FT-IR INVESTIGATION OF ION ASSOCIATION IN PLASTICIZED SOLID POLYMER ELECTROLYTES

FT-IR spectroscopy has been utilized to monitor ion association in pla sticized solid polymer electrolytes (SPEs). The SPEs were prepared fro m a random copolymer of ethylene oxide (EO) and propylene oxide (PO) a nd the salt lithium trifluoromethanesulfonate (lithium triflate, LiTf) . Tetraethylene glycol dimethyl ether (tetraglyme) and N,N'-dimethylfo rmamide (DMF) were chosen as model-plasticizers. Despite having a simi lar dielectric constant to that of the polymer host, epsilon similar t o 5, the incorporation of tetraglyme into the SPEs resulted in increas ed ion association. The addition of a higher dielectric constant solve nt, DMF, epsilon = 36.7, resulted in decreased ion association iu the SPE. The effects of salt concentration (0.05-1.25 mol dm(-3)) and temp erature (25-100 degrees C) upon ion association in SPEs were also inve stigated. At low salt concentrations, ion association was found to inc rease with temperature, however, at 1.25 mol dm(-3) the temperature de pendence of ion association was dominated by concentration effects. Th ere appears to be a maximum in the fraction of ''free'' ions at a LiCF 3SO3 concentration of 0.4 mol dm(-3), preceded by a minimum at approxi mately 0.2 mol dm-(3), consistent with the molar conductivity behavior previously observed in these electrolytes.