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

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 T g 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 T g 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 crystal/liquid interface. This effect arises because the slow-growing flanks of needle crystals are wetted and embedded by the liquid. This explanation has support from independent observations that polystyrene spheres (a model for slow-growing crystals) are embedded on the time scale of crystal growth.