Influence of Charge Interaction Strength and Counterion Size on the Structure and Dynamics of Simulated Telechelic Ionomer Melts

Ionomer melts offer the promise as structural materials in a variety of technologically important applications due to their unique physical properties, but a molecular understanding of their structure and dynamics is crucial for guiding the rational design of these materials. Here, we perform systematic simulations of coarse-grained telechelic ionomer melts having variable charge interaction strength and counterion size at a relatively high temperature well above the glass transition temperature, a thermodynamic condition that not only allows for the formation of ionic aggregates but also enables us to determine chain and ion diffusion. After demonstrating that cohesive interaction strength can be largely tuned by charge interactions and counterion size, we provide a detailed characterization of the structural properties of ionic aggregates. We find that telechelic ionomer melts with small counterions tend to form isolated, compact ionic aggregates with a relatively narrow size distribution, while those systems with larger counterions exhibit a wider distribution in the aggregate sizes, where the relatively large aggregates are more extended and branched. We also show that the formation of ionic aggregates has a significant influence on the chain conformational properties and that the chain conformations are dominated by bridges in the regime of strong charge interactions, but there is also a modest fraction of loop chains, where both ionic groups of the chain participate in the same aggregate. Finally, we discuss how the dynamics of telechelic ionomer melts respond to the formation of ionic aggregates, based on the end-to-end vector relaxation, stress autocorrelation function, mean square displacement, continuous ion-association function, and non-Gaussian parameter. We find that both chain and counterion dynamics are suppressed upon increasing the charge interaction strength and decreasing the counterion size, accompanied by an increase in the degree of non-Gaussianity. Our study sets the stage for investigating other important aspects of telechelic ionomer melts, such as nonlinear rheology and glass formation.