3. Aging phenomena

Aging is defined as the slow, irreversible alteration of a material under service conditions, involving its structure, morphology or composition, and leading to a loss of its functional properties. These changes may be due to instability inherent in the material or its environment. There are two main types of aging to consider:

• physical aging in the broadest sense, in which there is no alteration of the material's chemical structure, and polymer chains in particular maintain their integrity. In principle, this type of aging is reversible. A distinction can be made between mechanisms involving temperature alone, i.e. structural relaxation, which consists in densification of the amorphous phase when the polymer is kept at a temperature below T _g and recrystallization phenomena (cold crystallization)...