Investigating The Thermal And Kinetic Stability Of Thin Films Of Molecular Glasses
Stable Glasses (SGs) are very dense glasses with remarkable thermodynamic and kinetic stability, that are made by the process of Physical Vapor Deposition (PVD). These glasses are made directly into a low-energy state via surface-mediated equilibration (SME), during PVD. In this work, we are investigating the kinetic and thermodynamic stability of vapor-deposited films of N,N’-Bis(3-methylphenyl)-N,N’-diphenylbenzidine (TPD) of different thicknesses and substrate temperatures. We produced SGs of bulk (>200nm) TPD films with a broad range of stability and measured the density change of the films, which is a measure of the thermodynamic stability, upon thermal annealing. To study the kinetics of as-deposited glasses, we have optimized the process of solvent vapor annealing (SVA). When the as-deposited films of TPD are solvent annealed with toluene under the right conditions, a solvent front, analogous to a propagating thermal front, can be produced and measured by in-situ spectroscopic ellipsometry. The speed of the solvent front, limited by the relaxation of the medium, gave an indirect estimate of the kinetics of the film. For bulk films of TPD SGs, we observed the strong dependence of the thermodynamic stability on substrate temperatures. The kinetics of the bulk SG films on the other hand, depend not only on the substrate temperature but also on other factors such as the birefrigence of the films and mobility of the solvent front. The thermodynamics and kinetic stability of as-deposited SG films point towards the existence of two regimes in the formation of the SGs. In very thin films (<60nm), the density of the as-deposited films exceeds that of the supercooled liquid (SCL) line and follows a new SCL line, at lower temperatures. The transition from the high temperature SCL, to the low temperature SCL happens within a narrow range of substrate temperatures, where the kinetics of the films changes drastically by almost to an order of magnitude. This suggests the existence of a distinct phase that exists in very thin as-deposited films, with a liquid-liquid phase transition, below the glass transition, to the low temperature SCL.