Below Tg: aging phenomenon
Amorphous and glassy states of molecular and pharmaceutical compounds: general properties

On cooling, the liquid passes into a non-equilibrium state at T_g. At a temperature T < T_g, its structure is apparently frozen in the state it had at T_g. Molecular movements corresponding to α-relaxations are extremely slow, but nevertheless persist. This mobility enables the system to evolve very slowly towards the liquid equilibrium state. In practice, all the physical properties of glass evolve: this is the aging phenomenon. The lower the temperature T, the slower this evolution and the greater the imbalance to be corrected.