Kinetic phase diagram for crystallization of bimodal mixtures of oligomers as predicted by computer simulations

Abstract

We analyze the crystallization process of mixtures of long and short oligomers such as n-alkanes using a Sadler–Gilmer-type growth algorithm. The short chains are assumed to crystallize only in a fully stretched conformation, while the long chains take a pathway over a folded state. Increasing the concentration of long chains from zero to unity, the zero-growth temperature, which separates the stable growing phase from the non-growing phase, first decreases for low concentrations of long chains, reaching a minimum value at intermediate concentrations, and increases again for higher concentrations, as a result a growth-gap appears. Here, mixing long chains into the short chain liquid stops the growth process completely until it resumes at higher concentrations of long chains. This effect is explained by the particular kinetic pathway, the long chains take into the crystalline state. Since the folded state has to be passed at first, long chains are more likely to be desorbed again. At low concentrations, the long chains thus effectively dilute the short chain liquid leading to a decrease of the zero-growth temperature. At higher concentrations, the transition into the stretched state of the long chains leads more frequently to bonded pairs, which stabilizes the growth again due to the higher binding energy among fully stretched long chains.