(420b) Generic Coarse-Grained Modeling of Ion-Containing Polymers

Ion-containing polymeric materials are used in many applications in our everyday lives as well as in emerging areas, including as battery electrolytes, membranes for separations, and packaging. Given the synthetic ability to control molecular properties, including chain length, ion content, and chemical composition, that impact the overall material behavior, the rational design and optimization of future materials relies on understanding the fundamental composition/structure–property relationships at the molecular scale. In this regard, coarse-grained simulations, which focus on chain architecture and generic chemical features without including atomistic detail, are an attractive tool to efficiently consider a range of possible architectures and compositions quickly and develop such fundamental understanding. Of course, capturing the important local ion correlations as well as longer ranged features of these materials at the same time with simple models presents an inherent challenge, and one must carefully decide at what level to include various relevant features of ion-ion interactions and ion-polymer solvation. It is common to use simple bead-based polymer models with adjusted interaction strengths between beads based on their chemistry, as well as to include long-ranged Coulomb interactions between ions that are scaled by the dielectric constant of the medium. When considering polymers which strongly solvate ions such as for battery electrolytes, our group has also included a potential of form -S/r⁴ between ions and polymer beads, where the parameter S is related to the pure polymer dielectric constant but is often set empirically. With such a model, we can capture the overall effects of the Born solvation energy of the ions and reproduce several important features of these systems with only pairwise, spherically symmetric interactions.

Specifically, for salt-doped block copolymers which contain a conducting microphase and a mechanically strong microphase to allow for a robust battery electrolyte material, we use a larger S for the conducting phase to capture the selective solvation of ions into that phase. This allows us to reproduce experimentally observed behaviors, such as increasing ion diffusion with copolymer molecular weight and decreasing ion diffusion with ion concentration, that are difficult to capture with simple models. Recent studies applying this model to show the impact of ion size, polymer dielectric constant, tethering anions to the backbone, and tapering the copolymer composition profile will be discussed. One result of interest is that at low dielectric constant (low S), increasing size asymmetry between ions (reducing cation size at fixed cation-anion contact distance) significantly improves cation conduction due to the reduction in ion aggregation and in correlated cation-anion motion. However, at high dielectric constant, size asymmetry slows cation mobility due to strong preferential solvation, and cation transference number decreases. With this work, we aim to provide insight into the fundamental impacts of ion size, composition profile, and polymer dielectric strength on ion correlations, diffusion, and transference number, and ultimately into how to adjust molecular parameters to lead to improved conduction.