Dielectric and Conductivity Studies of Polymer Electrolytes
Dielectric and conductivity studies has been performed in order to study the microscopic structure and dynamics of ion conducting polymers. The investigated systems are various salts, e.g. LiCF3SO3, dissolved in poly(ethylene glycol), poly(propylene glycol) or copolymers which partly consists of these polymers. The ionic conductivity has been measured over a wide range of concentrations. The results show similar behaviour to traditional liquid solvents featuring low permittivity. The dielectric measurements show a rapid increase in the static relative permittivity which may be attributed to the formation of ion pairs. The values of the static relative permittivity for the solution is used in a thermodynamic treatment for estimating the fractions of dissociated ions and ions associated into pairs, triplets, quadrupoles and hexapoles. The calculations show that the fraction of dissociated ions increases with increasing concentration of salt, if the static permittivity of the solution, and not the solvent is used. The dielectric measurements show also a dispersion at about 1 MHz for NH4CF3SO3, dissolved in poly(propylene glycol) at room temperature. It is argued that ion pairs should be the cause of this dielectric loss and that the corresponding frequency might serve as a probe for the local flexibility of the polymer chain segments. It may also be an alternative way to estimate the mobility of the charge carriers.