The new insight into dynamic crossover in glass forming liquids from the apparent enthalpy analysis

Abstract

One of the most intriguing phenomena in glass forming systems is the dynamic crossover (T B), occurring well above the glass temperature (T g). So far, it was estimated mainly from the linearized derivative analysis of the primary relaxation time τ(T) or viscosity η(T) experimental data, originally proposed by Stickel J. Chem. Phys. 104, 2043 (1996)10.1063/1.470961; Stickel J. Chem. Phys. 107, 1086 (1997)10.1063/1.474456. However, this formal procedure is based on the general validity of the Vogel-Fulcher-Tammann equation, which has been strongly questioned recently T. Hecksher Nature Phys. 4, 737 (2008)10.1038/nphys1033; P. Lunkenheimer Phys. Rev. E 81, 051504 (2010)10.1103/PhysRevE.81.051504; J. C. Martinez-Garcia J. Chem. Phys. 134, 024512 (2011)10.1063/1.3514589. We present a qualitatively new way to identify the dynamic crossover based on the apparent enthalpy space (Ha′=dlnτ/d(1/T)) analysis via a new plot lnHa′ vs. 1T supported by the Savitzky-Golay filtering procedure for getting an insight into the noise-distorted high order derivatives. It is shown that depending on the ratio between the virtual fragility in the high temperature dynamic domain (m high) and the real fragility at T g (the low temperature dynamic domain, m m low) glass formers can be splitted into two groups related to f 1 and f > 1, (f m highm low). The link of this phenomenon to the ratio between the apparent enthalpy and activation energy as well as the behavior of the configurational entropy is indicated. © 2012 American Institute of Physics.