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.