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

Abstract

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 T_g 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 T_g 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.

Keywords:

• glass transition
• ion content
• ionic aggregate morphology
• ionomers
• poly(styrene-ran-cinnamic acid)

Acknowledgments

We acknowledge with gratitude that this work was supported by the Natural Science Foundation of the Hubei Province of China (2014CFA094), as well as by the National Natural Science Foundation of China (51703053), the Overseas High-Level Talents Scientific-Research Starting Fund of Hubei University of Technology, China (HBUT-science-[2005]2), and the Select Overseas Chinese Scholars Science and Technology Activities Foundation of the Ministry of Human Resources and Social Security of China ([2013]277).