Effect of thermal treatment on cation local structure in manganese-neutralized sulfonated polystyrene ionomers

Ionomers are copolymers consisting of primarily nonionic repeat units, with a small fraction (<15%) of ionic comonomer [1]. The marked changes in material properties observed upon incorporation of ionic comonomer are due to the aggregation of the ionic groups into microdomains. One ionomer which has been studied in detail by numerous investigators is lightly sulfonated polystyrene (SPS), produced by a post-polymerization sulfonation of polystyrene to place sulfonic acid groups at the para-positions of a fraction of the phenyl rings. The distribution of ionic sites along the polymer backbone is random. We have employed EXAFS to study the local cation environment in two MnSPS samples (2.6 and 7.6 mol % sulfonation) subjected to three thermal histories (solvent casting from THF/water, solvent casting followed by annealing above T_g, and compression-molding). The state of aggregation of the ionic groups was determined by the presence or absence of the ionomer peak in the SAXS pattern.