COOPERATIVE REORIENTATIONS, TRANSLATIONAL MOTIONS AND ROTATIONAL JUMPS IN VISCOUS-LIQUIDS

We have studied H-2 NMR relaxation of symmetric and nonsymmetric guest molecules dissolved in the glass former tricresyl phosphate (TCP). Correlation times for a liquidlike overall reorientation (tau(iso)) and for a solidlike rotational jump (tau(j)) are extracted and compared to the corresponding time constants of the host TCP which has been studied by P-31 NMR. While the liquidlike motion of the guests exhibits a strong slowing-down upon supercooling, the jump process stays fast and persists also in the glassy state. At high temperatures in the fluid liquid, guest and host molecules exhibit correlation times tau(iso) which are well distinguished: the smaller is the guest, the faster is its reorientation. However, in the highly viscous regime the time constants of guest and host molecules approach each other independent of molecular size. In this regime the guest molecules probe the glass transition of the host. Furthermore, the NMR results are compatible with assuming isotropic reorientation in the viscous and anisotropic motion in the fluid liquid. For some guests also translational correlation times have been obtained, and they tend to separate from those of the reorientational motion upon cooling; i.e. translational rotational coupling breaks down in the highly viscous regime. All in all, these observations support the idea of a change of transport mechanism in the supercooled liquid at a temperature 15-20% above the glass transition temperature.