COOPERATIVE DYNAMICS IN RELAXATION - A COUPLING MODEL PERSPECTIVE

Relaxation of identical constituents in complex systems, such as molecules in glass-forming viscous liquids, ions in vitreous ionic conductors or polymer chains in fully entangled polymer melts, is determined by cooperative processes of motions coupled together by interactions. Both experimental data and computer simulations of model complex systems support the physical picture and quantitative predictions provided by the coupling model. In this paper, the coupling model is summarized and examples of its accomplishments are given. The cooperativity arises mainly through coupling of the individual constituents and cannot be represented by a static distribution of relaxation times. The effect of the interactions, when averaged over the heterogeneous motions of all the relaxing constituents, produces a time-dependent rate for relaxation in linear response. The situation of nonequilibrium structural relaxation, such as enthalpy and volume recovery, is also discussed where the required averaging is then more complicated due to the interplay between the changing structure and the heterogeneous nature of the relaxing constituents.