Physical aging and molecular mobility of amorphous polymers

The classical approaches of the slow dynamics of liquid glass formers and glasses are recalled. Actually, it is well acknowledged that several features are common to glasses and glass forming liquids. For example, the quasi universal occurrence of secondary β slow (or so-called Johari–Goldstein) relaxation and its correlation with the α relaxation process has to be included in any physical model. This ingredient is now introduced in the coupling model proposed by Ngai. However, recent experimental data, mainly dealing with physical aging, show that some aspects remain unsolved and many questions are still to be answered.