Structural Changes in Semi-Crystalline Ethylene-Based Ionomers During the Heating Process

The structural behavior of ionic aggregates in semi-crystalline ethylene-based ionomers during heating was studied to understand the influence of different ionic groups on their properties. The ionomers were synthesized by neutralizing carboxylic acid groups with Zn and Na ions at varying ratios. Differential scanning calorimetry (DSC) revealed two distinct endothermic peaks, with the melting point being largely unaffected by the Zn/Na ion ratio. However, the melting enthalpy of <i>T</i>_<i>i</i> is maximum at Na/Zn ratios of 3:7 and 5:5, suggesting that crystallites preferentially grow in the presence of both ions are present. In situ wide-angle X-ray scattering (WAXS) identified temperature-dependent transitions, where monoclinic crystals melted near <i>T</i>_<i>i</i> and recrystallized into orthorhombic forms, which subsequently melted as the temperature approached <i>T</i>_<i>m</i>. Small-angle X-ray scattering (SAXS) analysis, employing the Yarusso-Cooper equation, indicated a temporary expansion of ionic aggregates near <i>T</i>_<i>i</i>, followed by further expansion near <i>T</i>_<i>m</i>. These expansions were attributed to the melting of polyethylene crystals, which were found to compress the aggregates under normal conditions. These findings provide insights into the relationship between ionic group composition, microstructural evolution, and thermal behavior in ethylene-based ionomers, with implications for their application in temperature-sensitive environments.