Fast Surface Crystal Growth on Molecular Glasses and Its Termination by the Onset of Fluidity

Abstract

Organic glasses can grow crystals much faster on the free surface than in the interior, a phenomenon important for fabricating stable amorphous materials. This surface process differs from and is faster than the glass-to-crystal (GC) growth mode existing in the bulk of molecular glasses. We report that similar to GC growth, surface crystal growth terminates if glasses are heated to gain fluidity. In their steady growth below the glass transition temperature T, surface crystals rise above the amorphous surface while spreading laterally and are surrounded by depressed grooves. Above T, the growth becomes slower, sometimes unstable. This damage is stronger on segregated needles (α indomethacin, nifedipine, and o-terphenyl) than on crystals growing in compact domains (γ indomethacin). This effect arises because the onset of liquid flow causes the wetting and embedding of upward-growing surface crystals. Segregated needles are at greater risk because their slow-growing flanks appear stationary relative to liquid flow at a low temperature. The disruption of surface crystal growth by fluidity supports the view that the process occurs by surface diffusion, not viscous flow. Compared to the bulk GC mode, surface crystal growth is disrupted less abruptly by fluidity. Nevertheless, to the extent that fluidity damages them, both processes are solid-state phenomena terminated in the liquid state.

Fast Surface Crystallization of Molecular Glasses: Creation of Depletion Zones by Surface Diffusion and Crystallization Flux

“…On the other hand, AFM is more accurate than SEM for measuring the height of crystals, since SEM relies on side-view observations at uncertain angles. Consistent with the previous finding, 17 we observed depletion zones around α IMC surface crystals on the initially flat glass surface. In cross-sectional views (Figure 1d), SEM shows that the depletion zone descends as it approaches the flank of an α IMC needle, approximately reaching a local minimum at the intersection.…”

“…In the case of γ IMC, the "tall crystal" problem encountered in the AFM analysis of α IMC is less severe, allowing the AFM tip to locate the crystal/glass interface. Real-time AFM also yielded the growth velocities of γ IMC surface crystals, which agree with those from optical microscopy; 17 for the crystal in Figure 4c, u = 0.9 nm/s at 317 K. As we discuss later, the depletion zones are well explained by surface diffusion near a moving interface responding to crystal growth.…”

“…The axial growth velocities from AFM agree with the reported growth velocities measured by optical microscopy. 12,13,17 Figure 2 shows the typical width w and height H of α IMC needles. Both are several hundred nanometers, and both increase slightly with the distance to the tip.…”

“…Surface crystals rise upward as they grow laterally 12 and are surrounded by depressed grooves or depletion zones that form on the initially flat amorphous surface. 17 The process of surface crystal growth is halted by a thin coating (e.g., 10 nm of gold or a few nanometers of polymer). 26 For the glasses of indomethacin and nifedipine, the velocity of surface crystal growth is proportional to the diffusivity of surface molecules independently measured through the decay of surface gratings.…”

Effect of molecular size and hydrogen bonding on three surface-facilitated processes in molecular glasses: Surface diffusion, surface crystal growth, and formation of stable glasses by vapor deposition

“…First, fast surface diffusion is responsible for fast surface crystal growth in many organic glasses. [7][8][9][10] For these systems, the surface crystal growth rate is nearly proportional to the surface diffusion coefficient. 6 A second consequence of surface mobility is the ability to prepare highly stable glasses by vapor deposition.…”