Effect of Small Molecular Additives on Growth Rates of Molecular Crystals from the Melt near Glass-Transition Temperature.

Frequently, small concentrations of additives can substantially modify crystal growth rates from solutions, while a substantial concentration of additives is required to likewise affect crystallization from the melt. Recently, it was discovered that even 1 wt % of polymeric additive can significantly either increase or decrease growth rates of molecular crystals depending on the differences between glass-transition temperatures T g of host crystals and polymers [C. Huang et al., J. Phys. Chem. B 2017, 121, 1963-1971]. A similar effect is illustrated here for small-molecule additives. It is shown that the growth rate of a molecular crystal increases if the T g of an additive is lower than that of a host crystal and growth rate decreases if the T g of an additive is higher than that of a host crystal. This effect is observed only for host crystals that can form intermolecular hydrogen bonds in the melt. We speculate that in this case, the additive can efficiently disrupt the hydrogen bond network and affect the liquid dynamics in the melt.