PVD Glasses: Beta Process Rejuvenation without Loss of Stability

Stable vapor-deposited films of organic glass-forming molecules transform to the supercooled liquid state in distinct ways compared to their liquid-cooled glass counterparts, featuring an elevated temperature for the onset of transformation upon heating the glass above T_g (T_onset) and a surface-initiated growth front mechanism. In this work, the beta relaxation process in stable glasses of methyl-m-toluate (MMT) is monitored with dielectric spectroscopy. For a glass deposited at 0.90T_g during annealing at 0.98T_g, the dielectric storage component increases significantly in the direction of the anticipated supercooled liquid. Surprisingly, this evolution occurs without any change in T_onset. This suggests that the beta process rejuvenates without any associated rejuvenation of the alpha (primary) relaxation, opening the possibility for new insights into the nature of the motions associated with the beta process and its relationship to the position of the glass in the potential energy landscape.