Ion Transport Mechanisms in Ionomers*

An understanding of dynamics in single-ion conducting polymers, such as ionomers with ionic groups covalently bonded to the polymer backbone, is needed to design these polymers for use as electrolytes. In melt ionomers, the ions tend to self-assemble into nanoscale ionic aggregates, and the morphology of these aggregates affects both the ion and chain dynamics. I will describe the ionic aggregate and ion dynamics in atomistic molecular dynamics (MD) simulations of precise polyethylene-based ionomers. In particular, the mechanisms for ion transport are the same as those found in previous coarse-grained MD simulations. In systems with isolated ionic aggregates, ions move through a process of aggregates merging, rearranging, and breaking up. In systems with percolated ionic aggregates, ions "shuttle" along the ionic aggregate. From new CG simulations, we find that the diffusion constant for ions in systems with percolated ionic aggregates is inversely proportional to an ion "stepping" time scale, which quantifies the cation-anion association lifetime.