Morphology of Lightly Carboxylated Polystyrene Ionomers

Lightly carboxylated polystyrene (CPS) ionomers have been synthesized via free-radical copolymerization of styrene and 4-vinylbenzoic acid, and their morphology and properties compared with those of lightly sulfonated polystyrene (SPS) ionomers. CPS ionomers neutralized with Na⁺ give SAXS patterns similar to those exhibited by SPS ionomers, suggesting rather complete aggregation of ionic groups. However, when neutralized with Zn²⁺, a poorly-ordered morphology is produced, with many ionic groups remaining unaggregated. Differences are also evident in the dynamic mechanical behavior, with NaCPS exhibiting a strong high-temperature relaxation peak whose temperature increases faster than that of the matrix glass transition, while ZnCPS shows a weak high-temperature peak whose temperature increases in step with the matrix transition. By using a weaker acid substituent, cation effects on morphology are amplified relative to those seen in SPS ionomers.