Relaxation in the glass transition regime interpreted in terms of the aggregate model

Abstract

The retardation time spectra in the glass transition range are discussed on hand of multidimensional deuteron NMR experiments, measurements of the dielectric loss and the T₁-relaxation of proton spins in liquids at different temperatures and different frequencies. The data can be described by applying the model of reversible aggregation. Every liquid is here characterized by a broad aggregate size distribution. Relaxation is interpreted as collective response of intra-aggregate Debye processes running off while the aggregate ensemble fluctuates. The relaxation time increases with the volume of the aggregate so that every relaxation time distribution is strictly related to the aggregate size distribution. The shape of relaxation mode distributions is similar, the width depends on how the molecular mechanisms couple to the stationary dynamics. The coupling changes on heating so that the width of the relaxation mode distribution decreases. A full description of the α-process needs to account for aggregate ensemble fluctuations, a bimodal relaxation mode spectrum and a heterogeneous kinetics.