Modeling the Differing Time Scales of Structural Recovery and Mechanical Relaxation Observed in Aging Experiments

When subjected to temperature changes below the glass transition Tg region, amorphous polymers exhibit evolution of both structure (volume or enthalpy) and mechanical response (physical aging). While historically it has been assumed that the glassy structure and the mechanical response evolve at the same rate, results from the NIST torsional dilatometer (and from other sources) have indicated that these properties can evolve at different rates, depending on the temperature history. The torsional dilatometer results have been modeled in two ways. First, it was assumed that the volume and mechanical response are governed by different clocks, with the principle of time-aging time superposition employed to evaluate an aging time shift factor ate from the torsional response, which was then compared to a structural shift factor aδ calculated from the evolution of the volume. These results were also investigated using a thermoviscoelastic model based on rational thermodynamics and configurational entropy; this model does not include an explicit assumption of separate time scales, but different time scales for the structure and mechanical properties appear to arise naturally from the formulation. The results from the thermoviscoelastic model show good qualitative agreement with the torsional dilatometer results, although more material data is needed to make an exact comparison.