In Situ Solution Crystallization Study ofn-C246H494: Self-Poisoning and Morphology of Polymethylene Crystals

An in situ study of solution crystallization kinetics and morphology of the long alkane n-C246H494 has been performed by interference optical microscopy using a T-jump hot stage. The crystallization temperatures (Tc) covered the full range of extended-chain and part of the once-folded chain range. A drastic change in crystal habit, from broad curved-faced “truncated lozenge” to needle-shape, occurs as Tc is lowered toward the extended- to folded-chain transition. Crystal shapes were fitted to the Mansfield ellipse and Tc dependencies of the following parameters were determined:  growth rates of {100} and {110} faces (G100 and G110), as well as the rates of secondary nucleation i100 and layer propagation v100 on {100} faces. With decreasing Tc, all four rates first pass through a maximum and then a minimum at the transition. G100 is more retarded by self-poisoning than G110. Furthermore, at the minimum i100 is impeded significantly more compared to v100, resulting in straight {100} facets. The fact that v100 passes through a minimum is the first evidence that the barrier to layer propagation (“substrate completion”) is entropic rather than surface energy based. Significantly, the changes in crystal habit with decreasing Tc correspond to similar albeit less drastic changes which occur in polyethylene with increasing Tc. It is thus suggested that self-poisoning also operates in polyethylene, and increasingly so at higher temperatures. Finally, the binary phase diagram C246H494−n-octacosane solvent exhibits classical behavior, reasonably well described by the Flory−Huggins theory.