Diffusion of solvent/salt and segmental relaxation in polymer gel electrolytes

Dynamic light scattering and broad band impedance spectroscopy have been performed on polymer gel electrolytes consisting of a mixture of propylene carbonate/lithium perchlorate (PC/LiClO4) stabilized with a polymer matrix, poly(methyl methacrylate) (PMMA). The salt concentration and temperature dependences of the conductivity are compared to the dynamical behaviour of the system. Complex relaxation dynamics is observed in the dynamic light scattering experiments with multiple relaxation processes with different temperature and wave vector dependencies. In focus here is a fast exponential decay attributed to diffusion of low molecular compounds and a slower and very broad relaxation process attributed to the segmental motion of the polymer backbone. The relation between the relaxation processes and the ionic conductivity is investigated, with special emphasis on the relation to the fast diffusive process. We show that the slowing down of this diffusion process is a more important limiting factor for ionic conductivity than the possible creation of ion pairs at higher salt concentrations. The physical picture that emerges from the results is that the conductivity mechanism is a diffusion of ions moving together with the solvent molecules, both of which are essentially decoupled from the segmental motion of the polymer backbone.