Indications for a change of transport mechanism in supercooled liquids and the dynamics close and below Tg

Abstract

The Van der Waals glass former m-tricresyl phosphate (m-TCP) is investigated by 31P nuclear magnetic resonance (NMR) and Raman scattering (RS) and polybutadiene (PB) is investigated by 2H NMR. Further, dielectric relaxation data, X D ″( ω), 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 ∼ 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 ( β s-process) for T ≲ T c. The β s-process is well distinguished from the fast β f-process observed by RS and NS and described by MCT. Indications for the β s-process are also found in glycerol where no β-relaxation peak is observed in dielectric relaxation. Presumably below T c the spectral density of the α-process exhibits peculiarities at high frequencies which also show up in the NMR relaxation ( T ⩾ T g). Below T g, T 1 is dominated by the β 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.