EPR investigation of HO. radical initiated degradation reactions of sulfonated aromatics as model compounds for fuel cell proton conducting membranes

In order to predict hydroxy radical initiated degradation of new proton con ducting polymer membranes based on polystyrene, polyethersulfone, polyether etherketone, or on polymers obtained by radiation grafting of styrene on di fferent fluoropolymers, eight sulfonated aromatics were chosen as model com pounds for EPR experiments, aiming at the identification of products of HO . radical reactions with these polymers. Photolysis of H2O2 was employed as the source of hydroxyl radicals. A detailed investigation of the pH profil e was carried out for p-toluenesulfonic acid. Besides benzyl- and hydroxy-c yclohexadienyl radicals at lower pH values, phenoxyl radicals were identifi ed, predominating in the pH range 10.5-13.0. A large number of new radicals give evidence of multiple hydroxylation of the aromatic rings, confirming reaction mechanisms proposed on the grounds of product analysis, but no evi dence of dimerisation was found. The result as regards stability of organic proton exchange membranes for fuel cells is, that all unsaturated bonds an d weakly bound atoms are subject to immediate attack by HO .. Ether links o pen by HO . ipso addition. Strategies for the reduction of membrane degrada tion should focus on a minimisation of HO . formation and of its access to the interior of the membrane.