Roles of individual and cooperative motions of molecules in glass-liquid transition and crystallization of toluene.

Deeply supercooled fragile liquid is known to be dynamically heterogeneous, where super Arrhenius behavior of shear viscosity and alpha and beta relaxation processes have been observed. To clarify origins of these behaviors, we have investigated correlations between microscopic molecular diffusion and macroscopic hydrodynamics of vapor-deposited toluene films by using time-of-flight secondary ion mass spectrometry. The molecules are intermixed gradually at around 70 K on the film deposited at 15 K, which is followed by an abrupt film morphology change at around the calorimetric glass-transition temperature (T(g)) of 117 K. For the film deposited at 100 K, intermixing of the molecules occurs at temperatures over 100-130 K, but no abrupt increase in diffusivity is recognizable at T(g). This result can be explained as dynamical heterogeneity or decoupling between translational diffusion and viscosity. The self-diffusion is thought to occur in a sub-T(g) region as a precursor state of the glass-liquid transition; that is, individual motion in the solid-like state evolves into cooperative motion of liquid-like state at T(g). The supercooled liquid nucleates at 147 K, although the crystal can grow even at 117 K provided that nuclei preexist. The origins of unusual glass transition behaviors of amorphous solid water are also discussed in comparison with this standard.