Unusual Glass Transition Breadths of Ionomers: Effects of Thermal Treatment and Charge-Carrying Side Chains
H Umana-Kossio and TD Nguyen and JRY Wang and MO de la Cruz and JM Torkelson, MACROMOLECULES, 55, 6536-6546 (2022).
Effective control over the thermal behavior and mechanical strength of polymeric materials has been highly sought for decades and continues to this day, particularly with the urgent demand for highly durable energy storage devices and soft electronics, to name a few. Here, we report a simple yet versatile approach to fine-tune the glass transition range of a family of ionomers via their side-chain structure and charge fraction. We analyze ionomers of poly(3-sulfopropyl methacrylate-ran-methyl methacrylate) that are synthesized by conventional free-radical polymerization. Using derivative heat flow curves from differential scanning calorimetry, we find that above a critical low charge-carrying side-chain fraction (f(q)), the glass transition temperature shifts to higher values and the glass transition breadth increases significantly in response to thermal treatment. After several thermal cycles, values of glass transition breadth as high as 90-104 degrees C were obtained, and the evolution from one glass transition regime to two distinct, contiguous glass transition regimes was evident. Quenched molecular dynamics simulations elucidate the roles of several key design parameters of the ionomers near the glass transition, specifically the importance of the charge-carrying polymer side chains. 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 and is accompanied by thermal treatment, the local ionic concentrations are more heterogeneous, and the distribution of the ionic cluster sizes becomes broader at the transition point. The resulting enhanced degree of compositional and dynamic heterogeneity leads to a shift in the supercooled liquid transition toward higher temperatures.
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