Crystal nucleation rates in glass-forming molecular liquids: D-sorbitol, D-arabitol, D-xylitol, and glycerol.

The rate of crystal nucleation has been measured in four glass-forming molecular liquids: D-sorbitol, D-arabitol, D-xylitol, and glycerol. These polyalcohols have similar rates of crystal growth when compared at the same temperature relative to Tg (the glass transition temperature), peaking near 1.4 Tg, while the nucleation rates J are vastly different. In D-sorbitol and D-arabitol, J reaches a maximum of ∼108 m-3 s-1 near 1.1 Tg, whereas J < 10-2 m-3 s-1 in D-xylitol and <1 m-3 s-1 in glycerol based on no nucleation in large samples after long waits. This confirms the fundamentally different mechanisms for nucleation and growth. Near Tg, both nucleation and growth slow down with a similar temperature dependence, suggesting a similar kinetic barrier for the two processes. This temperature dependence is significantly weaker than that of viscosity η, approximately following η-0.75. This indicates that viscosity is a poor representative of the kinetic barrier for nucleation, and a better choice is the crystal growth rate. Under the latter assumption, the classical nucleation theory (CNT) describes our data reasonably well, yielding σ = 0.013 J/m2 for D-sorbitol and 0.026 J/m2 for D-arabitol, where σ is the critical nucleus/liquid interfacial free energy. There is no strong indication that the CNT fails as the length scale for corporative rearrangement exceeds the size of the critical nucleus, as recently suggested for lithium disilicate.