INDICATIONS FOR A CHANGE OF TRANSPORT MECHANISM IN SUPERCOOLED LIQUIDS AND THE DYNAMICS CLOSE AND BELOW T-G

The Van der Waals glass former m-tricresyl phosphate (m-TCP) is investigated by P-31 nuclear magnetic resonance (NMR) and Raman scattering (RS) and polybutadiene (PB) is investigated by H-2 NMR. Further, dielectric relaxation data, X(D)''(omega), on glycerol are reanalyzed and the results are compared with those of NMR and RS. The fast dynamics of m-TCP are described by the mode coupling theory (MCT); a critical temperature, T-c similar to 260 K, is estimated. The existence of a crossover temperature is further supported by (i) a breakdown of translational-rotational coupling and (ii) an emergence of a third process (beta(s)-process) for T less than or similar to T-c. The beta(s)-process is well distinguished from the fast beta(E)-process observed by RS and NS and described by MCT. Indications for the beta(s)-process are also found in glycerol where no beta-relaxation peak is observed in dielectric relaxation. Presumably below T-c the spectral density of the alpha-process exhibits peculiarities at high frequencies which also show up in the NMR relaxation (T greater than or equal to T-g). Below T-g, T-1 is dominated by the beta(s)-process. This process exhibits a typical solid-state behaviour, e.g., an inhomogeneous distribution of Debye processes allows one to describe the NMR relaxation down to 50 K in the case of PB.