Solid-State Voltammetric Measurement of Plasticization Transport Enhancement in Ionically Conducting Poly(ethylene oxide) Films

The diffusion coefficient D(Os) of the electroactive metal complex [Os(phen)3)](2+) (phen = phenanthroline) and the ionic conductivity sigma(LiCF3SO3) of the salt LiCF3SO3 in films of the polymeric electrolyte PEO15/LiCF3SO3 (PEO = polyethylene oxide)) exposed to partial pressures of acetonitrile from P/P-0 = 0 to 0.8 increase as acetonitrile is sorbed into the polymer electrolyte. When compared with a free volume analysis, the changes in D(Os) and (sigma LiCF3SO3) are nearly identical. The temperature dependences of D(Os) and sigma(LiCF3SO3) in unplasticized PEO15/LiCF3SO3 and of the gas chromatographically measured coefficient for partitioning of acetonitrile into PEO15/LiCF3SO3 all display a discontinuity at the PEO15/LiCF3SO3 melting temperature. Acetonitrile appears to partition only into the amorphous part of the PEO15/LiCF3SO3 polymer. A model is presented that accounts for the enhancement of sigma(LiCF3SO3) induced by acetonitrile sorption ("plasticization") in terms of competitive displacement of Li+ coordination to and Li+ cross-linking of polyether chains by coordination to acetonitrile. There may also be some alteration in the degree of polymer crystallinity. Enhancement of D(Os) is proposed to occur as an indirect consequence of greater PEO chain segmental mobility resulting from diminution of Li+/polyether coordination. In this sense the plasticization is a specific chemical effect related to plasticizer-electrolyte coordination.