Surface mobility of molecular glasses and its importance in physical stability

Abstract

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.

Monitoring the recrystallisation of amorphous xylitol using Raman spectroscopy and wide-angle X-ray scattering

“…It has been shown that the surface crystallization is more rapid than bulk crystallization in amorphous glasses aged below their glass transition temperature (Zhu et al, 2010(Zhu et al, , 2008Wu et al, 2007;Wu and Yu, 2006;Yu, 2016). The reason for this can not only be found in the higher molecular mobility of surface molecules compared to bulk molecules, but also in the more pronounced structural relaxation of the surface as compared to the bulk (Hasegawa et al, 2009).…”

“…The rate of crystallization was higher for xylitol-silica dispersions measured with Raman as compared to WAXS. Bulk diffusion is promoted over surface diffusion at temperatures above T g while surface diffusion becomes instable due to viscous flow on the surface (Yu, 2016). Still, even at elevated temperatures, Raman measurements showed differences compared to WAXS that could be attributed to surface crystallization, since the measurement geometry of the Raman measurements favors detection of surface events.…”

“…In the present study, increasing the temperature caused the rate of crystallization to increase as measured with Raman. Increasing the temperature increases molecular motion and reduces viscosity, which both favor crystallization (Bhugra and Pikal, 2008;Yu, 2016). When the temperature was increased from T = 21 C to T = 29 C, the difference in rate of crystallization between pure xylitol and xylitol-silica dispersion became less significant.…”

“…In the present study, solid xylitol was molten and rapidly re-solidified in liquid nitrogen at a temperature approximately 170 K below Tg. When a melt is rapidly cooled to temperatures significantly below Tg, the molecular motion rapidly decreases and the system reaches a nonequilibrium state (Yu, 2016). Below Tg, recrystallization of the amorphous solid is emphasized on the particle surfaces.…”

“…The reason for this can not only be found in the higher molecular mobility of surface molecules compared to bulk molecules, but also in the more pronounced structural relaxation of the surface as compared to the bulk (Hasegawa et al, 2009). When the temperature exceeds the glass transition temperature, the viscosity of the sample decreases, and surface diffusion of molecules is disturbed by viscous flow of the material (Yu, 2016). This interrupts surface crystal growth, and as bulk diffusion is simultaneously promoted the difference between the rate of surface and bulk crystallization is expected to become smaller.…”