Surface mobility of molecular glasses and its importance in physical stability

Amorphous molecular materials (molecular glasses) are useful for drug delivery, bio-preservation and organic electronics. A central issue in developing amorphous materials is the stability against crystallization and other transformations that can compromise material performance. We review recent progress in understanding the stability of molecular glasses, particularly the role for surface mobility. Surface diffusion in molecular glasses can be vastly faster than bulk diffusion. This high surface mobility enables fast crystal growth on the free surface. In this process, surface crystals grow upward and laterally, with the lateral growth rate being roughly proportional to surface diffusivity. Surface mobility also influences bulk crystal growth as the process can create fracture and free surfaces. During vapor deposition, surface mobility allows efficient equilibration of newly deposited molecules, producing low-energy, high-density glasses that are equivalent to liquid-cooled glasses aged for thousands of years. Free surfaces can accelerate chemical degradation of proteins. Measures for inhibiting surface-facilitated transformations include minimizing free surfaces, applying surface coatings, and preventing fracture.