THE DYNAMIC STRUCTURE MODEL AND THE DECOUPLING OF SECONDARY RELAXATIONS NEAR THE GLASS-TRANSITION

We present a microscopic formulation of the memory effect as proposed by the dynamic structure model [17,18] by considering the hopping of a charged particle among two sites in the presence of a locally rearran ging structural environment. The state of the environment of each site is described by a structural variable, which prefers to assume differ ent values if the site is occupied or vacant, and it varies in time ac cording to an overdamped Brownian dynamics. We find that the dipolar r elaxation strongly depends on how the bare jump frequency nu of the pa rticle and the characteristic structural relaxation time tau(s) of its environment relate to each other. For nu tau(s) << 1 the imaginary pa rt of the dielectric susceptibility chi''(omega) exhibits a single pea k pattern, while for nu tau(s) greater than or equal to 1 a second, sm aller peak appears at higher frequencies, which becomes more separated from the first peak with increasing nu tau(s). It is suggested that t his behaviour provides an explanation for the decoupling phenomenon of secondary beta-relaxations from the main primary alpha-relaxations cl ose to a glass transition.