Kinetic and Calorimetric Fragility of Chalcogenide Glass-Forming Liquids: Role of Shear vs Enthalpy Relaxation

The kinetic and calorimetric fragility indices m of binary As-Se and Se-Te chalcogenide liquids with a wide range of fragility are determined using a combination of parallel plate rheometry, beam bending viscometry, and conventional differential scanning calorimetry (DSC). It is shown that both sets of measurements lead to consistent m values only if the validity of the assumptions often implicit in the methodology for the estimation of m are considered. These assumptions are (i) the glass transition temperature T_g corresponds to a viscosity of ∼10¹² Pa s and (ii) enthalpy and shear relaxation time scales τ_en and τ_shear are comparable near T_g. Both assumptions are shown to be untenable for highly fragile liquids, for which modulated DSC studies demonstrate that τ_en ≫ τ_shear near T_g. In these cases, the above-mentioned assumptions are shown to lead to consistently higher values for the kinetic fragility compared to its calorimetric counterpart.