Cation complexation and conductivity in crown ether bearing polyphosphazenes

An attempt has been made to understand the mechanism of ionic conductivity in polyphosphazene-salt complexes by the synthesis and study of systems with crown ether side groups and salts with different cations. Amorphous phosphazene polymers, bearing either (12-crown-4)-methoxy, (15-crown-5)-methoxy, or (18-crown-6)-methoxy pendent groups, either as single-substituent polymers or mixed-substituent species in a 1:3 ratio with 2-(2-methoxyethoxy)ethoxy groups, were synthesized and characterized. The polymers in which all the side groups are crown ether units have glass transition temperatures higher than other oligo(ethyleneoxy) polyphosphazenes. They generate relatively low ionic conductivities at ambient temperatures when complexed with lithium triflate or lithium perchlorate. This suggests that the cation carries a significant part of the current in ether-type polymers. The ambient temperature ionic conductivity of the cosubstituent polyphosphazenes, as well as of poly[bis(2-(2-methoxyethoxy)ethoxy)phosphazene] (MEEP) (3), when complexed with MClO₄ (M = Li, Na, K, Rb, Cs), was measured. The ionic conductivity is reduced when a favorable 1:1 or 2:1 crown ether-cation complex is formed The thermal behavior of these polymer-salt complexes was also investigated. The polymers exhibit an increased glass transition temperature when a favorable 2:1 crown ether-cation complex is formed The relationships between the ionic conductivity and the glass transition temperature of the host polymer electrolytes and the stability of the crown ether-cation complexes formed are discussed.