The segmental dynamics of a polymer electrolyte investigated by coherent quasielastic neutron scattering

Abstract

The structural dynamics of a polymer electrolyte model material, poly(prolyene oxide) (PPO)–LiClO4 (and PPO for reference), has for the first time been studied using coherent quasielastic neutron scattering. By a combination of neutron spin echo and inverse time-of-flight techniques we investigate the relaxation function in an experimental time window 10−12≲t≲10−8 s at a momentum transfer corresponding to the distance between neighboring interchain segments. We find that the relaxation of the correlation between neighboring chains is slower and more stretched in the polymer salt complex compared to the pure polymer. The data can, for both PPO and PPO–LiClO4, be described by a stretched exponential function with temperature independent stretching parameters. While the relaxation times follow the macroscopic viscosity for the former, they do not for the latter. The slower relaxation in PPO–LiClO4 compared to PPO and the failure of the viscosity scaling in PPO–LiClO4 may be explained in terms of a temperature dependent effective molecular weight induced by cations acting as cross links between chains. We discuss the origin of the extra stretching of the relaxation in the polymer salt complex under the aspect of heterogeneity, comparing it with data in the literature. We find that the stretching to the major part is intrinsic or at most due to heterogeneities on an atomic length scale. The molecular length scale of the experiment allows for the first time a direct connection to the renewal time in the dynamic disordered hopping model for ion transport in polymer electrolytes.