Initial stages of rejuvenation of vapor-deposited glasses during isothermal annealing: Contrast between experiment and simulation

Physical vapor deposition can prepare organic glasses with high kinetic stability. When heated, these glassy solids slowly transform into supercooled liquid in a process known as rejuvenation. In this study, we anneal vapor-deposited glasses of methyl-m-toluate for 6 h at 0.98Tg to observe rejuvenation using dielectric spectroscopy. Glasses of moderate stability exhibited partial or full rejuvenation in 6 h. For highly stable glasses, prepared at substrate temperatures of 0.85Tg and 0.80Tg, the 6 h annealing time is ∼2% of the estimated transformation time, and no change in the onset temperature for the α 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 using 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.