Microscopic Morphology and Dynamics of Bridged and Pendant Ionomers

We performed molecular dynamics (MD) simulations using a coarse-grained model of ionomers with distinct chain architectures, in which both types of ions are incorporated in the polymer chain (bridged structure) and compared their dynamics and structures with the conventional pendant ones where counterions are freely added to the system. Our results showed that the glass transition temperatures of both structures are comparable and slightly higher than the corresponding control system. Mesoscale ordering existed within large, percolated aggregates in the bridged structure and between distinct aggregates seen in the pendant structure. Also, long-range order was seen in the percolated ionic network of the bridged structure. Dynamical heterogeneities were seen even at temperatures above T_g in both structures, which persist at higher temperatures for the bridged ionomers, indicating the preservation of the ionic network at higher temperatures in the bridged ionomer. Analysis of the dynamics of the physical bonds and the intermediate scattering function showed a linear correlation between the lifetime of the ionic bonds and the dynamical relaxation time over a wide temperature range. A comparison of these time scales showed that in the bridged ionomers, ions have collective dynamics, unlike the pendant structure in which ion-pair dissociation occurs before escaping the local environment. Furthermore, our results show that the ISF data in both structures can be collapsed onto a master curve that confirms the applicability of the time-temperature superposition principle in these structures. Overall, this study shows that incorporating ions in the backbone provides an alternative route to control the morphology and dynamics of the ionomers.