Aging volume recovery

McKenna. G.. Schultheisz. C., and Lcterrier. Y., Volume recovery and phys. ageing Dilatometric evidence for different kinetics. 9th Inti, Conf Deformation, Yield fracture of Polymers 4 1994 Cambridge. 31 I 314 (1994). 

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. 

Figure 2.16 Time-ageing time superposition for long-term prediction of volume recovery effects on the behaviour of polymers.

The simplest volume recovery experiment is the down-jump. Volumetric data collected at a series of aging temperatures are normalized in order to examine the relative departure from equilibrium 8 which is defined as... 

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]. 

Let us examine a volume recovery experiment and a creep experiment under progressive aging at the same temperature. Then the shift functions b (A,A ) and aj should be comparable if the prehistories are approximately the same and the change in free volume is the origin of aging effects. 

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. 

The simplest volume recovery experiment performed is the down-jump. In this experiment, a material initially above Tg and at equilibrium is subjected to a temperature down-jump to an aging temperature Ta below Tg. The isothermal evolution of volume at T, as indicated by the downward arrow in Figure 1, is monitored with time via length or volume dilatometry. Figure 2 shows tjq)i-cal data replotted from Kovacs data (9) for a series of aging temperatures for poly(vinyl acetate). These curves, called intrinsic isotherms, are plotted as the relative departure from equilibrium 5 versus the logarithm of time, with S defined as... 

The third type of volume recovery experiment, termed the memory or crossover experiment, demonstrates that the relaxation behavior is dependent on the prior thermal history of the material. In this experiment, the material is aged partially into equilibrium at temperature Ti and then heated to a temperature Ta where the volume initially lies on the equilibrium line. Because the volume departure from equilibrium at Ta is initially zero, no evolution of volume might be expected. However, the observation is that the volume increases away from equilibrium and then decreases back to equilibrium. The thermal history involved in the memory experiment is depicted schematically in Figure 4. The experimental results of Kovacs for poly(vinyl acetate) (PVAc) (9) are shown in Figure 5 for several Ti, in addition to the down-jump response. The memory experiment is attributable to a distribution of relaxation times for example, if at T the fastest relaxing regions reached equilibrium density by the time the jump... 

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