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 for time scales on the order of hundreds of years. Previous simulations suggest that the stability differences observed in these PVD glass films are the result of enhanced mobility near the free surface, such that molecules are allowed to sample a greater number of configurations before becoming trapped by subsequent deposited layers. However, much remains to be learned about the effects of properties like strong intermolecular interactions and molecular shape 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 closely observe changes in local film properties during the deposition process and make comparisons between the behavior of the deposited film surface and equilibrium liquid. Notably, for molecules containing sites with largely disparate interaction energies, we observe that surface segregation and clustering due to local phase separation cause a marked reduction of surface diffusion and inhibit relaxation near the free surface, resulting in less stable PVD glasses.

Varying molecular shape and chemical interactions in simulated vapor deposited glass films

2019 Spring National Meeting

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