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+.