Effect of Local Dynamics in Salt-Doped POEM as a Function of Side-Chain Length: Insights from Semi-Generic Coarse-Grained Modeling

Abstract

Salt-doped poly(oligo-oxyethylene methyl ether methacrylate) (POEM), in which poly(ethylene oxide) (PEO) side chains are attached to a methacrylate backbone, has attracted attention as a solid polymer electrolyte because its architecture reduces crystallinity compared with linear PEO. Prior work showed that changing the length of the PEO side chains can lead to more than an order-of-magnitude difference in ionic conductivity. We employed a semi-generic model to study this and related effects, aiming to provide deeper insights into ion conduction mechanisms to guide further experimental study. We consider POEM as a poly(methyl methacrylate) (PMMA) backbone grafted with PEO side chains and included glass transition temperature (Tg) differences by modifying the like–like interactions. We applied a stiff angle potential to create a freely rotating chain and modified the equilibrium bond angle to match the Kuhn length and density of PEO or PMMA. By further adjusting the angle potential strength between the PMMA backbone and PEO side chains, we were able to reproduce the Tg trends of neat POEM with different side-chain lengths. Our model successfully reproduces several key observations from previous experiments and simulations. These include the gradient in segmental dynamics along the PEO side chains, the preference of Li+ ions to coordinate with PEO units located in the middle of the side chains, and the trend in ionic conductivity versus temperature and side-chain length. We also found that, upon removing the angle potential between the backbone and side chains, the system fails to reproduce the expected conductivity trend with chain length; this suggests that the gradient in PEO segmental dynamics along the side chain is induced by the significantly slower dynamics of the backbone, and this coupling is a critical factor governing the chain-length-dependent conductivity in POEM.