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

An in situ study of solution crystallization kinetics and morphology of the long alkane n -C 246 H 494 has been performed by interference optical microscopy using a T -jump hot stage. The crystallization temperatures ( T c ) 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 T c is lowered toward the extended- to folded-chain transition. Crystal shapes were fitted to the Mansfield ellipse and T c dependencies of the following parameters were determined: growth rates of {100} and {110} faces ( G 100 and G 110 ), as well as the rates of secondary nucleation i 100 and layer propagation v 100 on {100} faces. With decreasing T c, all four rates first pass through a maximum and then a minimum at the transition. G 100 is more retarded by self-poisoning than G 110 . Furthermore, at the minimum i 100 is impeded significantly more compared to v 100, resulting in straight {100} facets. The fact that v 100 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 T c correspond to similar albeit less drastic changes which occur in polyethylene with increasing T c . It is thus suggested that self-poisoning also operates in polyethylene, and increasingly so at higher temperatures. Finally, the binary phase diagram C 246 H 494 − n -octacosane solvent exhibits classical behavior, reasonably well described by the Flory−Huggins theory.