Effects of Microstructure Formation on the Stability of Vapor Deposited Glasses

Glasses formed by physical vapor deposition (PVD) are an interesting new class of materials, exhibiting properties thought to be equivalent to those aged for thousands of years. Exerting control over the properties of PVD glasses formed with different types of glass forming molecules is now an emerging challenge. In this work, we study coarse grained models of organic glass formers containing fluorocarbon tails of increasing length, corresponding to an increased tendency to form microstructures. We use simulated PVD to examine how the presence of the microphase separated domains influences the ability to form stable glasses. This model suggests that increasing molecule tail length results in decreased kinetic stability and a shift towards out of plane orientation of the molecules in PVD films. We find that the relaxation time near the surface of ordinary glass films formed by these molecules remains essentially bulk-like, and the surface diffusion is markedly reduced due to a trapping mechanism where the tails are unable to move between local phase separated domains on our simulation time scales. Together, these results are consistent with theories which suggest surface mobility is the driving force behind enhanced PVD glass properties.