The role of the free surface state in the formation of simulated physically vapor deposited glasses

Glass films created using physical vapor deposition (PVD) have been recently shown to exhibit enhanced stability properties as compared to ordinary glasses formed via standard liquid-quenching. However, the mechanism behind the formation of PVD stable glasses has yet to be fully uncovered. In this work, we use molecular dynamics simulations to study a coarse-grained model glass-former based on the molecule 9-(3,5-di(naphthalen-1-yl)phenyl)anthracene (α,α-A), within which we vary the strength of the dihedral potential on the respective side-groups to systematically study the effects of intramolecular degrees of freedom on molecular packing. By studying the PVD process as it is occurring, we've observed a denser, lower-energy free surface state that gradually expands into its bulk-like state as the film is formed. By looking at PVD glass formation with this new perspective, we can gain insight into the role of the free surface, the substrate, and the effect each has on molecular packing and mobility, both in and out of plane.