Initial Stages of Rejuvenation of Vapor-Deposited Glasses during Isothermal Annealing: Contrast Between Experiment and Simulation

TL;DR

This study investigates how vapor-deposited glasses of methyl-m-toluate rejuvenate during isothermal annealing, revealing differences between experimental results and computer simulations, especially in highly stable glasses.

Contribution

It provides new insights into the rejuvenation process of vapor-deposited glasses, highlighting contrasts between experimental observations and simulation predictions.

Findings

Partial or full rejuvenation observed in moderately stable glasses.

Highly stable glasses show increased high-frequency relaxation without loss increase.

Simulations do not replicate the rejuvenation seen in experiments for stable glasses.

Abstract

Physical vapor deposition can prepare organic glasses with high kinetic stability. When heated, these glassy solids slowly transform into the supercooled liquid in a process known as rejuvenation. In this study, we anneal vapor-deposited glasses of methyl-m-toluate (MMT) for six hours at 0.98Tg to observe rejuvenation using dielectric spectroscopy. Glasses of moderate stability exhibited partial or full rejuvenation in six hours. For highly stable glasses, prepared at substrate temperatures of 0.85Tg and 0.80Tg, the six-hour annealing time is ~2% of the estimated transformation time, and no change in the onset temperature for the {\alpha} relaxation process was observed, as expected. Surprisingly, for these highly stable glasses, annealing resulted in significant increases in the storage component of the dielectric susceptibility, without corresponding increases in the loss component. These changes are interpreted to indicate that short-term annealing rejuvenates a high frequency relaxation (e.g., the boson peak) within the stable glass. We compare these results to computer simulations of the rejuvenation of highly stable glasses generated by the swap Monte Carlo algorithm. The in silico glasses, in contrast to the experiment, show no evidence of rejuvenation within the stable glass at times shorter than the alpha relaxation process.