Anomalous Glass Transition Breadths of Random Ionomers*

Efficient control over the thermal behavior of polymeric materials is desired for shape memory and acoustic dampening materials. In this work, we report a simple yet versatile approach to tune the glass transition range of a family of random ionomers via their side-chain structure and charge fraction. We analyze random ionomers of poly (3-sulfopropylmethacrylate-ran-methyl methacrylate) by differential scanning calorimetry (DSC) and find that beyond a critical charge fraction (f_q) the glass transition temperature shifts to higher values and the glass transition breadth gets broadened significantly in response to thermal treatment. Non-equilibrium Molecular Dynamics (MD) simulations elucidate the roles of several key design parameters of the ionomers near the glass transitions, specifically the importance of the charged species and the polymer side chain. Analysis of energetics and structural relaxation dynamics reveals the effects of strong ionic correlations in a low dielectric constant medium and the side chain mobility on the transition from liquid to supercooled liquid. As f_q increases beyond a critical value, the local ionic concentrations are more heterogeneous, and the distribution of the ionic cluster sizes gets broader at the transition point. The resulting enhanced degree of compositional and dynamic heterogeneity leads to a shift in the supercooled liquid transition towards higher temperatures.