Fragility, spatially heterogeneous dynamics, and enhanced translational self-diffusion in low molecular weight glassformers

Recent measurements of self-diffusion coefficients in prototypical small molecule glassformers reveal behaviors believed to be universal for low molecular weight fragile glasses. These behaviors include enhanced translational motion and a self-diffusion controlled crystallization regime. Similar enhancements in self-diffusion relative to the Stokes-Einstein relation have been found for two glassformers of similar fragility: $o$-terphenyl with around 100 times faster self-diffusion than expected at T$_{g}$ +3 K, and \textit{tris}-naphthylbenzene, with around 400 times faster diffusion at T$_{g}$. The temperature range of enhanced motion also overlaps a range of self-diffusion controlled crystal growth. The origin of enhanced diffusion is often attributed to spatially heterogeneous dynamics thought to exist in fragile glassformers. Relaxation time distributions in the literature for $o$-terphenyl and \textit{tris}-naphthylbenzene, and their relationships with enhancement of self-diffusion, fragility, and heterogeneous dynamics are discussed.