Direct determination of kinetic fragility indices of glassforming liquids by differential scanning calorimetry: Kinetic versus thermodynamic fragilities

Abstract

A calorimetric method of obtaining directly the fragility of liquids from the fictive temperatures of variably quenched glasses, is outlined. “Steepness indexes” m, have been determined for a group of molecular liquids of diverse character, and vary in the range 50–150. The values obtained mostly agree well with those from earlier studies using dielectric relaxation, heat capacity spectroscopy, and viscosity data. In our method there is the advantage that the fragility is determined from the relaxation process that is basic to the calorimetric glass transition temperature measurement, namely, that of the enthalpy. The calorimetric measurements also yield the liquid and glass heat capacities, and entropies of fusion, permitting relationships between thermodynamic and kinetic responses to be examined simultaneously. We study glycerol, dibutylphthallate, 9-bromophenanthrene, salol, orthoterphenyl, propylene carbonate, decalin and its nitrogen derivative decahydroisoquinoline, and find the latter two to be the most fragile liquids known, m =145 and 128 respectively. Surprisingly, of the liquids studied, decalin has the smallest increase in heat capacity at the glass transition. By contrast, the strongest liquid, glycerol, has the largest increase. However, the thermodynamic fragility of decalin, assessed from the scaled rate of increase of the excess entropy above Tg, is found to be high, due to the unusually small value of the excess entropy at Tg. Conversely, the entropy-based fragility for glycerol is the lowest. Thus the correlation of kinetic and entropy-based thermodynamic fragilities reported in recent work is upheld by data from the present study, while the basis for any correlation with the jump in heat capacity itself is removed.

Anomalously large isotope effect in the glass transition of water

“…This is typical for vitrifying systems and well known (e.g., from DSC measurements). Changing the heating rate 100 times shifts the onset temperature by ∼6.3% in H 2 O and ∼4.3% in D 2 O, which is comparable to the ∼3.8% shift observed in glycerol (15) and ∼3% shift observed in salicin (16) (recalculated to the same 100 times change in the heating rate). However, changing the heating rate affects the dielectric onset temperatures in H-LDA and D-LDA differently (Fig.…”

Deformation of glass forming metallic liquids: Configurational changes and their relation to elastic softening

“…The steepness of the viscosity dependence on temperature in the vicinity of the glass transition, i.e., the liquid fragility, has been known to be associated with the stored configurational enthalpy since the early work of Angell and co-workers. [5][6][7] Furthermore, the effect of strain rate on viscosity induced by mechanical deformation has also been linked to changes in configurational enthalpy. 8 However, attributing the deformationally induced softening of liquids to a unique functional relation between shear modulus and stored configurational enthalpy is a concept that has just recently been brought to attention.…”

Comment on “Direct determination of kinetic fragility indices of glassforming liquids by differential scanning calorimetry: Kinetic versus thermodynamic fragilities” [J. Chem. Phys. 117, 10184 (2002)]

“…An aim of the work of Wang et al 1 is establishing a connection between kinetic fragility ͑a relaxation property͒ and thermodynamics. Toward this end, Angell 4,5 had suggested that the magnitude of the heat capacity increment at the glass transition, ⌬C p (T g ), should correlate with the fragility of a glass former.…”