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.