Effect of Thermal Treatment on Cation Local Structure in Manganese-Neutralized Sulfonated Polystyrene Ionomers

Two sets of manganese-neutralized sulfonated polystyrenes, at 2.6 and 7.6 mol % sulfonation, were subjected to three sample treatments: solvent casting from tetrahydrofuran/water, solvent casting followed by annealing, and compression molding. Both as-cast samples were void of ionic aggregates, as evidenced by the absence of a peak in their small-angle X-ray scattering (SAXS) patterns. Upon annealing, the SAXS peak appeared, indicating the formation of ionic aggregates. The compression-molded 7.6 mol % material also exhibited the SAXS peak, while the compression-molded 2.6 mol % material did not, possibly due to a slower rate of aggregation in the lower ion content material. The local coordination structure about the Mn²⁺cation was examined by extended X-ray absorption fine structure (EXAFS) spectroscopy and found to be nearly identical for all six samples despite the differences in the state of aggregation. That the local structure is unaffected by the microdomain structure reflects the difference in size scale probed by SAXS and EXAFS.