  Glasses formed by physical vapor deposition (PVD) are an interesting new
class of materials, exhibiting properties thought to be equivalent to those of
glasses aged for thousands of years. Exerting control over the structure and
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
in the supercooled liquid influences the ability to form stable glasses. This
model suggests that increasing molecule tail length results in decreased
thermodynamic and kinetic stability of the molecules in PVD films. The reduced
stability is further linked to the reduced ability of these molecules to
equilibrate at the free surface during PVD. We find that as the tail length is
increased, the relaxation time near the surface of the supercooled equilibrium
liquid films of these molecules are slowed and become essentially bulk-like,
due to the segregation of the fluorocarbon tails to the free surface. Surface
diffusion is also markedly reduced due to clustering of the molecules at the
surface. Based on these results, we propose a trapping mechanism where tails
are unable to move between local phase separated domains on the relevant
deposition time scales.
