Polymer electrolytes derived from polynorbornenes with pendent cyclophosphazenes: Poly(ethylene glycol) methyl ether (PEGME) derivatives

A novel elastomeric polymer electrolyte was developed by the linkage of poly(ethylene glycol) methyl ether (PEGME) side chains and norbornene-based units to a cyclotriphosphazene ring, followed by polymerization of the norbornene units by ring-opening metathesis polymerization (ROMP). This yielded an organic polymer backbone with ion co-ordinative cyclic phosphazene side groups. Adjustment of the norbornene content in the starting monomer allowed the degree of branching and cross-linking to be controlled and, in turn, the mechanical properties of the material to be tuned. The polymers were complexed with fixed molar ratios of LiN(SO₂CF₃)₂ and LiSO₃CF₃ and examined as solid polymer electrolytes. Polymer gel electrolytes were then formulated by the addition of variable amounts of propylene carbonate to the polynorbornenes. Films of solvent-free and solvent-containing polymers were homogeneous, flexible and self-standing materials. The ionic conductivities of the solvent-free polymers were in the range of 4×10^-5 S/cm at 30 °C in the presence of 40 mol% of LiSO₃CF₃ or LiN(SO₂CF₃)₂. The conductivities of the solvent-containing (gel) systems increased with increases in the propylene carbonate content to a value of 2×10^-3 S/cm at 30 °C in the presence of 50 wt.% of propylene carbonate and with the use of LiN(SO₂CF₃)₂ at a constant O:Li⁺ ratio of 8:1. Even with this high concentration of propylene carbonate, the electrolyte was a solid elastomer rather than a viscous liquid. In general, the conductivities are twice as high with the use of LiN(SO₂CF₃)₂ compared to LiSO₃CF₃. The T_g values were determined by DSC analysis, and these decreased with increased amounts of propylene carbonate in the system.