Physical aging volume recovery

Simon, S. L., Plazek, D. J., Sobieski, J. W., and McGregor, E. T., Physical aging of a polyether-imide volume recovery and its comparison to creep and enthalpy measurements, J. Polym. 

Cowie, J. M. G., Elliott, S., Ferguson, R Simha, R Physical aging smdies on poly(vinyl acetate) - enthalpy relaxation and its relation to volume recovery. Polymer Communications, 28(11), pp. 298-300 (1987). 

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. 

If these physical aging effects that occur during creep measurements are due to the decrease of free volume, the time-age shift function must be directly related to the change in free volume. In recent years several expressions have been proposed to describe volume recovery behavior [4,5,8,20-25] in which the temperature and structure dependence of the retardation times is based on activation processes [21], configurational entropy [22,23] or free volume [4,5,8,20,24,25]. 

While at a temperature below Tg, some degree of molecular motion does occur. With time the molecular system does approach the true equilibrium state, i.e., the equilibrium values of volume, enthalpy or other state function variables. Physical aging represents a thermodynamic drive towards the equilibrium glassy state, and the recovery process involves a decrease in free volume as the material ages. 

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... 

The decrease in volume during physical aging is specifically known as volume recovery or volume relaxation. Volume recovery experiments include down-jump, up-jump, and memory experiments. The results of these experiments, which are shown and discussed later, demonstrate that structural recovery is both nonlinear, nonexponential and path-dependent. 

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