Blend miscibility of sulfonated polystyrene ionomers with polystyrene: Effect of counterion valency and neutralization level

Elastic recoil detection (ERD) was used to characterize the phase behavior of blends involving neutralized poly(styrene-ran-styrenesulfonate) ionomers (P(S-SS_x)-M) and deuterated polystyrene (dPS). The lightly sulfonated ionomers (acid mole fraction x = 0.007) were neutralized with various cations (M): sodium (Na⁺), barium (Ba²⁺), and zinc (Zn ²⁺). The dPS:P(S-SS_0.007)-M blends have a higher upper critical solution temperature (UCST) than the dPS:P(S-SS_0.007) blends, indicating that neutralizing the acid copolymer reduces blend miscibility. The UCST is higher when P(S-SS_0.007) is neutralized (125%) with divalent cations, Ba²⁺ and Zn²⁺, rather than with a monovalent cation, Na⁺. In addition, as the level of neutralization increases from 25% to 125%, the miscibility in the dPS;P(S-SS_0.007)-Zn blends decreases; this was not observed in the dPS:P(S-SS_0.007)-Na blends. Complementary linear viscoelastic measurements were performed on a copolymer and ionomers with a higher acid content. Upon neutralization, the storage moduli at lower frequencies indicate slower polymer chain relaxations and the self-assembly of ionic functional groups. The specific interactions that produce these physical cross-links also impede blend miscibility.