On the ion association at low salt concentrations in polymer electrolytes; a Raman study of NaCF3SO3 and LiClO4 dissolved in poly(propylene oxide)

Abstract

Raman spectra of poly(propylene oxide) (PPO) complexed with LiClO4 and NaCF3SO3, respectively, reveal the presence of both ion pairs and solvated ions. The amount of dissociated and paired ions has been calculated using a two-component band analysis of the ν1(A1) anion vibration. The degree of dissociation is approximately constant in the low concentration range of monomer (i.e., oxygen) repeating unit to salt ratio O:M=1000–30. For larger concentrations, it gradually decreases with increasing salt content. Association constants for the two systems are estimated from the concentration quotients at low salt contents using the Debye–Hückel theory. Extrapolation to zero concentration indicates a dissociation minimum at concentrations lower than O:M=1000. Large and positive values of ΔS0 (100±40 and 120±40 Jmol−1 K−1 for PPO–NaCF3SO3 and PPO–LiClO4, respectively) are obtained for the ‘‘free’’ ion–ion pair equilibria, which may reflect the increasing configurational freedom of polymer chains upon forming ion pairs. Comparisons with conductivity data suggest that single ions rather than charged multiplets are responsible for the conduction. The increase of ion association at high concentrations is too small to explain the conductivity drop at O:M<20. The σ drop is mainly due to reduced mobility as reflected in increasing values of viscosity.