Linear viscoelasticity of sulfonated polystyrene ionomers based on bivalent cations

Linear viscoelasticity is analyzed for unentangled sulfonated polystyrene ionomer samples, SPS-X, neutralized with transitional bivalent metal ions (with X = Zn2+, Mn2+, Co2+, and Ni2+). Two model systems, with the degree of gelation slightly above the gel point (1.0 mol. %) and close to the full gelation point (1.6 mol. %), are chosen. For either system, the amplitude of plateau is insensitive to, while the terminal relaxation time is highly dependent on, the degree of neutralization. The terminal relaxation time first increases, but then decreases with the increase in the degree of neutralization. This trend is discussed with respect to the difference in dissociation rates between the acidic, ionic associations and free salts that coexist in the ion aggregates. The transition point, where the terminal relaxation time achieves the highest value, occurs at the stoichiometric point for Zn2+, but is higher than the stoichiometric point for Mn2+, Co2+, and Ni2+. Both the ion content at the transition point and the terminal relaxation time follow the order of SPS-Ni > SPS-Co > SPS-Mn > SPS-Zn. This order contrasts with SPS-X with alkali counterions where the transition point is consistently the stoichiometric point and the terminal relaxation time increases with the decrease in the counterion size. We attribute this order to the different filling statuses of the electrons in the 3d orbital for these SPS-X samples.