Aging structural recovery modeling

Of particular importance to structural recovery models is the idea that the apparent mobility may depend on the temperature/pressure/stress path rather than on the instantaneous state of the material. This differs from all of the phenomenological equations for the characteristic relaxation time, including equation (19), which are presently used in other models of structural recovery. The dependence of the relaxation on the imique state has been investigated by Strvik (146), and although it was claimed that the relaxation time only appeared to depend on the instantaneous volume, there is considerable scatter in the data and the relationship appears to depend not only on the final aging temperature but also on the sample history (ie, the magnitude of down- and double-temperature jumps). The scatter may be due to the measurements of volume and relative relaxation time being made in separate instruments such that the thermal history of the samples were not identical. Alternatively, it may be an indication of a more complicated... 

Although the basic phenomenology associated with glasses is known and models exist which describe structural recovery and physical aging, researchers are not yet able to predict a priori experimental observations. 

In this article on physical aging, the phenomena associated with structural recovery and physical aging are described, starting with discussion of volume recovery, enthalpy recovery, viscoelastic properties, and failure. There are more in-depth reviews of the phenomena and models associated with the glass transition... 

Below Tg (4,5) co-operative molecular processes are usually assumed to be inactive. However, physical ageing implies that conformational changes may be still able to occur if rather infrequently. Structural relaxation processes are observed to be non-exponential and are represented by a continuous distribution or stretched exponential form (d). Thermorheologically simplicity (TRS) implies that the molecular relaxation process has the same form at different temperatures (7) and the validity of this assumption is addressed in this paper. Isobaric volume recovery (8,9) has been described by a single parameter mc el, however all fi ee volume models (10,11) have limitations and a distribution of hole sizes and relaxation times leading to a pseudo-linear theory is a more realistic model(72). Comparison of data fi om various techniques should throw light on the molecular nature of physical agdng. 

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