Comment on 'Water’s second glass transition, K. Amann-Winkel, C. Gainaru, P. H. Handle, M. Seidl, H. Nelson, R. Böhmer, and T. Loerting, Proc. Natl. Acad. Sci. (U.S.) 110 (2013) 17720.', and the sub-Tg features of pressure-densified glasses

It is known that ambient pressure DSC scan of a glass formed by cooling a liquid under a high pressure shows a sub-Tg endotherm with a Cp peak, an exothermic minimum, and finally the true Tg-endotherm of the glass (A. Weitz, B. Wunderlich, J. Polym. Sci. Polym. Phys. 12 (1974) 2473). Amann-Winkel et al. (Proc. Natl. Acad. Sci. (U.S.) 110 (2013) 17720) reported that the ambient pressure DSC scan of eHDA, the expanded high density amorphous ice, cooled under a pressure of 0.07 GPa shows onset of a Cp endotherm at 116 K. Unaware of the studies of pressure-densified glasses, they regarded this onset as a second glass transition of water at 116 K involving translational mobility. We show that: (i) their eHDA at 0.07 GPa and 140 K was ultraviscous water of high density, and the ex situ DSC scans were obtained for the recovered glassy solid that formed on cooling this water under 0.07 GPa pressure to 90 K, (ii) decrease in the onset temperature of the 116 K endotherm they found on decreasing the cooling rate, and used as a typical signature of glass-transition, is not a signature of glass transition – and this rules out the possibility of this endotherm being a glass transition endotherm – nor are the methods claimed to be standard are standard or reliable methods, (iii) the DSC scans of the recovered glassy solid at ambient pressure are qualitatively similar to those found for pressure-densified glasses, and (iv) the experimental procedure and data analysis of the dielectric relaxation study is too inaccurate to reliably support the view that water has a second Tg. We conclude that the rise in Cp at 116 K is the sub-Tg endotherm of pressure-densified glassy water, i.e., water has only one Tg in the DSC scan at ambient pressure. We also describe other findings on pressure-densified glasses, and the possibility of their sub-Tg mobility affecting certain irreversible processes, and suggest experiments for investigating why a glass formed by cooling under high pressure has different properties at ambient pressure than a glass formed by cooling at ambient pressure.