How does a glass become a supercooled liquid? Extreme conditions yield new perspectives

Kinetic facilitation describes the idea that a mobile region in a glass or a liquid can induce the mobility of an adjacent region, which had initially lower mobility, and was not expected to rearrange under those conditions. This mechanism has been used to explain the transformation of ultrastable organic thin films. In this case, a supercooled liquid front forms on the more mobile regions of the films, i.e. surfaces and interfaces, and propagates across the film, consuming a glass that otherwise would have remained almost unchanged for a much longer period of time. To the date, this is the clearest example of kinetic facilitation as the mechanism controlling the transition of a glass. However, this same mechanism has been used to describe the bulk transformation of a very stable glass. It has been predicted theoretically and observed in simulations that the bulk transformation into the supercooled liquid glass of very stable glasses will start at regions of very low stability and these regions will grow transforming into the liquid the adjacent glass. For this mechanism to dominate the transformation it is necessary to have a big contrast in mobility between the different regions of the glass. This can be achieved either by preparing glasses of very high stability or by pushing the transition to high temperatures, where the contrast in mobility between glass and liquid will increase. The use of fast scanning nanocalorimetry allows us to shift the transition to high temperatures, reproducing the necessary conditions for observing this type of mechanism for the transformation of the glass, even for glasses which are not ultrastable. We present here experimental results on the bulk transformation of vapor deposited glasses of different stabilities, showing how the growth of these more mobile regions plays an important role in the softening process in all cases.