Crystallization of Organic Glasses: How Does Liquid Flow Damage Surface Crystal Growth?

Abstract

Organic glasses can grow crystals much faster on a free surface than in the interior, a result of the high mobility of surface molecules. A puzzling property of this process is that it is active in the glassy state, but disrupted as the glass is heated above the glass transition temperature Tg to become a fluid, despite the large increase of mobility. To understand this phenomenon, high-resolution microscopy was used to observe surface crystal growth in amorphous indomethacin (IMC) in two polymorphs (α and γ). α-IMC represents the general case of strong disruption of surface growth by fluidity, while γ-IMC represents the opposite situation where the effect is weak. We observed that heating above Tg causes liquid flow toward the crystals of both polymorphs. The essential difference between the polymorphs is that γ-IMC grows as compact domains and its uniformly advancing growth front is unperturbed by the onset of liquid flow. In contrast, α-IMC grows segregated needles and liquid flow strongly alters the c...