Scaling between the rotational diffusion of tracers and the relaxation of polymers and glass formers

The rotational dynamics of subnanometric molecular tracers in glass formers with low molecular weight and polymers is discussed by presenting electron spin-resonance and time-resolved fluorescence studies. It is shown that th e reorientation of the tracer is fully or partially coupled to the host rel axation processes. The degree of coupling is well described by the fraction al scaling tau alpha tau(i)(xi i) between the correlation time tau of the t racer and the host timescales tau(i). The crossovers between the different regimes are sharp. Remarkably, one crossover occurs systematically very clo se to the alpha-beta splitting point of the dielectric relaxation or the cr itical temperature predicted by the mode-coupling theory of the glass trans ition.