Sub‐Sub‐ T g Enthalpy Relaxation in a B ₂ O ₃ Glass

Enthalpy relaxation processes in a B,O 3 glass were investigated calorimetrically in the temperature region from well below to near the glass‐transition temperature T g . The low‐temperature ( T a T g ‐ 100 K) or sub‐sub‐ T g anneals stabilize the glass structure. On heating, the annealed sample shows an excess endothermic peak above the annealing temperature T a , and gradually recovers the original enthalpy without heating through T g . The enthalpy relaxation evolves in a continuous manner with annealing time as seen in the recovery process. As T a approaches T g , a gradual transformation of the sub‐sub‐ T g behavior to that characteristic of the sub‐ T g anneal occurs. The relaxation spectrum exhibits a broad distribution of relaxation times with activation energies of ∼ 10 5 J/mol. The so‐called β distribution commonly used in the analysis of the sub‐ T g relaxation processes is seen to be too narrow to describe the sub‐sub‐ T g behavior. Possible mechanisms for the sub‐sub‐ T g relaxation phenomena are proposed.