Peculiarities of fluctuations in glass-forming liquids

Abstract

The present work is devoted to the investigation of the peculiarities of fluctuations in glass-forming liquids. To justify our reasons we will anlayze the character of molecular light scattering in silicate glass, that was the object of experimental study. Our approach to the problem of fluctuations in glasses and liquids near the glassification point is based on the following general motivations. Highly-viscous and glass-like states of glass-forming liquids are in the metastable region, which correspond to the crystal state of a system in the equilibrium. The behavior of a system is determined by relations between two characteristic times: τw(T) and τe, where the former is the waiting time for the critical nucleus and τe is the time for cooling the system from the melting temperature to the spinodal temperature Ts at the same pressure. If τe <(<<) τw(Ts) the system smoothly passes to the glass-like state. Here two factors are very important. In metastable region of a system the nuclei of the new phase (crystal) appear. The fraction volume φ(T), occupied by them, depends on the cooling rate. The value of φ is the less the more quick cooling the system is. However, below the spinodal in accordance with thermodynamics the homogeneous states of a system are impossible. The existence of the lasts contradicts the stabilty conditons. From here we conclude that typical glass-like states in a system should be microinhomogeneous. Similar conclusion is confirmed by the results of experimental study of the structure. Now we should note that the strongly anisotropic interaction between nonspyherical molecules of glass-forming liquids leads to considerable coupling between the translational and orientational degrees of freedom in a system. In other words the changes of density is accompanied by the corresponding modulation of molecular orientations. Due to strong increasing the shear viscosity at approaching the glassification point some configurations of the centers of mass and the orientations of molecules froze. More exactyly, only weak thermal deviations of coordinates of molecules from their local equilibrium values are possible. The radical change of the global molecular configuration requires exponentially large time. Thus, the combination of frozen and thermal fluctuations is the distinctive feature of glass-like state of a system. It is necessary to note that different ways of cooling of a system lead to nonequivalent frozen fluctuations.