Microscopic dynamics of glass-forming polymers

The microscopic dynamics of glass-forming materials, especially polymers, is a topic of strong current interest and often heated debate. In this review an overview of the most common dynamical phenomena in glass-forming polymers will be presented: α relaxation, β relaxation, 'boson peak', and 'fast process' (β fast relaxation). The experimental results presented will be mainly from inelastic and quasielastic neutron scattering. Several mostly unanswered questions arise from the experimental findings, e.g.: Is the α relaxation of heterogeneous or homogeneous origin? Is there any relation between the ubiquitous low-temperature vibrational properties of glasses and the α relaxation? Different theoretical and model approaches will be mentioned, e.g. mode-coupling theory, the coupling model, the vibration–relaxation model. The limitations of these attempts will be demonstrated leading to the conclusion that currently there is no theory describing the microscopic dynamics of polymers completely over the whole temperature range from the glassy to the liquid (rubbery) state. Nevertheless, for certain temperature ranges or individual phenomena, especially the β relaxation, successful concepts could be developed.