Transformation of Stable Glasses into Supercooled Liquids: Growth Fronts and Anomalously Fast Liquid Diffusion

Physical vapor deposition onto substrates near 0.85T(g) can prepare organic glasses with low enthalpy, high density, and high thermal stability. Isotopically labeled multilayer films of tris(naphthyl)benzene and indomethacin stable glasses were prepared and secondary ion mass spectrometry was used to study the evolution of these materials upon heating above T-g. In contrast to ordinary glasses, when stable glasses are held above Tg they transform to a liquid via a growth from mechanism. In these experiments, growth fronts are initiated at the free surface of the glass and in some cases at the glass/substrate interface or an internal interface in the glass. For tris(naphthyl)benzene, the velocity of this growth front is observed to be nearly independent of the stability of the glass. Diffusion in the liquid that results from the growth front is initially 2-5 times faster than for the equilibrium supercooled liquid at the same temperature; the nature of this liquid is unclear. Under some circumstances, the slow evolution of this unusually mobile liquid into the equilibrium supercooled liquid can be observed.