Viscosity-conductivity coupling and ionic mobility in liquid polymer electrolytes

Viscosity and conductivity measurements have been made on solutions of NaCF$\sb3$SO$\sb3$, LiCF$\sb3$SO$\sb3$, LiClO$\sb4$, NaClO$\sb4$, and NaSCN in poly (propylene oxide), PPO, of molecular weight 4000. Viscosity data conform very well to VTF behavior and yield, in all but one special case, ideal glass transition temperatures which track the measured glass transition temperatures as well as in the case of glassforming aqueous solutions. Conductivity data measured over the same temperature range do not conform to VTF behavior as well as the viscosity data. The differences in the temperature dependence of the two processes are demonstrated by Walden rule plots. Some of the solutions have been examined spectroscopically by an independent group for the extent of ion association and in two cases the deviations of conductivity from viscosity-dictated behavior predict almost quantitatively the strong temperature dependence of the free ion concentration found spectroscopically up to the high-temperature limit of the measurements. The temperature and salt concentration dependence of the relative time scales of structural and conductivity relaxation in NaCF$\sb3$SO$\sb3$ PPO(4000) solutions has been examined. The ratio of the structural and conductivity relaxations approaches unity as salt content increases, which is consistent with a transition from a super-coupling of the two modes at dilute concentrations to the apparently decoupled behavior of a fast ion glass when extrapolated to higher salt contents. The first known measurements of polymer-salt solution viscosities in the vicinity of the glass transition temperature are reported and a decoupling of the viscous and enthalpy relaxations is noted upon small additions of salt to pure PPO(4000). This decoupling is observed to diminish as salt content increases