Mechanisms of Ion Diffusion as a Function of Microstructure in Ionomer Melts

Understanding how the mechanisms and relative rates of ion transport depend on melt morphology and polymer architecture is a fundamental issue in ionomers. The low dielectric constant of the polymer backbone drives microscale aggregation of charged-groups and counterions, which can greatly impact ion conductivity. For some melts, aggregates can percolate and provide pathways for free ions to move independent of polymer dynamics; in other cases, isolated aggregates are favored and transport happens through infrequent collision-exchange events. We perform molecular dynamics simulations of coarse-grained ionomer melts that span these morphologies to understand charge mobility as a function of polymer architecture and background dielectric constant. For percolated networks, counterion diffusion depends weakly on these variables and results from discrete step motions along more static pathways of bound charged-groups. In contrast, charge mobility varies widely for systems that form clusters as the probability of ion exchanges is sensitive to the length scale and dielectric nature of the polymer-backbone regions between clusters. We explicitly measure the time scales of these distinct mechanisms and show that they directly underlie diffusion coefficients of the charged species.