Phase behavior and conductivity in poly(ethylene oxide)–sodium thiocyanate systems

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Abstract
Observed phases and transition temperatures in mixtures of poly(ethylene oxide) and sodium thiocyanate (PEO–NaSCN) are described by a binary phase diagram. The components of this system are semicrystalline PEO which can dissolve salt up to a mole fraction Xs=0.023, and a crystalline complex of composition PEO3.5NaSCN (Xcc=0.22). The salt-deficient PEO component is normally formed by a eutectic reaction with eutectic temperature Te=63 °C. Conductivity of PEO–NaSCN mixtures is calculated assuming that only ions in the amorphous, liquidlike regions contribute to charge transport. This model predicts a positive discontinuity in conductivity on eutectic melting, and a sharp maximum in conductivity at low salt concentrations, in agreement with observed behavior.