Crystallization near Glass Transition: Transition from Diffusion-Controlled to Diffusionless Crystal Growth Studied with Seven Polymorphs

Abstract

A remarkable property of certain glass-forming liquids is that a fast mode of crystal growth is activated near the glass transition temperature T_g and continues in the glassy state. This growth mode, termed GC (glass-crystal), is so fast that it is not limited by molecular diffusion in the bulk liquid. We have studied the GC mode by growing seven polymorphs from the liquid of ROY, currently the top system for the number of coexisting polymorphs of known structures. Some polymorphs did not show GC growth, while others did, with the latter having higher density and more isotropic molecular packing. The polymorphs not showing GC growth grew as compact spherulites at all temperatures; their growth rates near T_g decreased smoothly with falling temperature. The polymorphs showing GC growth changed growth morphologies with temperature, from faceted single crystals near the melting points, to fiber-like crystals near T_g, and to compact spherulites in the GC mode; in the GC mode, they grew at rates 3−4 orders of magnitude faster with activation energies 2-fold smaller than the polymorphs not showing GC growth. The GC mode had rates and activation energies similar to those of a polymorphic transformation observed near T_g. The GC mode was disrupted by the onset of the liquid's structural relaxation but could persist well above T_g (up to 1.15 T_g) in the form of fast-growing fibers. We consider various explanations for the GC mode and suggest that it is solid-state transformation enabled by local molecular motions native to the glassy state and disrupted by the liquid's structural relaxation (the α process).