Diblock copolymers, composed of two chemically distinct components that can microphase separate into various ordered structures, are attractive for applications such as solid electrolytes for batteries because one microphase can allow transport of ions or small molecule penetrants while another provides mechanical strength or other desired properties. Depending on the segregation strength, composition of the copolymer, and ion content, microphases including lamellae, bicontinuous network morphologies such as the double gyroid phase, and hexagonally packed cylinders can be formed. We use coarse-grained modeling to determine how these experimentally controllable variables impact morphology and ion transport. The diffusion or conduction of ions through the conducting microphase is generally slower than through the analogous homopolymer and is thought to depend on the distribution of ions within the conducting phase, among other factors. To understand these effects, we consider a wide range of systems with various ion-polymer and ion-ion interaction strengths. Our model reproduces the experimental trend of increasing ion transport with copolymer molecular weight, and this trend is more dramatic as ions are solvated in one polymer block more strongly or as the ion-ion interactions get stronger. The degree to which ions are locally concentrated, quantified by their average number of nearest ion neighbors out to a distance of approximately three diameters, is a good predictor of the ion diffusion constant. Specifically, systems whose ions are more locally aggregated (have more ion neighbors) have a reduced diffusion constant. We also calculate the degree of correlation in cation and anion motion, which can significantly reduce conductivity versus that predicted from the Nernst-Einstein limit.

Modeling ion conduction through block copolymers: Morphology, ion solvation, and correlated ion motion

2019 Fall National Meeting

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