Graft-type polymer electrolyte membranes for fuel cells prepared through radiation-induced graft polymerization into alicyclic polybenzimidazoles

Graft-type sulfonated polybenzimidazole was prepared through radiation-induced graft polymerization of styrenes into an alicyclic polybenzimidazole film and subsequent sulfonation. The alicyclic polybenzimidazole, ChPBI, was prepared from trans-1,4-cyclohexanedicarboxylic acid and 3,3′-diaminobenzidine using typical polycondensation. Anisotropic domains with a size of several tens of micrometers were found in the ChPBI films casted from LiCl-containing N,N-dimethylacetamide. Irradiation of the ChPBI membranes with a 220 kGy dose of γ-rays created radical species with mean lifetimes of two days. The treatment of this membrane with a 50/50 (v/v) mixture of 1-propanol and styrene produced polystyrene graft chains. Sulfonation of the resulting grafted membrane with ClSO3H occurred selectively on the polystyrene grafts. The sulfonated films showed proton conductivity on the order of 10−3–10−2 S/cm with an ion exchange capacity between 2.1 and 2.9 mmol/g. SEM-EDX analysis of the membrane indicated the presence of macrophase separated domains up to 1 μm in diameter. The proton conductivity of the membrane did not decrease for 600 h at 120 °C in liquid water.