NMR studies of ionic mobility and molecular mobility in polymer electrolytes

Abstract

The pulsed field gradient spin echo technique has been used to measure the self-diffusion coefficients of both the cation and anion in LiCF 3SO 3 PEO n systems as a function of concentration and temperature. In addition, the ionic conductivities were determined by ac conductivity measurements. The temperature dependence of both conductivity and ion diffusivities could be very well described by the Vogel-Tamman-Fulcher equation. Predicted values for ionic conductivity were obtained from the NMR diffusivities using the Nernst-Einstein equation and compared with those from direct measurement. It is clear that at higher reduced temperatures and/or lower salt concentrations, there is an increasing degree of ionic association or correlated motions of neighbouring cations and anions which give rise to deviations from the Nernst-Einstein equation. The molecular mobility of the polymer chains in these systems has also been studied by NMR measurements of the proton transverse relaxation behaviour. It has been found that the addition of salt does not affect the critical entanglement molecular weight of the polymer but it does increase the segmental relaxation time. Below the entanglement molecular weight the polymer chain dynamics can be described by the Rouse model. Above the critical entanglement molecular weight, a model due to Brereton can be used, and the NMR data have been shown to be consistent with a constant chain length between entanglements, the relaxation times varying with salt concentration in a manner predicted from the conductance measurements. It is concluded that the dissolved salt increases the energy barriers to polymer segmental motion, but not the entangled structure of the polymer.