The glass-softening temperature range and non-Arrhenius dynamics: the case of vitrified water

The temperature-width, DeltaT, of the glass-softening endotherm over which the time-dependent relaxational contribution to a property is fully gained, is significantly affected by, (i) the parameters of the Vogel-Fulcher-Tammann equation, (ii) the distribution of relaxation times and its temperature dependence and (iii) the spontaneous structural relaxation during heating through the glass-softening region. Their consequences on DeltaT have been analyzed in terms of both the dynamic heat capacity which is affected by (i) and (ii), and the differential scanning calorimetry which is also affected by (iii). The analysis and experimental data show that DeltaT alone may not be useful in characterizing the extent of its non-Arrhenius behavior. Therefore, the recently proposed qualitative relation between the extent of non-Arrhenius behavior and DeltaT/T-g, and the related 'master plot' from which this extent may be determined, seem inadequate. This removes the basis for the recent inference [Nature, 398 (1999) 492] that supercooled water's molecular kinetics tends to become Arrhenius at low temperatures. (C) 2000 Elsevier Science B.V. All rights reserved.