Fast contribution to the activation energy of a glass-forming liquid

Abstract

This paper presents physical-aging data for the silicone oil tetramethyl-tetraphenyl trisiloxane. The density and the high-frequency plateau shear modulus G∞ were monitored following temperature jumps starting from fully equilibrated conditions. Both quantities exhibit a fast change immediately after a temperature jump. Adopting the material-time formalism of Narayanaswamy, we determine from the dielectric loss at 0.178 Hz the time evolution of the aging-rate activation energy. The relative magnitude of the fast change of the activation energy differs from that of the density, but is identical to that of G∞. In fact, the activation energy is proportional to G∞ throughout the aging process, with minor deviations at the shortest times. This shows that for the silicone oil in question the dynamics are determined by G∞ in—as well as out of—equilibrium.