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

摘要

The translational diffusion coefficient D-trans for rubrene, 9,10-bis(phenylethynyl)anthracene (BPEA), and tetracene in the fragile molecular glass-former sucrose benzoate (SB) (T-g=337 K) was studied as a function of temperature from T-g+3 K to T-g+71 K by use of the holographic fluorescence recovery after photobleaching technique. The values of D-trans 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/eta, where eta is the shear viscosity. In going from the crossover temperature T-x approximate to 1.2T(g) to T-g, D-trans eta/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 D-trans similar to T/eta(xi), with xi=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 alpha alpha beta-tris(naphthyl)benzene in a plot of enhancement versus the relative probe size parameter rho(m)=(m(p)/m(h))(1/3), where m(p) and m(h) are, respectively, the molecular weights of the probe and host solvent. The plot clearly shows a sharp increase in enhancement of translational diffusion at rho(m)approximate to 1. By applying temperature shifts, D-trans for probe diffusion in SB and the dielectric relaxation time tau(D) 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 D-trans similar to 1/tau(D)(T+Delta T), where tau(D)(T+Delta T) is the temperature-shifted dielectric relaxation time. The results from this study are discussed within the context of dynamic heterogeneity in glass-forming liquids. (c) 2007 American Institute of Physics.