A new series of solid polymer electrolyte materials based on the poly(organophosphazene) system has been designed and synthesized. The new polymers contain linear or branched oligoethyleneoxy side chains. The polymers were characterized by 31 P, 13 C, and 1 H-NMR spectroscopy, gel permeation chromatography, differential scanning calorimetry, and elemental analysis. The ambient temperature (25°C) ionic conductivities of the polymers complexed with lithium triflate were measured by complex impedance analysis. The polymers that bear linear oligoethyleneoxy side chains [NP{O(CH2CH2O)n-CH3}2], have low glass transition temperatures that range from -84 to -75°C. These polymers have properties that are similar to those of the classical counterpart poly[bis(2-(2-methoxyethoxy)ethoxy)-phosphazene. They have low dimensional stabilities and undergo viscous flow even at room temperature. The polymers with branched oligoethyleneoxy side chains (podands) have similar glass transition temperatures, in the range of -82 to -79°C. However, the bulk dimensional stabilities of the branched polymers are significantly higher than those of the corresponding linear side chain series. The branched side chain polymers resist viscous flow and readily form thin, free-standing films. The podand polymers also dissolve lithium triflate to form ionically conducting materials with conductivity levels similar to those of the polymers bearing linear side chains.

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.

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A method for the introduction of sulfone or sulfoxide functional groups into the side groups of polyphosphazenes has been developed. This procedure involves the prior introduction of thioether-containing side groups into phosphazenes followed by oxidation of the sulfur atoms by H2O2 or m-chloroperbenzoic acid (MCPBA). This method was first explored at the level of model small molecule cyclic species as a prelude to the polymer oxidation reactions. The attractive forces generated by sulfone or sulfoxide functional groups produce alkyloxy-substituted polyphosphazenes with relatively high glass transition temperatures (up to +19 °C). The potential of these materials as polymer electrolytes, both in the solid state and in systems with added propylene carbonate, was explored by means of impedance analysis conductivity studies. The competition between the polymer and the solvent for lithium ions was also investigated.