Glass softening in the limit of high heating rates: Heterogeneous devitrification kinetics on nano, meso, and micrometer scale

Abstract

When heated rapidly, glasses often devitrify heterogeneously, i.e., by a softening front that originates at the surface of an amorphous film. Yet the fundamentals of this devitrification regime are not completely understood; depending on experimental conditions, the reported front propagation distances differ by an order of magnitude. Using a high-resolution fast scanning calorimetry technique, we have investigated the softening of glassy methylbenzene films with thicknesses between 30 and 1400 nm. We confirm first that, in all films, the devitrification process begins with the formation of a softening front that propagates through the films over distances of ∼50 nm and that the front propagation kinetics at this stage follow an Arrhenius law. However, we also show that, in films with thicknesses above 165 nm, the front propagation does not terminate with the onset of bulk softening. Specifically, increasing the films’ thicknesses above 165 nm yields sharp, clearly discernible endotherms that precede the bulk softening endotherms and that are consistent with a two-fold increase in the enthalpic barrier to front propagation at a well-defined critical temperature. We term this phenomenon “Arrhenius discontinuity” and use reaction rate and continuum front dynamics theories to explain its origins and the physical nature of the resulting distinct heterogeneous devitrification processes. Finally, we discuss the findings in the context of recent theoretical, computational, and experimental studies of heterogeneous devitrification by other research groups.

Crossing the Frontier of Validity of the Material Time Approach in the Aging of a Molecular Glass

“…16,27−29 When the temperature step amplitude, and thus the distance from equilibrium, is increased, deviations from the TN prediction start to be perceptible. 15,18 An extreme illustration is the heterogeneous equilibration dynamics observed in ultrastable glasses 30 which display an exceptional stability. 31,32 Their annealing close to the glass transition temperature T g can be compared to an extremely large temperature step experiment.…”

“…This material time flows at a rate controlled by an out-of-equilibrium relaxation time that itself ages. The performance of this phenomenological model has motivated recent studies aiming at investigating the potential deeper physical meaning of the material time. ,− When the temperature step amplitude, and thus the distance from equilibrium, is increased, deviations from the TN prediction start to be perceptible. , An extreme illustration is the heterogeneous equilibration dynamics observed in ultrastable glasses which display an exceptional stability. , Their annealing close to the glass transition temperature T g can be compared to an extremely large temperature step experiment. In strong contrast with the homogeneous picture of the TN formalism, their aging dynamic was shown to be controlled by the nucleation and growth of equilibrated regions. − …”