Heterogeneities at short and long time scales of the dynamics of a probe molecule in different model glass formers

The correlations, namely dynamic heterogeneity, and different relaxation time and length scales were investigated in two model glass formers. Decoupling phenomena were observed on the transport coefficients, which would correspond to different averages over these heterogeneities. The temperature dependence of the rotational dynamics of a nearly cylindrical molecular tracer in phenyl salicylate (SALOL) was investigated over the short time scale of the Longitudinal Detected Electron Spin Resonance spectroscopy (LODESR). A comparison with the dynamics of the same probe dissolved in either o-terphenyl (OTP) or SALOL is also reported on the short and long time scales of the rotational relaxation, the latter being provided by linear Electron Spin Resonance (ESR) studies. The rotational dynamics, at short and long times, exhibited a similar behavior in both glass formers with exponential relaxation and coupling to the viscous flow at higher temperatures, and with selective decouplings on cooling. Interestingly, by proper scaling, the dynamics at long times and the viscosity of the host matrices collapse in a single curve in a wide temperature range. The scaling does not work at short times. (c) 2006 Elsevier B.V. All rights reserved