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.

Introduction

Glasses are amorphous materials that combine the mechanical strength of crystals and the spatial uniformity of liquids. Compared to crystals, glasses are more easily fabricated to be spatially homogeneous from macroscopic to nearly molecular dimensions. Such spatial uniformity is the basis for their wide applications in optics and contributes to the superior strength of metallic glasses [1], [2]. While better-known glasses are inorganic and polymeric, organic glasses of relatively low molecular weights (“molecular glasses”) are being explored for applications in drug delivery [3], [4], [5], bio-preservation [6], [7], and organic electronics [8], [9]. Pharmaceutical scientists take advantage of the high solubility of amorphous solids for the delivery of poorly soluble drugs.

An important subject in the study of molecular glasses is physical stability [5], [10], [11]. Glasses are non-equilibrium solids formed by cooling liquids, condensing vapors, or evaporating solutions while avoiding crystallization. A common feature of these processes is the kinetic arrest of a fluid structure. In glass formation by liquid cooling (Fig. 1), molecular motions slow down with falling temperature and eventually the system falls out of equilibrium at the so-called glass transition temperature T_g, forming a solid glass. Glasses are thermodynamically driven to crystallize and to “age” toward the equilibrium liquid state, both processes leading to changes in structure, properties, and performance.

A recent progress in understanding glass stability is the finding that molecular glasses have extremely high surface mobility and this property causes problems of poor stability and paradoxically, provides a tool for preparing glasses with vastly improved stability. Here we discuss this recent progress. Section 2 reviews recent measurements of surface diffusion on molecular glasses. Section 3 discusses the role of surface mobility in the physical stability of molecular glasses. We show that surface mobility is directly responsible for fast crystal growth on free surfaces, and may be involved in bulk crystal growth through the creation of voids and free surfaces. In vapor deposition, surface mobility allows efficient equilibration of newly deposited molecules and the formation of stable glasses with exceptionally low energy and high density. We also consider the methods for stabilizing molecular glasses against surface-facilitated transformations.