Translational diffusion in sucrose benzoate near the glass transition: Probe size dependence in the breakdown of the Stokes-Einstein equation

The translational diffusion coefficient Dtrans for rubrene, 9,10-bis(phenylethynyl)anthracene (BPEA), and tetracene in the fragile molecular glass-former sucrose benzoate (SB) (Tg =337 K) was studied as a function of temperature from Tg +3 K to Tg +71 K by use of the holographic fluorescence recovery after photobleaching technique. The values of Dtrans vary by five to six orders of magnitude in this temperature range. Contrary to the predictions of the Stokes-Einstein equation, the temperature dependence of probe diffusion in SB over the temperature range of the measurements is weaker than that of T, where is the shear viscosity. In going from the crossover temperature Tx ≈1.2 Tg to Tg, Dtrans T increases by factors of 2.4±0.2 decades for rubrene, 3.4±0.2 decades for BPEA, and 3.8±0.4 decades for tetracene. The decoupling between probe diffusion in SB and viscosity is characterized by the scaling law Dtrans ∼T , with =0.621 for tetracene, 0.654 for BPEA, and 0.722 for rubrene. Data for probe diffusion in SB are combined with data from the literature for probe diffusion in ortho-terphenyl and ααß -tris(naphthyl)benzene in a plot of enhancement versus the relative probe size parameter ρ m = (mp mh) 13, where mp and mh are, respectively, the molecular weights of the probe and host solvent. The plot clearly shows a sharp increase in enhancement of translational diffusion at ρ m ≈1. By applying temperature shifts, Dtrans for probe diffusion in SB and the dielectric relaxation time τD can be superimposed on a single master curve based on the Williams-Landel-Ferry equation. This suggests that the dynamics of probe diffusion in SB is described by the scaling relationship Dtrans ∼1 τD (T+ΔT), where τD (T+ΔT) is the temperature-shifted dielectric relaxation time. The results from this study are discussed within the context of dynamic heterogeneity in glass-forming liquids.