Below Tg: non-linearity
Amorphous and glassy states of molecular and pharmaceutical compounds: general properties

Below T _g , relaxations are actually more complex than described above. This is because, during aging, enthalpy and molar volume decrease slowly. This gradual densification leads to an increase in the hindrance to molecule movement. It's easy to see why the structural relaxation of aged glass requires more cooperativity due to this molecular settling. In fact, it has long been observed that viscosity depends on the degree of structural relaxation ...