Microscopic Morphology and Dynamics of Polyampholyte and Cationic Ionomers

Molecular dynamics (MD) simulations are performed using a coarse-grained model of polyampholyte ionomers in which both types of ions are incorporated in the polymer chain and, consequently, the ion-pairs form bridges across the chains (i.e., bridged ion-pair) and compared their dynamics and structures with the conventional cationic ionomers, where only cations are incorporated into the polymer chain (e.g., pendant ion-pair). While the glass transition temperatures of both structures are comparable, mesoscale ordering within large, string-like aggregates is observed in the polyampholyte (bridged) ionomer instead of square-like aggregates in the cationic (pendant) ionomer. Both systems show dynamical heterogeneities at temperatures above Tg, which persist at higher temperatures for the bridged ionomers. A linear correlation between the lifetime of the ionic bonds and the dynamic relaxation time over a wide temperature range is observed. A comparison of these time scales showed that ions have collective dynamics in the bridged ionomers. Furthermore, the intermediate scattering function data obtained at different temperatures can collapse onto master curves, which confirms the applicability of the time–temperature superposition principle in these structures. Overall, this study shows that modifying the chain connectivity of the ion-pairs provides an alternative route to control the morphology and dynamics of ionomers.