1. General aspects and analysis of the phenomenon

There are two main mechanisms of physical aging.

a) Structural relaxation processes (in the broadest sense). The cause of these processes is the material's own instability, since aging can be described as an evolution towards equilibrium. These phenomena do not necessarily involve interaction with the environment; their kinetics depend only on thermodynamic parameters (temperature T, stress σ); the material is not the seat of mass transfer, only the spatial configuration of macromolecules is modified. Three cases can be distinguished:

• orientation relaxations (materials spontaneously tend to become "disoriented");

• post-crystallization (for semi-crystalline polymers that are incompletely crystallized at the outset);

• structural relaxation in glassy amorphous...