Influence of Ionic Interaction Strength on Glass Formation of an Ion-Containing Polymer Melt

A deep understanding of the glass formation of ion-containing polymers is crucial for applications of high technological relevance, but it remains a significant challenge to develop a molecular theory to predict thermodynamic and dynamic properties of these materials. Here, we perform extensive molecular dynamics simulations of an ion-containing polymer melt having variable ionic interaction strength to address the essential phenomenology of thermodynamic and dynamic properties upon approaching the glass-transition temperature. Both thermodynamic and dynamic properties are shown to be strongly influenced by ionic interaction strength. The characteristic temperatures of glass formation increase with increasing ionic interaction strength in a nearly linear fashion, a trend that can be rationalized based on changes in the cohesive energy density induced by ionic interactions. The growth of fragility with increasing ionic interaction strength is found to be somewhat nonlinear. Moreover, we demonstrate that the string model of glass formation provides an excellent description of the relationship between the structural relaxation time and the string length in our ion-containing polymer melt having variable ionic interaction strength, implying that the entropy theory provides a promising tool for investigating the glass formation of ion-containing polymers if ionic interactions are incorporated properly. The string model also allows us to establish the general trends of the enthalpy and entropy of high temperature activation in variation with ionic interaction strength, fundamental energetic parameters that play a key role in the entropy theory of glass formation. Our simulation results should be helpful for developing a general framework to understand the glass formation of ion-containing polymers.

Automated summary

Extensive molecular dynamics simulations of ion-containing polymer melts show that both thermodynamic and dynamic properties are strongly influenced by ionic interaction strength, with characteristic temperatures of glass formation increasing linearly with increasing strength. Additionally, the growth of fragility with increasing ionic interaction strength is somewhat nonlinear. The string model of glass formation is found to provide an excellent description of the relationship between structural relaxation time and string length, indicating the potential of entropy theory in studying the glass formation of ion-containing polymers.