Relation between structural and conductivity relaxation in PEO and PEO based electrolytes

Abstract

The effect of a lithium salt on the general polymer dynamics of poly( ethylene oxide) (PEO) and how these dynamical alterations affect the ionic conductivity and structural relaxations in (PEO)(4):LiClO4 have been investigated. The study was based on differential scanning calorimetry (DSC) and dielectric relaxation studies. The DSC studies indicated an increased glass transition temperature of (PEO)(4):LiClO4 as compared to that of PEO. The polymer-salt complex exhibited enhanced conductivity of sigma(D.C). = 3.2 x 10(-7) S . cm(-1) at room temperature (298 K) due to the presence of mobile Li+ ions. Impedance data have been scaled and analyzed under conductivity and modulus formalisms over wide ranges of frequency and temperature for both PEO and (PEO)(4):LiClO4. For PEO, the analysis of the scaled formalisms indicates that both the conductivity and the structural relaxation mechanisms are temperature-independent above the melting point of PEO. However, below the melting point, the nucleation and growth of spherulites and also the formation of 'interphase' regions in PEO result in a hindered long range D.C. conductivity, which, in turn, leads to a decoupling of the D.C. conductivity and the structural relaxation. However, for (PEO)(4):LiClO4, by comparing the conductivity relaxation data with the calorimetric glass transition and analysis of scaled formalisms, a direct coupling between Li+ ion motions and polymer segmental dynamics is observed for the polymer electrolyte.