The thermal stability of PSU and SPSU was studied by TGA. The decomposition curves of PSU and SPSU with both sulfonating reagents are shown in Fig. 2. As the SPSU: TMSCS/ClSO3H molar ratio increased, the thermal stability of the polymer decreased.However, the results indicated that the thermal stability of SPSU is adequate within the conceivable temperature range for PEMFC application.
TGA curves of SPSU exhibited a three-step degradation pattern compared to the single-step degradation of PSU. The weight loss at temperatures ranging from 50 to 200 C corresponds to the loss of absorbed water, which interacts with the hydrophilic sulfonic groups. The second weight loss between 200 C and 400 C was due to the desulfonation process, which involves evolution of SO2 and SO gases. Finally, the third weight loss above 450 C is related to main chain polymer decomposition. Similar results were observed for other sulfonated high performance polymers [24]. The second weight loss was used to assess the DS of the modified polymers using equation (8).