Inverse coarse–graining methodologies to understand ion transport in block copolymer electrolytes

Date

2017-09-19

Authors

Sethuraman, Vaidyanathan Mathamangalath

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Abstract

This research is focused on two fronts (i) developing multiscale simulation strategies for multicomponent polymers which can generate self assembled morphologies at both mesoscopic and atomistic length scales (ii) understanding the conformational attributes and dynamics of polymers in structured morphologies to understand the ion–transport mechanisms in block copolymer electrolytes.

First part of the work is devoted in developing strategies to create equilibrated block copolymer morphologies below ODT with hard repulsive potentials. To this end, ordered morphologies with the help soft repulsive potentials are generated which possess equilibrated long range order within very short computational time. A rigorous mapping between the interaction parameters of the hard and soft potentials is then utilized to obtain the intermolecular interaction parameter of the soft potential corresponding to the target hard potential repulsion parameter. Subsequent to establishing the long range structure, short repulsive potential (within a coarse-grained framework) is reintroduced and equilibrated to generate ordered morphologies using hard repulsive potentials. Further to this, both topological and dynamic properties in ordered lamellar phases were characterized. The topological constraints are seen to increase with increasing degree of segregation. On characterizing the local dynamics of polymeric segments, we found that inhomogeneities exist in the spatially local dynamics and the length scale of perturbation of such inhomogeneities is controlled by the interfacial width of the block copolymer.

The last part of the work involved the generation of ion–doped block copolymer melts at the atomistic level and to compare the results obtained therein with those for pure homopolymeric melts. To this end, we employed a multiscale simulation method to generate PS–PEO block copolymer doped with LiPF₆ ions. Our results demonstrate that the cation-anion radial distribution functions (RDF) display stronger coordination in the block copolymer melts compared to pure PEO homopolymer melts. Radial distribution functions isolated in the PEO and PS domains demonstrate that the stronger coordination seen in BCPs arise from the influence of both the higher fraction of ions segregated in the PS phase and the influence of interactions in the PS domain. Further, the cation-anion RDFs display spatial heterogeneity, with a stronger cation-anion binding in the interfacial region compared to bulk of the PEO domain. Investigations into the ion transport mechanisms in PS-PEO block copolymer melt reveal that ions exhibit slower dynamics in both the block copolymer (overall) and in the PEO phase of the BCP melt. Such results are shown to arise from the effects of slower polymer segmental dynamics in the BCP melt and the coordination characteristics of the ions. Polymer backbone-ion residence times analyzed as a function of distance from the interface indicate that ions have a larger residence time near the interface compared to that near the bulk of lamella, and demonstrates the influence of the glassy PS blocks and microphase segregation on the ion transport properties. Ion transport mechanisms in BCP melts reveal that there exist five distinct mechanisms for ion transport along the backbone of the chain and exhibit qualitative differences from the behavior in homopolymer melts.

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