NMR and ion conductivity studies on cross-linked poly(ethyleneoxide-propyleneoxide) and branched polyether doped with LiN(SO₂CF₃)₂

Two solvent-free cross-linked polymer electrolytes, a random ethyleneoxide and propyleneoxide copolymer, poly(EO-PO) and a random ethyleneoxide and 2-[2-(2-methoxyethoxy)ethoxy]ethylglycidylether copolymer, poly(EO-GE) doped with LiN(SO₂CF₃)₂ in the ratio 10:1 (polymer oxygen:lithium) were studied using ionic conductivity and ¹H, ¹⁹F and ⁷Li NMR measurements in the temperature range of ∼303-353 K. Correlation times for the polymer segmental and the lithium hopping motions of the order of 10^-10-10^-8 s were determined from the minima in the temperature dependence of the spin-lattice relaxation of the polymers (¹H) and lithium (⁷Li). At the same temperature such motions are faster in the poly(EO-PO) than in the poly(EO-GE) systems and the temperature dependencies of these motions are larger in the poly(EO-PO) system. Since the relaxation behaviors of the anion (¹⁹F) were consistent with a single component, the self-diffusion coefficients of the anion were measured using the pulsed-gradient spin-echo (PGSE) NMR method. Although the diffusion data were consistent with a single isotropically diffusing species at each temperature, the measured diffusion coefficients were dependent on Δ (the timescale of the diffusion measurement) but became constant at long Δ values. We surmise that at long Δ the anion has sufficient time to diffuse amongst enough microdomains of the polymer electrolyte system to complete a 'true average'. During shorter times, the anion appears to diffuse faster. The activation energies obtained from the apparent diffusion coefficients at longer Δ values agreed with those obtained from ionic conductivity measurements for the two polymer electrolytes. The anion diffusion in poly(EO-PO) is faster than in poly(EO-GE) at every temperature.