were determined with differential scanning calorimetry. With an increase in the degree of grafting and sulfonation, the glass-transition temperature increased, whereas the crystallinity percentage decreased. The tensile strength and elongation decreased with the degree of grafting and sulfonation. (c) 2005 Wiley Periodicals, Inc.Polymer electrolyte membranes were prepared via the grafting of styrene and acrylic acid onto fluorinated ethylene propylene copolymer with a preirradiation technique and subsequent sulfonation. The thermal and mechanical properties of the grafted membranes and their sulfonated derivatives were dependent on the degree of grafting. The grafted membranes showed a two-step degradation pattern, whereas their sulfonated derivatives showed a three-step degradation pattern. The glass-transition temperature and crystallinity percentage of the membranes