopropylated poly(benzimidazole)

A novel proton-conducting polymer, sulfopropylated poly(benzimidazole) (PBI-PS), was synthesized by the ring-opening reaction of 1,3-propanesultone on the reactive N-H groups of PBI. Cast films of PBI-PS obtained from a dimethylsulfoxide solution of PBI-PS exhibited high water uptake, adequate mechanical strength, and flexibility. The temperature dependence of proton conductivity for PBI-PS films hydrated at 90% R.H. showed high proton conductivity of the order of 10−3 Scm−1 in the temperature range from room temperature to 140 °C, which is superior to those of conventional proton conducting polymers such as Nafion. The physical behavior of absorbed water in hydrous PBI-PS films was evaluated by differential scanning calorimetry (DSC), indicating that the absorbed water in PBI-PS mainly consisted of freezing bound water.