Slow molecular dynamics in binary organic glass formers

We investigate systems benzene in polystyrene as model for binary glass formers by applying multidimensional H-2 NMR experiments. It is demonstrated that the molecular larger component of these systems essentially behaves as a neat glass former, whereas the small benzene molecules show a completely different scenario. Their 6-fold axes still reorientate isotropically below the glass transition of the mixed system. This dynamics is characterised by a very broad distribution of correlation times. Within the distribution an exchange between fast and slow reorientating molecules is detected which takes place on the same time scale as the reorientation itself. (C) 2000 Elsevier Science B.V. All rights reserved.