Elastic interpretation of the glass transition in aluminosilicate liquids

One of the remaining puzzles of the glass transition is the origin of a glass-forming liquid's "fragility," which quantifies the departure of its relaxation time from Arrhenius-activated kinetics. According to the shoving model proposed by Dyre, fragility is controlled by the instantaneous shear modulus of the liquid, since any flow event requires a local volume increase, and the related activation energy is equal to the work done in shoving aside the surrounding atoms. Here, we present an in situ high-temperature Brillouin spectroscopy test of the shoving model near the glass transition of eight aluminosilicate glass-forming systems. We find that the measured viscosity data agree qualitatively with the measured temperature dependence of shear moduli, as predicted by the shoving model. However, the model systematically underpredicts the values of fragility for our aluminosilicate liquids. This suggests that the dynamics of the glass transition are governed by additional factors beyond the evolution of the shear modulus, such as configurational entropy. We have also compared the glass transition temperature (T _g,vis) obtained from viscosity (temperature at 1012 Pa s) with the onset temperatures of the decrease in elastic moduli (T _g,elas) and increase in the thermal expansion coefficient (T _g,CTE) during heating. While we find an approximate one-to-one correlation between T _g,vis and T _g,CTE, it is clear that the elastic moduli probe a different frequency response of the glass structure, since T _g,elas is systematically lower than T _g,vis.