Effects of internal degrees of freedom on simulated vapor deposited glass films

Recently, amorphous films of small molecules formed via physical vapor deposition (PVD) have been shown to exhibit remarkable thermodynamic and kinetic properties, equivalent to those that have been aged on the order of hundreds of years. Previous results suggest that the stability differences observed in these PVD glasses are the result of enhanced mobility near the free surface, such that molecules are allowed to sample a greater number of configurations before being trapped by subsequent deposited layers. However, much remains to be learned about the effects of properties like molecular shape and rotational freedom on the stability and morphology of these PVD amorphous packings. By using molecular dynamics simulations to mimic the PVD process of coarse-grained small molecules, we can make fine-tuned changes to these properties and then closely observe changes in films, both during and after the deposition process. Notably, we can then potentially make a connection between changes in internal degrees of freedom and the corresponding molecular entropy and changes in PVD film properties. Thus far, we suggest that molecules exhibiting greater entropy in their supercooled liquid state create more stable PVD glass films.