Fast ion conduction in mixtures of salt with comb-shaped polymers based on macrocyclic and linear polyethers

Abstract

In the first part of this work samples of the poly(vinyl ether) having 3 ethylene oxide units in the side chain were crosslinked to varying degrees and doped with LiClO₄. The glass transition, Tg, was increase by about 20 K at 11% crosslinking, but the levels of conductivity remained the same as those for tha uncrosslinked material up to 5% crosslinking. The temperature dependence of the a.c. conductivity was predominately non-Arrhenius and the data were analysed using the Vogel-Tammann-Fulchar (VTF) equation and the Adam-Gibbs configurational entropy model. Both approaches appear valid at low crosslink densities but may not be applicable at higher levels of crosslinking. Activation energies associated with the segmental motion in the polymer ware around 55 kJ mol*. Conductivity levels increased significantly when the number of EO units in the side chain increased to 5 (mono-dispersed) or 6/7 (average number of EO units). Ionic conductivity in these samples doped with salts are noticeably higher than those reported for MEEP or polysiloxane electrolytes. The temperature dependence of the a.c. conductivity of these polymers doped with LiClO₄ and NaCl0₄ were predominately Arrhenius at low to medium salt concentrations, over the temperature range studied. Activation energies associated with the segmental motion in these systems were also calculated. In the second part of this work, a series of polyphosphazenes with pendant crown ethers ware prepared. A 16-crown-5 unit was attached to the polymer backbone using methylene spacer units to modify the initial Tg of the polymer. Spacers with 0, 3 and 6 methylene units were examined and the Tgs were observed to decrease with the increasing of the spacer units. The conductivity of mixtures of these polymers with LiClO₄ and NaCl0₄ were measured as a function of temperature and it was found that the conductivity was highest for the system with the longest spacing unit which also has the lowest Tg. Also for comparison a lS-crown-5 and 12-crown-4 derivative with one methylene spacing unit ware prepared and examined. The dependence of conductivity on temperature of these materials was found to be non-Arrhenius and the data were analysed according to VTF equation and Adam-Gibbs configurational model. This predicts that (Tg - T,) has a value of about 50 K, but only in the case of n - 0 does this criterion coma close to being satisfied and this may not to be the best model to use in these systems.