Effects of Ion Content on the Ion Aggregation Morphology and Glass Transition of Poly(styrene-ran-cinnamic acid) (SCA) Zn-Salt Ionomer

Poly(styrene-ran-cinnamic acid) (SCA) Zn-salt ionomers (SCA‒Zn) bearing increasing contents (0, 0.23, 0.55, 1.3, 3.0, and 5.5 mol%) of Zn2+ ions were successfully synthesized by solution neutralization of the SCA with Zn2+ ions at room temperature. As the ion content of the SCA‒Zn increased from 0 to 5.5 mol%, the size and the aggregation number of the ionic aggregates did not change significantly, because the increase in ion content led to a monotonic increase in the number density of ionic aggregates, while the number of ions participating in the aggregates to form ionic aggregates was essentially fixed. Furthermore, the Tg values of the SCA‒Zn increased slowly as the Zn2+ ion content increased, and the linear increase rate with Zn2+ ion content was ∼0.8 °C mol−1, which was quite different from the reported increase rate in the Tg values of the matrix phase with Na+ ion content of 3–6 °C mol−1. This appeared to be caused by the steric hindrance effect of the adjacent groups of –COO− ions in the SCA and the long-distance electrostatic attraction. In addition, the degree of ionization of the SCA‒Zn was too low to cause large changes in thermal stability.