The Role of Intramolecular Relaxation in the Stability, Optical Birefringence, and Structural Anisotropy of Vapor Deposited Glasses*

Stable glasses (SGs) made by physical vapor deposition are characterized by improved density and kinetic stability compared to liquid-quenched glasses. Additionally, SGs are usually shown to be optically birefringent and structurally anisotropic. However, the relationship between molecular structure and SG properties remains obscured. Here, we choose two molecules: α,α-A and α,α-Phen, which differ by a substituent, anthracenyl (A) versus phenanthrenyl (Phen), on the 3,5-di(naphthalen-1-yl)benzene moiety (diarylbenzene). The A–diarylbenzene bond presents a higher rotational barrier than the Phen–diarylbenzene bond, making α,α-A molecule more rigid than α,α-Phen. We examine the stability, optical birefringence, and packing anisotropy of SGs formed by the two molecules as a function of deposition temperature, using Ellipsometry and Grazing Incidence Wide Angle X-ray Scattering. It is observed that α,α-A SGs, compared to α,α-Phen SGs, are more birefringent, with a larger anisotropic "layering peak" in the direction perpendicular to the substrate. These effects can be attributed to the α,α-A molecule's rigidity, which reduces its range of enhanced mobility at the surface and its ability to effectively pack, resulting in the layering behavior.