Abstract
The temperature dependence of the viscosity of glass is a major concern in glass research. The apparent activation energies obtained from Arrhenius plots often show unusual values larger than bond energies, which makes the interpretation of the activation energy difficult. In this study, a reasonable interpretation of the apparent activation energy is obtained along similar lines as those adopted in solid state physics and chemistry. In contrast to the widely held view that the transition occurs at the reference temperature T 0 according to the Vogel–Fulcher–Tammann formula, in this work the structural change observed at the calorimetric temperature T g is considered as a transition from the liquid to solid phases. The energy barrier for atom rearrangements significantly changes in the transition range with width ΔT g . This change in the energy barrier alters the manner in which the apparent activation energy constitutes the Arrhenius form. Analysis of available experimental data shows that the real value of energy barrier is significantly smaller than the apparent activation energy, and the obtained values are in the reasonable range of energy expected for chemical bonds. The overestimation of the apparent activation energy depends on the ratio T g /ΔT g , which is larger for fragile glasses than for strong glasses. Importantly, the linear term in the temperature dependence of the energy barrier does not appear in Arrhenius plots. This explains why the temperature dependence of viscosity for strong glasses obeys well the Arrhenius law, despite that the temperature dependence of energy barrier is expected for every glass.
Highlights
The temperature dependence of the viscosity of glass is a major concern in the field of glass research
A theoretical model was proposed by Johnson et al, who considered a formula for the energy barrier based on the elastic moduli of the solid state and extended it to the liquid state [83, 85]
This structural change causes a change in the energy barrier Eb in a continuous manner, and this causes η(T ) to deviate from the Arrhenius law
Summary
There exists a large gap in comprehension of the glass state between glass physics and other areas of solid-state physics. Even in the glass state, atoms are migrating with extremely slow velocities From this view, Tg cannot be the temperature of a phase transition. Glass is an equilibrium state: it is viewed as a nonequilibrium state in glass literature This is the natural consequence of the preceding conclusion. 4. The transition state is a mixture of liquid and solid phases: it is viewed as a supercooled-liquid state after a sufficiently long time in glass literature. A natural consequence of the above-described views is that the energy barrier Eb for atomic movement depends on the structure, that is, Eb = Eb({Rj}), and it depends on temperature [34] The pre-exponential factor is too large: the absolute values of log η0 are greater than 100 for organic glasses [23]
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