Proton exchange membranes by radiation grafting of styrene onto FEP films. I. Thermal characteristics of copolymer membranes

Thermal behavior of radiation‐grafted and sulfonated poly(tetrafluoroethylene‐hexafluo‐ropropylene)‐g‐polystyrene membranes was evaluated using thermogravimetric analysis. A three‐step degradation pattern was observed from thermograms of all the membranes and was attributed to the dehydration, desulfonation, and degradation of the FEP backbone. The evidence for desulfonation was reflected in the loss in ion exchange capacity of the heat‐treated samples. The membranes were found to undergo loss in ion exchange capacity beyond ∼ 200°C. No trace of sulfonic acid groups as the ion exchange sites was found above ∼ 350°C. The swelling of heat‐treated membranes was also reduced considerably. However, the loss in swelling was much higher than that in the ion exchange capacity of the corresponding membrane. The results were explained in terms of cross‐linking reaction occurring in the membrane matrix during the desulfonation process that imparts rigidity in the membrane matrix.