Self-diffusion of supercooled tris-naphthylbenzene.

We have measured self-diffusion coefficients for deeply supercooled tris-naphthylbenzene using secondary ion mass spectrometry. Isotopically labeled multilayer films were prepared by vapor deposition. For samples deposited within a few K of T(g), the evolution of the concentration profile was observed to be Fickian on all accessible length and time scales. Diffusion is enhanced by a factor of approximately 100 at 338 K when compared to the prediction of the Stokes-Einstein model. In combination with previous neutron reflectivity experiments, these measurements quantify the length scale at which dynamics cross over to non-Fickian behavior in supercooled TNB. Comparisons are made between this diffusion data and other measurements of dynamics in supercooled TNB, including probe diffusion and crystal growth rates. An earlier report of self-diffusion coefficients for TNB was in error because the samples were prepared by vapor deposition at T(g) - 50 K, creating very stable glasses in which the concentration profile does not evolve by Fickian diffusion.