Studying the kinetic stability of molecular glasses using solvent vapor annealing*

Vapor deposition enables access to low-energy stable glasses (SGs). Upon heating above the onset temperature of transformation (well above nominal glass transition temperature), SGs transform via thermal transformation fronts, analogous to those seen in crystals, indicating their increased kinetic stability. However, it is difficult to directly observe these fronts in SGs with low stability or in liquid quenched glasses, where the glass can transform homogeneously. Here, we perform solvent vapor annealing (SVA) as an alternative approach and demonstrate that the mechanism by which the solvent penetrates a molecular glass film depends on the solvent vapor pressure. Above a certain threshold, the solvent front moves linearly into the film (Case II diffusion). By monitoring the solvent front's velocity, we can directly compare the kinetic stability for films deposited at a wide range of temperatures, deposition rates, and with various thicknesses. The results correlate well with thermal growth fronts, when comparisons are possible, but SVA can be used on a wider range of glasses with more accuracy, as it can avoid the bulk transformation mechanisms. We discuss the variations of kinetic stability when the thermal stability and density are varied across a range of film thicknesses.