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 Tg values were determined by DSC analysis, and these decreased with increased amounts of propylene carbonate in the system.