Strong elasticity anisotropy in molecular glasses*

Although glass is often considered isotropic, anisotropic glasses can be prepared by physical vapor deposition of rod-shaped organic molecules. Those glasses feature tunable molecular orientations and anisotropic properties that are beneficial for applications such as organic semiconductors. While the deposition-condition-dependent molecular orientation and the associated optical birefringence of molecular glasses have been investigated, little is known about their mechanical anisotropy, which, however, is needed for understanding their strength upon mechanical loading in packaging and application. In this work, we use micro-Brillouin light spectroscopy to determine the elastic stiffness tensors of three glass films of itraconazole vapor-deposited at substrate temperatures (T_sub) of 330, 315, and 290 K, respectively. The five independent elastic constants in each tensor demonstrate the strong influence of the molecular orientation on the elastic anisotropy. The in- and out-of-plane Young’s moduli of the high T_sub sample, which features a vertical molecular orientation, exhibit a record high anisotropy ratio of 2.1 among molecular systems.