2. Aging through structural relaxation (in the broadest sense)

When a plastic material is cooled, at the end of its forming operations, molecular mobility decreases by several orders of magnitude during crystallization or vitrification. The changes in macromolecular network configuration required to reach thermodynamic equilibrium then become too slow for this equilibrium to be achieved within the time scale imposed by the cooling rate. The material then freezes in a metastable state, as shown in figure 1 .

At the operating temperature T _u , the material is therefore out of equilibrium. If it has some residual mobility, this will enable it to slowly evolve towards equilibrium (represented by the straight line...