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.