EXAFS studies of various sulfonated and carboxylated cadmium ionomers

Four sulfonate ionomers and one carboxylate ionomer, all neutralized with cadmium, were studied using the technique of extended X-ray absorption fine structure (EXAFS) spectroscopy. The cadmium in all four sulfonate ionomers had essentially identical EXAFS oscillations, which suggests that the influence of the identity of the polymer matrix on the local aggregate structure is minimal. In contrast, the cadmium in the carboxylate ionomer had a substantially different local environment, showing that, as expected, the local environment of the cadmium cation depends on anion type. For the sulfonate ionomers, simulations of the EXAFS oscillations with reasonable fitting parameters using the crystal structure of cadmium methanesulfonate dihydrate fit the experimental patterns very well. This agreement between theory and experiment strongly suggests that the atomic arrangement around the cadmium atom in the ionomer is the same as the arrangement around the cadmium atom in the small-molecule analogue. In the carboxylate ionomer, small but significant differences existed between the experimental pattern and the pattern generated theoretically using atomic positions derived from the small-molecule analogue cadmium acetate dihydrate. The reason for the difference is unclear; however, if the atomic positions in the ionomer were only slightly different from those:in the crystal structure, this could have led to the disagreement between the experimental data and the fit.