Anharmonic and relaxation effects in polymer electrolytes with singly charged cations

A short review of the structural phase behavior of polymer electrolytes based on complexes of poly(ethylene) oxide (PEO) and alkaline metal salts is presented. These polymeric blends are semicrystalline and show a degree of crystallinity which is strongly affected by the salt concentration and the cation size. The experimental evidence obtained by thermal and mechanical studies indicates that relevant interactions exist between the polymer backbone and the salt, which is not acting as a conventional second phase filler. In the low concentration region, the salt dissolves in the amorphous fraction of the host polymer up to a limiting solubility, giving rise to an enhancement of the non-crystallinity of the system and to a shift of the glass transition toward higher temperatures. The behavior of the dynamic elastic modulus, E?, over a wide temperature interval (120-300 K) in PEO-NaSCN solid blends has been interpreted in terms of contributions due to the anharmonicity of vibrational modes and the ?a- or primary cooperative relaxation. The strength of the ?a-relaxation shows the highest values when the addition of the sodium salt gives rise to the lowest degree of crystallinity in the system.