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 the reorientation of the tracer is fully or partially coupled to the host relaxation processes. The degree of coupling is well described by the fractional scaling tau alpha tau(i)(xi i) between the correlation time tau of the tracer and the host timescales tau(i). The crossovers between the different regimes are sharp. Remarkably, one crossover occurs systematically very close to the alpha-beta splitting point of the dielectric relaxation or the critical temperature predicted by the mode-coupling theory of the glass transition.