Influence of Terminal Phenyl Groups on the Side Chains of Phosphazene Polymers:  Structure−Property Relationships and Polymer Electrolyte Behavior

A series of single- and mixed-substituent polyphosphazenes with a combination of 2-(2-methoxyethoxy)ethoxy and 2-(2-phenoxyethoxy)ethoxy side groups were synthesized and evaluated to determine the effect of variations in side group ratios on the physical properties and ionic conductivity of gels. The 2-(2-phenoxyethoxy)ethoxy side groups increased the T g values of both the solid polyphosphazenes and the gel electrolytes relative to the control polymer poly[bis(2-(2-methoxyethoxy)ethoxy)phosphazene] (MEEP) but decreased the ionic conductivity. Specifically, as the content of 2-(2-phenoxyethoxy)ethoxy side groups increased, the glass transition temperatures ( T g ) also increased, from −79 °C for MEEP to −21 °C for poly[bis(2-(2-phenoxyethoxy)ethoxy)phosphazene]. A gel polymer electrolyte derived from MEEP had an ionic conductivity of 3.9 × 10 -4 S/cm at 25 °C, one of the highest measured for a MEEP-type system. The gel electrolyte derived from the polymer with all phenyl-terminated substituents had an ionic conductivity of 1.8 × 10 -5 S/cm at 25 °C, and those from mixed-substituent polymers increased with increasing 2-(2-methoxyethoxy)ethoxy side group content. The gel electrolyte derived from a polymer with a 60:40 ratio of 2-(2-methoxyethoxy)ethoxy to 2-(2-phenoxyethoxy)ethoxy side groups gave the best compromise between dimensional stability and conductivity, with a T g of −61 °C and an ionic conductivity of 1.2 × 10 -4 S/cm.