Volume recovery in epoxy glasses subjected to torsional deformations: the question of rejuvenation

Torsional dilatometry was used to examine the mechanical properties of an epoxy glass during physical ageing, i.e. after a quench from above to slightly below the glass transition temperature. Volume changes in the sample were measured simultaneously with the viscoelastic responses in stress relaxation experiments as functions of the deformation magnitude. The torque relaxation obeys time-ageing time superposition where the shift factor, a_te, increases with ageing time until the sample reaches mechanical equilibrium at t* ≈ 10⁴ s, independent of the magnitude of the strain. In this epoxy, the torsional deformations induce volume expansions which relax on a time-scale similar to those of the torque relaxation. However, the volume recovery responses cannot be superposed at different ageing times by a simple shift along the time axis. Mechanical stimuli only momentarily disrupt the volume evolution following a quench. The underlying volume recovery kinetics, which are much slower than the mechanical torque or volume relaxation, remain unaltered. The facts that t* is independent of the magnitude of the strain and that the volume recovery after a quench remains unaltered, in spite of the imposition of mechanical deformations, support the argument that mechanical stimuli neither alter the underlying (non-equilibrium) thermodynamic state of the glass nor erase physical ageing.