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

Abstract

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. When a nonionic monomer is copolymerized with a small amount of ionic comonomer, profound changes in prop- erties are observed relative to the nonionic parent polymer. The ionic copolymers, termed ionomers, typically contain less than 10 mol % of a sulfonic or carboxylic acid-bearing monomer and are neutralized with metal cations. The marked increases in, for example, modulus and tear and abrasion resistance and the dramatic alterations in ion- transport propertie~l-~ that occur in these materials are now accepted as being due to aggregation of the ionic groups4i5 into microdomains. Because of the small size of these ionic aggregates, diffraction techniques can reveal little regarding their in- ternal structure. On the other hand, extended X-ray ab- sorption fine structure (EXAFS) spectroscopy is a powerful technique for determining the local atomic environment about a specific atom and has been profitably applied to numerous ionomers.6-17 The EXAFS signal is a modula- tion of the X-ray absorption coefficient on the high-energy side of an element's absorption edge, and it contains in- formation on the coordination shells surrounding that atom. The type and number of atoms in a shell, as well as the distance to and disorder within a shell, are reflected in the EXAFS data. As such, EXAFS can probe the en- vironment around ions that reside in ionic aggregates, as well as dispersed ionic groups and even ions in solution.1° One ionomer that has been studied in detail by nu- merous in~estigators~~~ ~~'~~~~~~