Pore-Collapse in Amorphous Solid Water: a Dynamics Study

Vapor-deposited amorphous ice, so-called amorphous solid water (ASW), exhibits complex structural and morphological transformations upon heating. A network of micropores, present at the deposition temperature (80 K), collapses at 100-145 K, and a glass transition takes place simultaneously above 120 K. Here, we separate the two processes by allowing the micropores to collapse upon heating, which is monitored by small-angle X-ray scattering (SAXS) experiments. The combined micropore collapse and glass transition dynamics are studied using X-ray photon correlation spectroscopy (XPCS). After cooling back down and heating a second time, we see remaining pores collapsing only near Tg. Our analysis reveals both diffusive and ballistic processes attributed to pore collapse dynamics. Fast processes (ca 100 Å2/s) occur only when both micropore collapse and glass transition are at play simultaneously. In other words, both processes impact on each other and lead to a speed-up. The glass transition dynamics mainly features a slow diffusive process with a diffusion coefficient of around 1 Å2/s and lower. This value is in nice agreement with other work on thin and on bulk samples.