Phase behaviors of solid polymer electrolytes: applicability of an extended Debye-Huckel theory

Poly(ethylene oxide) (PEO) with three different salts, ZnCl2, ZnBr2 and ZnI 2 has been characterized at various temperatures and compositions by optica l microscopy and differential scanning calorimetry. A molecular thermodynam ic model based on both an extended Flory-Huggins theory and a Debye-Huckel theory modified by Guggenheim is developed to interpret the liquidus curves in the resulting phase diagram. The proposed model considers not only shor t-range specific interactions between the salt ions and the polymer or solv ating base groups (oxygen in PEG) of the polymer leading to loose complex f ormation but also long-range electrostatic forces between salt ions. Quanti tative description according to the proposed model is in good agreement wit h experimentally observed transition temperature of a given system includin g an eutectic point. (C) 1999 Elsevier Science Ltd. All rights reserved.