Calorimetric Features of High-Enthalpy Amorphous Solids and Glass-Softening Temperature of Water

Abstract

In the differential scanning calorimetry (DSC) scans of the annealed states of vapor-deposited, hyperquenched, and crystal-amorphized solids, the glass-softening or T g endotherm is interrupted by the crystallization exotherm, thus making the T g endotherm appear like a rounded peak. This apparent peak is occasionally misidentified as a sub-T g peak, which appears in the 0.7T g to 0.8T g range in the DSC scan of glasses preannealed at a specific temperature, T ann. To help prevent such misidentification, we provide four criteria for distinguishing the apparent peak of the T g endotherm from a sub-T g peak: (i) Crystallization occurs in the T g endotherm range or at T just above it, and not in the sub-T g peak range. (ii) The T g endotherm onset occurs at T > T ann, but the sub-T g peak onset occurs at T ann. (iii) Unannealed glasses show a T g endotherm; only preanneal ed glasses show the sub-T g peak, and the height of the sub-T g peak and its area vary when T ann and the annealing time are varied. (iv) Slope of the DSC scan may decrease, become zero or become negative prior to the onset of the T g endotherm, but it does not decrease and remains positive prior to the onset of sub-T g peak. The annealed state of hyperquenched glassy water is known to crystallize in the 142−150 K range of its endotherm, the onset temperature of the endotherm is higher than T ann, its unannealed state shows the T g endotherm, and the slope of the scan prior to the onset temperature is not always positive. These observations remove the basis for a recent conjecture that the T g of water is between 165 and 180 K, and is unobservable. Available calorimetric data on vapor-deposited, hyperquenched, and mechanically amorphized solids have shown a T g endotherm partially superposed by the crystallization exotherm, but T g itself does not change. It is proposed that intermediate states formed during the occurrence of disorder−order transitions in solids and in proteins can be studied in the time domain by using their hyperquenched glassy and mechanically amorphized states.

The amorphous state of pharmaceuticals obtained or transformed by milling: Sub-Tg features and rejuvenation

“…This involves polymers [15][16][17][18], metals [19,20], oxides [21] and silicates [22,23]. Such features have also been discussed by Johari [24,25] and Yue and Angell [26] in relationship with the discussion concerning the localisation of the glass transition of water. Glasses showing these features have been obtained by a variety of techniques involving fast cooling such as that used in vapor deposition [20] and electrospray technique [27], but also in the production of glass fibers [22,23,26].…”

Enthalpy relaxation in hyperquenched glasses of different fragility

“…The change in the relaxation spectrum resulting from annealing at different time and temperature shows a number of features, which have been discussed thoroughly for basaltic glass [6]. The most prominent of these is the sub-T g endotherm, which has been detected for a number of glass systems [2,6,[8][9][10] and which has provided new information on the glass transition of water [11].…”

Entropy increase in the amorphous-to-crystalline phase transition in zirconium tungstate

“…Accordingly, it is not obvious that a glass transition should precede recrystallization in such case. Anyway, it is well known that the glass transition endotherm is not always observed in thermal analysis, as it very often is weak and spread out in temperature [23].…”