Dynamics of Charged Polyionic Liquids

Citation

Stewart, Christopher Jenkins (2025) Dynamics of Charged Polyionic Liquids. Master's thesis, California Institute of Technology. doi:10.7907/6js0-8r29. https://resolver.caltech.edu/CaltechTHESIS:05232025-215105217

Abstract

Polymerized ionic liquids (PILs) exhibit complex ion transport dynamics that are central to advancing technology in energy storage and efficient energy conversion. In this work, we probe the behavior of charged polymer systems in the solvent‐free limit using a coarse‐grained Gaussian core model that explicitly incorporates long‐range electrostatic interactions. Our simulations span a wide range of chain lengths, from monomeric units to highly entangled polymers, revealing how both intrachain and interchain interactions govern key properties such as the radius of gyration, relaxation time, and diffusivity. Notably, charged polymers adhere to classical reptation scaling, indicating that electrostatic forces do not inhibit standard polymer melt scaling behavior. We quantify these effects by evaluating both the Onsager transport coefficients and the direct drift response under applied electric fields, thereby linking molecular trajectories to macroscopic ion conductivity.

Our findings show that as the chain length increases, the motion of polymerized ions becomes increasingly correlated, a trend that stabilizes ion conductivity despite decreasing diffusivity. This study demonstrates that the complex interplay between correlated motion and cooperative chain dynamics results in a relatively stable conductivity that increases over short chain length, plateaus in the transition regime, before decreasing in the fully entangled regime — contrary to idealized predictions based solely on diffusivity. By explicitly modeling the microscopic interactions and accounting for both hydrodynamic and electrostatic effects, we provide a physically grounded framework that captures the emergent behavior of these charged systems. In doing so, our work offers a robust platform for the rational design of next-generation PIL electrolytes, distinguishing itself from phenomenological models through clear, simulation-based insights.

Thesis Files