Slow and fast methyl group rotations in fragile glass-formers studied by NMR

The spin-lattice relaxation times of the selectively ring deuterated, fragi le glass-formers propylene carbonate and toluene were compared with those m easured for species which were specifically labeled at the methyl groups. I t was found that the dynamics of the CD3 group is strongly decoupled from t hat associated with the primary response of toluene, while for propylene ca rbonate the degree of decoupling is relatively weak. The experimental resul ts could be described successfully using a model which takes into account t he ring dynamics as well as those of the methyl group. (C) 2000 Elsevier Sc ience B.V. All rights reserved.