Vibrational and thermal characterization of Nafion membranes substituted by alkaline earth cations

The vibrational spectra of Nafion^® membranes with alkaline earth cations as counterions have been obtained by photoacoustic infrared spectroscopy (PAS-IR) and Fourier transform Raman spectroscopy (FT-Raman). The symmetrical stretching band of the sulfonate group (∼1060 cm⁻¹) decreases with increasing atomic mass of the countercation, excepting for Mg²⁺, in both techniques. The thermal behavior of Nafion membranes by means of differential scanning calorimetry (DSC) curves showed a dependence on the water content and a similar profile between Nafion-Mg²⁺ and -Ca²⁺, and between Nafion-Sr²⁺ and -Ba²⁺, for the first scanning curve. An ageing effect was also observed on Nafion by DSC curves. High-resolution thermogravimetry (TG) curves under nitrogen atmosphere showed similar decomposition profiles. It was determined from TG curves that there is a decreasing water content with increasing ionic radius of cation, and the thermal stability increasing from Nafion-Mg²⁺ to -Ba²⁺.