o-Terphenyl Self-Diffusion Near the Glass Transition Temperature

Self-diffusion coefficients (D$_{T}$) have been obtained for the fragile glassformer o-terphenyl from the glass transition temperature (T$_{g}$= 243 K) to T$_{g}$+ 32 K. Compared to the predictions of the Stokes-Einstein equation, we observe substantially enhanced translational diffusion ( $>$ 2 decades), which is a strong indicator of spatially heterogeneous dynamics. The values of D$_{T}$ are measured by isothermally annealing deuterio and protio o-terphenyl thin films. Samples are prepared as vapor-deposited glassy bilayers (100 - 1000 nm) with an initially sharp interface. When a sample is annealed, diffusion blurs the interface while supercooled o-terphenyl vaporizes at the film's surface. A mass spectrometer records deuterio and protio o-terphenyl concentrations over time, providing a profile of the diffusion that has occurred in the film. Isotope and thermal history effects on D$_{T}$ have also been investigated. Our results qualitatively agree with dynamic facilitation models and random first order transition theory.