THE MOLAR CONDUCTIVITY BEHAVIOR IN POLYMER ELECTROLYTES AT LOW-SALT CONCENTRATIONS - A RAMAN-STUDY OF POLY(PROPYLENE GLYCOL) COMPLEXED WITH LICF3SO3

Raman scattering measurements have been carried out on poly(propylene glycol) complexed with LiCF3SO3 salt over a wide salt concentration range, the ether oxygen to alkali metal cation ratios (O:M) ranging from 4820:1 to 12:1. The relative concentrations of solvated anions, anion-cation pairs and ionic aggregates have been calculated from an analysis of the symmetric anion stretch. The degree of association is found to be almost constant in the O:M range 4820:1-40:1 whereafter it increases with increasing salt concentration. The results show that the dramatic increase reported for the molar conductivity in the O:M range 1000:1-40:1 cannot be explained by the redissociation of contact ion pairs or the formation of conducting triplets. Instead the major contribution to the conductivity increase seems to be a concentration dependent enhancement of the ionic mobility. A percolation based ionic hopping process involving exchange between ions in pairs and dissolved ions, either free or coordinated to ether oxygen sites, is advanced as a possible microscopic mechanism.