Manipulating the properties of stable organic glasses using kinetic facilitation

In contrast to ordinary glasses, when highly stable organic glasses are annealed at temperatures above T(g), they transform heterogeneously into the liquid state by a constant velocity propagating front that initiates at the free surface. The evolution of this growth front has been interpreted as kinetic facilitation, i.e., efficiently packed molecules become un-jammed only when mobility is available in adjacent regions. Here we use physical vapor deposition to prepare highly stable glasses of indomethacin in which mobile regions are either added to or eliminated from the samples in an attempt to use the kinetic facilitation concept to manipulate the properties of these materials. The addition of higher mobility layers in the interior of a thin stable glass film or at the substrate surface is shown to initiate new growth fronts, thus demonstrating that kinetic facilitation occurs independently of free surface mobility. Conversely, capping the free surface with a higher T(g) stable glass stops the growth front, apparently by eliminating surface mobility, thus increasing sample stability by slowing the transformation to the supercooled liquid.