Glass Transition and Relaxation Phenomena

Glass-forming liquids of different nature, including polymers, exhibit a variety of relaxation processes, ranging from the main α relaxation process, with pronounced temperature dependence, to mildly activated secondary relaxations and vibrations. The present chapter, after providing an overview on the nature of these processes, aims to unveil how they trigger the transformation of a supercooled liquid into a glass, the so-called glass transition or vitrification, and the subsequent evolution of the so-formed non-equilibrium glass, addressed as physical aging. Firstly, the paradigmatic view, exclusively relating vitrification and physical aging to the α relaxation, is presented. The last part of the chapter is dedicated to the discussion of recent findings rather pointing toward complex behavior of both vitrification and physical aging, where the role of non- α relaxation processes is demonstrated in a variety of experimental conditions and glass formers. The new picture paves the way to a revisited comprehension of both non-equilibrium phenomena in glasses as well as the equilibrium thermodynamics, where the existence of multiple free energy minima must be accounted for.