Correlation between the temperature range of cooperativity and the fragility index in ion conducting polymers

The effect of the plasticizer in the ion transport properties of a neat poly-(ethylene oxide), PEO-based ionomer, is investigated by the use of the bond strength–coordination number fluctuation (BSCNF) model of the viscosity. The plasticized forms of PEO-based ionomer considered contain 6 wt.% of six miscible solvents with a wide range of dielectric constants. Recently, based on a relationship derived from the BSCNF that reproduces accurately the Vogel–Fulcher–Tammann (VFT) equation, we have proposed an expression for the fragility index dependence of the normalized temperature range of cooperativity (Tc − Tk) / Tg, where Tk is the Kauzmann temperature and Tc is a crossover temperature that demarcates two distinct dynamical regimes (low-T and high-T) above the glass transition temperature Tg. In the present work, the scope of such an expression is extended to the study of some ion conducting polymers. The values of the fragility index of the materials in consideration here are calculated from the VFT equation. These values of the fragility index enable us to evaluate the characteristic parameters B* and C* of the BSCNF for each material sample. The excellent agreement of the theoretical expression with the experimental data implies that by contrasting the PEO-based ionomer with plasticized forms, the normalized temperature range of cooperativity remains VFT-like. This result reinforces the idea that although the ionic conductivity increases dramatically with plasticization, the mechanism of ion transport remains unchanged.