THERMAL AND THERMOOXIDATIVE DEGRADATION OF POLY(ETHYLENE OXIDE)-METAL SALT COMPLEXES

Thermal and thermooxidative degradation studies of electrolytes based on poly(ethylene oxide) (PEO) have been extended to include a range of inorganic salts to elucidate the role of the salt. In confirmation of earlier studies, inorganic salts generally reduce the thermal stability of PEO in an inert atmosphere but protect the polymer against thermal oxidation. These effects are explained, in part, by the strength of the interaction between the metal cation M+ and the oxygen atoms in the polymer backbone. The greater the charge density of M+, the greater its destabilizing influence during degradation under nitrogen and the greater its stabilizing effect in an oxidizing atmosphere. This arises because complexation tends to reduce the strength of backbone C-O bonds but also retards peroxidation at adjacent C-H bonds. The anions also influence the decomposition reactions both by screening the cations and by direct chemical interaction. In the presence of oxygen, acetaldehyde, methanol, and CO2 are the most abundant low-boiling products from both PEO and its complexes. Ethylene, which is abundant when PEO-salt complexes are pyrolyzed under N2, is absent. Methyl and ethyl formates are also formed in small amounts from complexes but not from pure PEO, though the latter does evolve small amounts of other esters and dioxane. Mechanisms of thermooxidative degradation accounting for these observations are proposed.