Unusual Glass Transition Breadths of Ionomers: Effects of Thermal Treatment and Charge-Carrying Side Chains

Abstract

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 freeradical 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 °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.

Eliminating the Tg-Confinement Effect in Polystyrene Films: Extraordinary Impact of a 2 mol % 2-Ethylhexyl Acrylate Comonomer

“…The glass transition breadth is also a key signature of the glass transition behavior in polymers. − Figure plots the composition dependence of T g,breadth for bulk and 15-nm-thick films. Bulk films of the copolymers considered in Figure exhibit T g , breadth values of 21–24 °C; the lack of any significant composition dependence of T g,breadth is in accordance with the fact that expansivity of bulk films is affected little or not at all by composition.…”

A Second Glass Transition Observed in Single-Component Homogeneous Liquids Due to Intramolecular Vitrification

“…Similarly, in ionomers consisting of a neutral chain backbone and charged groups, ionic clustering results in inhomogeneities and a broadened and even a double glass transition. 6 A similar example is the apparent double glass transition observed in some polymers caused by the emergence of partial crystallinity (for example, in polyethylene). 7 Finally, a double glass transition associated with a liquid−liquid transition has been observed in yttrium− aluminum oxide glasses.…”

“…However, in these mixtures, the multiple glass transitions are associated with inhomogeneities and the vitrification of chemically distinct components of the mixture. Similarly, in ionomers consisting of a neutral chain backbone and charged groups, ionic clustering results in inhomogeneities and a broadened and even a double glass transition . A similar example is the apparent double glass transition observed in some polymers caused by the emergence of partial crystallinity (for example, in polyethylene) .…”

Glass transition temperature of styrene‐based ionomer controlled over a very wide temperature range by nonpolar plasticizer content and ion content

“…1,2 Multiplets of ionomers restrict the mobility of polymer chains extending outward from the multiplets. [3][4][5][6][7][8][9][10][11][12][13][14][15][16] Therefore, ionomers are composed of three parts: ionic aggregates, regions of polymer chains whose mobility is reduced by multiplets, and regions with few or no ionic group, in which the mobility of polymer chains is not reduced by multiplets (so-called matrix regions). As for the restricted mobility region, its average size is relatively small, so the glass transition (T g ) of the restricted mobility region is not observed, and only the T g of polymer matrix appears; at this stage, the ionomer behaves like a single-phase material.…”

“…The multiplet formation is due to Coulombic interactions between ion pairs and polarity differences between ion pairs and polymer chains 1,2 . Multiplets of ionomers restrict the mobility of polymer chains extending outward from the multiplets 3–16 . Therefore, ionomers are composed of three parts: ionic aggregates, regions of polymer chains whose mobility is reduced by multiplets, and regions with few or no ionic group, in which the mobility of polymer chains is not reduced by multiplets (so‐called matrix regions).…”