Relaxation processes in polymeric electrolytes: Effect of the cation size and of the thermal history

Abstract

In this study, we present the dependence of the relaxation processes from the size cation and thermal history in complexes of polyethylene oxide (PEO) and alkali metal salts (sodium and potassium thiocyanates). The internal friction behavior as a function of temperature and frequency reveals the presence of local (γ) and cooperative (a _a ) molecular relaxations. The region of the γ-relaxation, which shows a single mechanical loss peak in pure PEO and in PEO-NaSCN, is strongly modified by the addition of potassium salt to PEO and two close peaks are present. Samples, prepared by a slow evaporation of the solvent, have a semi-crystalline structure. A “quenching” procedure imposed on them gives rise to the same structure, but enhances the amorphous phase and shifts the glass transition to lower temperatures, probably as an effect of decreasing constraints of the crystalline phases on the amorphous regions. Finally, different thermal histoires also produce changes on the low temperature γ-relaxation, which are discussed accordingly with the revealed phase behavior.