Diffusion-controlled and diffusionless crystal growth in liquid o-terphenyl near its glass transition temperature

Abstract

o-terphenyl is one of the organic liquids in which a fast mode of crystal growth is activated near the glass transition temperature T(g) and continues deep in the glassy state. This growth mode, termed glass-crystal (GC), is not limited by molecular diffusion in the bulk liquid, in contrast to the diffusion-controlled growth at higher temperatures. The GC mode has been previously described as abruptly emerging near T(g) and having a constant growth rate at a fixed temperature, two features important for testing its various explanations. We report here that the GC mode already exists in the equilibrium liquid of o-terphenyl up to 1.15T(g) (T(g)=246 K) in the form of loose, fast-growing fibers and that its growth rate is constant at T(g)+2 K, but decreases by 30% in 10 h at T(g)-13 K, during which time the glass' fictive temperature decreases by 6 K. The slow down of GC growth becomes less noticeable over time so that fast growth is still observable after long annealing. The fiber growth, similar to the fully activated GC growth that yields compact spherulites, is also not limited by bulk diffusion. Crystal growth in the GC mode has a comparable activation energy as liquid desorption but a much faster rate, properties in common with polymorphic conversions. The time dependence of GC growth is not readily explained by the effect of physical aging on the thermodynamic driving force of crystallization, the liquid desorption, the primary structural relaxation, or a secondary relaxation. The secondary dielectric relaxation observed by dielectric spectroscopy in glassy o-terphenyl disappears too quickly for its molecular motions to be responsible for GC growth.

Stability of Amorphous Pharmaceutical Solids: Crystal Growth Mechanisms and Effect of Polymer Additives

“…After the temperature is returned to 267 K, new GC growth was initiated predominately on the tips of the actively growing, far-reaching fibers. Xi et al reported a similar observation for GC growth in OTP (30).…”

Fast Surface Crystallization of Amorphous Griseofulvin Below T g

“…Thus, as previously postulated, negligible bulk molecular motion at temperatures well below T g cannot guarantee the stability of amorphous solids. Moreover, in addition to surface crystallization, diffusionless glassy crystal growth has also been documented in several organic systems near and below T g (20)(21)(22). These modes of crystallization cause a sudden increase in crystallization rate near or below T g compared to the crystallization rate from the supercooled liquid.…”

Crystallization of Organic Glasses: Effects of Polymer Additives on Bulk and Surface Crystal Growth in Amorphous Nifedipine

“…One such growth mode (the glass-to-crystal or GC mode) exists in the bulk and can cause an increase of crystal growth rate by orders of magnitude with a temperature drop by a few K (6)(7)(8)(9)(10). Another growth mode occurs at the surface and can lead to crystal growth substantially faster at the surface than in the interior (11)(12)(13)(14); this surface growth mode can be halted by a coating only a few nm thick (12).…”