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

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 0.7 T g to 0.8 T 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.