The role of intramolecular relaxations on the structure and stability of vapor-deposited glasses

Abstract

Stable glasses (SGs) are formed through surface-mediated equilibration (SME) during physical vapor deposition (PVD). Unlike intermolecular interactions, the role of intramolecular degrees of freedom in this process remain unexplored. Here, using experiments and coarse-grained molecular dynamics simulations, we demonstrate that varying dihedral rotation barriers of even a single bond, in otherwise isomeric molecules, can strongly influence the structure and stability of PVD glasses. These effects arise from variations in the degree of surface mobility, mobility gradients, and mobility anisotropy, at a given deposition temperature (Tdep) At high Tdep, flexible molecules have access to more configurations, which enhances the rate of SME, forming isotropic SGs. At low Tdep, stability is achieved by out of equilibrium aging of the surface layer. Here, the poor packing of rigid molecules enhances the rate of surface-mediated aging (SMA), producing stable glasses with layered structures in a broad range of Tdep. In contrast, the dynamics of flexible molecules couple more efficiently to the glass layers underneath, resulting in reduced mobility and weaker mobility gradients, producing unstable glasses. Independent of stability, the flattened shape of flexible molecules can also promote in-plane orientation orderat low Tdep. These results indicate that small changes in intramolecular relaxation barriers can be used as an approach to independently tune the structure and mobility profiles of the surface layer and thus the stability and structure of PVD glasses.

Role of Molecular Layering in the Enhanced Mechanical Properties of Stable Glasses

“…Although the specific origins of molecular layering are unknown, our previous studies in a spherically shaped molecular glass, 9-(3,5-di(naphthalen-1-yl)phenyl)anthracene (α,α−A), have shown a strong correlation between this feature and an increased index of refraction in the direction normal to the film surface (vertical direction). 14,25 As such, we have interpreted this feature to be indicative of the tighter packing of molecules in this direction. Furthermore, our recent experimental data suggests that layering likely emerges due to an accelerated rate of aging of the near surface.…”

“…This is consistent with the fact that deposition below T g yields more stable glasses with higher density. 25,37 The difference in maximum load between different samples in Figure 2A,C is to maintain a maximum depth of less than 10% of the total film thickness, which was slightly different between these two samples. This maximum indentation range was chosen to avoid the influence of the substrate on the measurements.…”

“…Furthermore, our recent experimental data suggests that layering likely emerges due to an accelerated rate of aging of the near surface. 25 As the surface molecules fall out of equilibrium upon further deposition, they can still age rapidly 27 while being constrained by the rigid out-of-equilibrium regions of the film. The influence of the free surface structure in templating anisotropy is also evident in the vapor deposition of liquid-crystalline (LC) molecules, which can adopt highly tunable LC phases that are not observed in liquid-quenched states.…”

“…32 In a recent study, we demonstrated that intramolecular degrees of freedom, such as barriers for dihedral relaxations, can modify the equilibrium shape of a molecule, as well as the stability of their PVD glasses. 25,37 The specific roles of stability and anisotropic structure in the mechanical properties of vapor-deposited glasses remain understudied. Previously, Brillouin light scattering measurements showed higher elastic moduli and sound velocity in SGs than in liquid-quenched glasses 12,16,38 and indicated anisotropy in these properties.…”

“…We have recently shown that there is a significant difference in the structure and stability of vapor-deposited films of these two molecules stemming from the differences in their molecular shape and flexibility, which affects the details of their free surface equilibration rate at low T dep . 25,37 To understand this phenomenon, in Figure 4 we plot hardness alongside the relative density change (Δρ) and normalized layering peak intensity, as reported in our recent publication. 25 We first focus on the high-T dep region (0.95T g < T dep < T g , highlighted in orange).…”