7Li NMR diffusion coefficient measurements on the Li+-doped ABA triblock copolymer poly(ethylene glycol) distearate

The Li+ diffusion and phase behavior of the lithium-doped poly(ethylene glycol) distearate were studied by 7Li NMR, DSC, and polarized-light optical microscopy. The spectra for highly Li+-doped samples (oxygen-to-lithium ratio y≤9) show 7Li quadrupolar powder patterns even at temperatures above the melting point, what is attributed to reduced segmental mobility due to local ordering. These patterns disappear above y-dependent temperatures, when the samples undergo order–disorder transitions, as determined recently by SAXS. Similar effect is observed in the behavior of the diffusion coefficient versus temperature, where the curve slopes increase by a factor ∼2 above the order–disorder transition. Additionally, when the temperature is fixed at 50 °C and the oxygen-to-lithium ratio is changed from 5 to 16, the ordered phase is observed only for samples with y<9, and below this y value a strong decrease in the 7Li diffusion coefficient is observed. The order present in samples with y≤9 at 50 °C decreases the segmental mobility and reduces the conductivity, which is directly confirmed by the 7Li NMR diffusion coefficient measurements.