Counterion effects on ion mobility and mobile ion concentration of doped polyphosphazene and polyphosphazene ionomers

A physical model of electrode polarization is applied to dielectric (impedance) data from two poly(methoxyethoxy-ethoxy phenoxyphosphazene) systems with nearly identical chemical structures, one composed of an ionomer with a single mobile cation and the other composed of a salt-doped polymer with mobile cation and mobile anion. Quantitative comparison of the ion mobility and mobile ion concentration, based on chemical structure, is achieved. Both conductivity and ion mobility are reduced to common curves by normalizing T with T-g, indicating that T-g of the polymer matrix is a major factor controlling ion diffusion. Even with the use of normalized temperature, both the mobility of ions and the mobile ion concentration in the doped polymers are similar to 10 times larger than those in the ionomers. These factors arise from faster diffusion of the anion and the local environment surrounding ion pairs. Also, Arrhenius and VFT parameters associated with mobile ion concentration and ion mobility, respectively, reveal differences in activation energies between ionomer and doped polymer that are due to interactions between the ion pairs and polymer segments.