Fast Scanning Calorimetry Studies of the Interface-facilitated Devitrification Mechanism of Model Molecular Glasses

Using Fast Scanning Calorimetry capable of rapid heating rates in excess of 100,000 K/s, we have investigated the interface-facilitated devitrification kinetics of micrometer-scale, ordinary and vapor-deposited amorphous films of several organic compounds. Under conditions of rapid heat loading, glass softening follows zero-order kinetic rate law with an Arrhenius-like temperature dependence during transformation. Similar to the devitrification mechanism of stable vapor-deposited glasses, the devitrification of rapidly heated ordinary glassy films begins at the surface and progresses into the sample bulk via a propagating devitrification front. Confinement of constituents at the devitrification front interface is indicated by the very high apparent activation energy barriers for front propagation in all of glassy films. This implies that the softening mechanism is controlled by self-diffusion of constituents in a transient glassy phase that is nearly identical in structure to the initially prepared glassy film. We will discuss the observed devitrification kinetics in the framework of an extended Wilson-Frenkel model of non-equilibrium, heterogeneous phase transitions.