Preparation of polymer electrolyte membranes consisting of alkyl sulfonic acid for a fuel cell using radiation grafting and subsequent substitution/elimination reactions

Polymer electrolyte membranes (PEMs) consisting of alkylsulfonic acid were successfully synthesized by radiation-induced graft polymerization of methyl acrylate (MA) into a poly(ethylene-co-tetrafluoroethylene) (ETFE) film, followed by sulfonation at an α-carbonyl carbon of the MA units in the grafting chain using an equimolar complex of chlorosulfonic acid and dioxane (ClSO3H-complex). Sulfonation reaction with ClSO3H-complex in the grafted chains of acrylic acid derivatives (acrylic acid and N,N-dimethylacrylamide) other than MA in the film could not be performed. The ion-exchange capacity (IEC) and ion conductivity of the MA-grafted PEM were controlled in the respective ranges of 0.54–1.47 mmol/g and 0.014–0.14 S/cm by changing the grafting degree. It has been confirmed by titrimetric and gravimetric analyses of sulfonic and carboxylic acids in the film that the sulfonation reaction is accompanied with decarboxylation of the MA units equimolar to the substituted sulfonic acids (substitution of carboxylic acid with sulfonic acid), and that the sulfonation degree was ca. 30%.