Aging of Johari-Goldstein Relaxation in Structural Glasses

Using frequency-dependent dielectric susceptibility measurements we characterize the aging in two supercooled liquids, sorbitol and xylitol, below their calorimetric glass transition temperatures, T$_{g}$.~ In addition to the alpha relaxation that tracks the structural dynamics, the susceptibilities of both liquids possess a secondary Johari-Goldstein relaxation at higher frequencies.~ Following a quench below T$_{g}$, the susceptibility slowly approaches equilibrium behavior.~ For both liquids, features of the Johari-Goldstein relaxation display a dependence on the time since the quench, or aging time, that is very similar to the age dependence of the alpha peak. However, one can not assign a single fictive temperature to both the alpha and Johari-Goldstein relaxations.~ For example, the peak frequency of the Johari-Goldstein relaxation remains constant during aging for sorbitol while it increases with age for xylitol, inconsistent with a decreasing fictive temperature.~ This behavior contrasts with that of the high frequency tail of the alpha peak whose shape and position track the aging of the main part of the peak.