A study of the effect of the polymer network and the solvent on the ion conductivity in gels

Ion-conducting gels were produced by photo-polymerisation of acrylates in the presence of Lithium hexafluorophosphate (LiPF6) in various solvents. The ionic conductivity of various mixtures containing propylene carbonate and polyethylene glycol diacrylate was measured as a function of temperature before and after polymerisation. The change observed in the conductivity upon polymerisation was explained using a modified free volume theory. The free volume within the mixtures was calculated from the measured density and the van der Waals density of the materials. Using this theory, plots relating the conductivity to the free volume within the mixtures were produced. The measured data were found to cluster around a master curve, explaining the temperature dependence of the conductivity and its reduction upon polymerisation in terms of free volume within various systems. The mobility of the lithium ions within the mixtures before and after polymerisation was studied using Li-7 NMR T-1 relaxation time measurements. The diffusion coefficients of all the components in the mixtures were also measured using the pulse field gradient NMR technique. The phase structure of the gels and the mobility of the ions were found to be dependent on the polarity of the polymer and the type of solvent used. Transport numbers obtained for Li+ were found to be much lower upon polymerisation in the case of mixtures containing PEGDA and PC whereas other mixtures showed a much lower decrease. (C) 1999 Elsevier Science B.V. All rights reserved.