The methyl group as a built-in probe of the glassy dynamics in propylene carbonate

Deuteron nuclear magnetic resonance was employed to study primarily the gla ssy phase of the fragile glass-former propylene carbonate; selectively deut erated at the methyl group. The analysis of temperature dependent line shap es allowed us to map out the energy barrier distribution below the glass tr ansition temperature. The results were compared to measurements of spin-rel axation times. We determined the relative width of the barrier distribution and the extent of the decoupling between the methyl group rotation time an d the structural relaxation time near the glass transition. These, findings are discussed in the context of what has been reported for secondary relax ation processes in other glass-formers. Stimulated echo experiments on prop ylene carbonate demonstrate that, while in the supercooled liquid small ang le reorientations dominate, in the glass the methyl group performs 120 degr ees (or 60 degrees) jumps.