FTIR Investigation of Ion Environments in PEO-based ``Single Ion'' Polymer Conductors

The infrared and Raman spectra of ionomers and electrolytes containing sulfonate groups have been previously studied in great depth. If the cation - anion interaction is relatively strong, the doubly degenerate asymmetric SO3- stretching mode becomes split into two components, while the symmetric SO3- stretching mode displays bands that can be assigned to the ``free'' (solvated) anion, neutral pairs and aggregates. In this paper we investigate a series of PEO-based polyester copolymer ionomers, with sulfonate anions covalently bound to the polymer chains, using FTIR spectroscopy. Ion content is systematically varied by changing the ratio of ionic to non-ionic isophthalate groups while keeping a fixed PEO segment molecular weight of 600. In the FTIR spectra of these ionomers, a splitting of the asymmetric mode is observed and only a single symmetric stretching band characteristic of some form of aggregated structure is apparent. There is no evidence that ``free'' or solvated ions are present. In addition, various ethylene oxide modes of vibration are shifted in frequency in a similar manner to those observed in studies of complexes of known structure, indicating their involvement in the formation of the aggregate.