Cooling physical aging

Figure 4.4 shows a dilatometric or calorimetric experiment to show structural relaxation (physical aging) and glass transition hysteresis. The sample is cooled from T0 to T it is kept at Tj for a certain time and heated again to T0. During the cooling step, the material vitrifies at B, resulting in an abrupt decrease in both the expansion coefficient and the specific heat. 

Figure 4.5 shows the variation of the expansion coefficient or the heat capacity in the same experiment described in Fig. 4.4. The area of the peak (which is endothermic in the calorimetric experiment) can be considered as a quantitative measurement of the extent of physical aging. If, having attained point H, the sample is cooled again to C and rapidly reheated, the manifestation of physical aging (the peak shown in Fig. 4.5) is erased and the sample is said to be rejuvenated. 

Figure 3.1. Schematic illustration of temperature dependences of the specific volumes of amorphous materials. This figure also illustrates the effects of the nonequilibrium nature of glass structure, which results from kinetic factors. Glass 1 and Glass 2 are specimens of the same polymer, but subjected to different thermal histories. For example, Glass 1 may have been quenched from the melt very rapidly, while Glass 2 may either have been cooled slowly or subjected to volumetric relaxation via annealing ( physical aging ) in the glassy state.

Upon cessation of cooling below Tg the enthalpy and volume slowly but incessantly decrease toward their equilibrium values. During this decrease other kinetic properties, such as the rate of creep, slow down. This deceleration of kinetic processes is called physical aging and is reflected in Figure 5.1 by the... 

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