Segmental Dynamics and Ion Association in PEO-Based Single Ion Conductors

We use quasi-elastic neutron scattering (QENS) to characterize polymer dynamics in solid polymer electrolytes (SPEs) that function as single ion conductors. In this case, the SPE combines a PEO spacer with a comonomer that introduces an acid group that serves as the anion. In these single ion conductors (ionomers), only the cation is not covalently bonded to the polymer chain. We study polymer dynamics as a function of ion content, as controlled by varying the ratio of nonionized to ionized comonomers. Even in nonionic polymers (no acid groups), we observe two segmental processes, which we interpret as motion of PEO segments in the spacer midpoint and near the isophthalate groups. In ionized samples, cross-linking between ionic groups considerably slows the dynamics of PEO segments near the isophthalate group. We compare the results to the PEO/LiClO4 system by comparing both polymer dynamics and conductivity as a function of ion content. The optimal ion content for ionomers is half that of the salt system, which we explain based on the differing behavior of polymer dynamics in the two systems.