Molecular dynamics simulation of the effect of a side chain on the dynamics of the amorphous LiPF6-PEO system

The effect on structure and dynamics of adding single, short poly( ethylene oxide) (PEO) side chains to amorphous long chain PEO has been probed by molecular dynamics (MD) simulation at 290 K. The MD box contained a long PEO backbone of 186 EO units; the side-chain lengths studied in salt-free systems involved 5-9, 11, 15, 19 and 23 EO units. Salt ions were then added in the form of Li+ and PF6- ions to maintain an Li : EO ratio of 1 : 20 for side-chain lengths of 5, 7, 9, 11 and 15 EO units. Both long-chain and side-chain PEO were CH3-terminated. The potential needed to describe the backbone-side-chain linkage was calculated by quantum mechanics. The overall mobility of the PEO backbone decreased on the addition of a side chain, with a minimum at 6-8 EO side-chain units. On the other hand, lithium-ion diffusion increased on adding a side chain, with a maximum in D(Li+) for 5-7 EO side-chain units.