Exploring The Relationship Between Stability and Anisotropy in Vapor Deposited Stable Glasses

Glasses prepared through physical vapor deposition (PVD) can adopt preferred molecular orientations and exhibit strong stability as compared to the liquid-quenched materials. The extent of this stability depends on deposition conditions, molecular geometry, and film thickness. In this work, we investigate the appearance of anisotropic molecular orientation in PVD glasses of 9-(3,5-Di(naphthalen-1-yl)phenyl)anthracene (αα-A) as a function of film thickness and deposition temperature. Spectroscopic ellipsometry reveal that thin films of αα-Α have a higher density and stronger birefringence than the bulk. Further, grazing-incidence wide-angle X-ray scattering measurements show a pronounced orientational anisotropy of the intramolecular motifs, in thin films of αα-A, particularly at low deposition temperatures. A strong layering feature is also seen at intermediate deposition temperatures, which is stronger in thin films than in bulk. We will discuss the relation of the increased stability of thin films (30-100 nm) as compared to bulk films (≥ 200 nm) to the measured structural anisotropy. We discuss how this enhanced anisotropy at low temperatures may arise from thermodynamic and kinetic factors that influence molecular packing during deposition.