Cation Coordination and Motion in a Poly(ethylene oxide)-Based Single Ion Conductor
KJ Lin and JK Maranas, MACROMOLECULES, 45, 6230-6240 (2012).
We study a poly(ethylene oxide)-based Na single ion conductor using molecular dynamics simulation (MD). The simulations are carried out with a nonpolarizable united-atom force field with scaled partial charges. This is the first time a force field is assembled for such system, and the model is consistent with experimental observables. The simulations show a wide range of cation states, with more than 75% of cations associated with the counterions. Ion aggregates become more prevalent and longer as temperature increases at the expense of the number of solvated cations. The larger ion clusters form chain-likes structure, consequently the Na ions at the edge of the cluster have a higher chance to encounter flexible PEO chains than those in the middle. PEO mobility increases with distance from the bound anion, resulting in dynamic segregation. Slow Na and EO are spatially correlated and most of the slow Na ions belong to ion aggregates. Because of the chain-like structure, the edge/end Na ions have a higher chance to escape. This allows the chain-like aggregate to serve as a charge conduction pathway. We observe a mechanism that utilizes the chain-like aggregates to transfer positive charge without cations moving equivalent distances, thus providing a conduction method that is decoupled from polymer motion.
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