The thermal behavior of perfluorosulfonated membranes of three equivalent mass (EW=910, 1000 and 1100 g eq−1) has been studied for membranes in acid form and in the alkali metal countercations substituted samples. The water contents
of the membranes decrease progressively with increasing EW and the countercations charge density. The monovalent cations substitutions increase the membranes thermal stability. DSC
curves show a single endothermic peak around 120�C that give low peak temperature for low EW and high peak temperature for large cations size. The membrane mechanical properties changed for different EW and temperatures of membranes. Stress-strain analysis showed that K+ substituted membranes at both temperatures present a highest YM compared to the other alkali cation substitutions. The thermal
properties of perfluorosulfonated membranes depend on the water contents, cation size, temperature and also on EW value.