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 t he 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 be fore 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 th e 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 polymeris ation in terms of free volume within various systems. The mobility of the l ithium ions within the mixtures before and after polymerisation was studied using Li-7 NMR T-1 relaxation time measurements. The diffusion coefficient s 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 mobil ity of the ions were found to be dependent on the polarity of the polymer a nd the type of solvent used. Transport numbers obtained for Li+ were found to be much lower upon polymerisation in the case of mixtures containing PEG DA and PC whereas other mixtures showed a much lower decrease. (C) 1999 Els evier Science B.V. All rights reserved.