Dewetting of Thin Films of a Molecular Glass Well Below its Glass Transition Temperature

摘要

Enhanced mobility below the glass transition temperature (T_g) has been observed polymer glasses as well as small molecule glasses. The existence of the mobile surface layer in these glasses has a strong effect on thermodynamical and mechanical properties of these glasses in nanometer length scales. Most studies focused on the existence, and the effect of this layer on structural properties, have been performed in thin, entangled polymer films. This is primarily due to the fact that these films due to their high viscosity are stable and resist dewetting above T_g for long enough periods of time for most characterization studies. The emerging application of small molecule glasses in organic electronics and the ability to produce exceptionally stable glasses using physical vapor deposition has motivated researchers to study these effects in thin films of small organic molecules. However, it is increasingly more challenging to perform structural characterization studies on these materials due to instability above T_g. In this work, we take advantage of the dewetting phenomena under various conditions to elucidate the effect of mobile layer on the properties of thin films and on the formation of exceptionally stable glasses. We show that dewetting in thin-spin coated film is mostly by nucleation and growth. The hole growth can be used to estimate viscosity below Tg as a function of film thickness. These studies show enhanced dynamics in thin films up to 10nm. In contrast, during vapor deposition the films are unstable and mobile even during the deposition. A competition between dewetting and deposition results in spinodal dewetting patterns that elucidate the evolution of thin films into exceptionally stable glasses.