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-C₂₄₆H₄₉₄ 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 (G₁₀₀ and G₁₁₀), as well as the rates of secondary nucleation i₁₀₀ and layer propagation v₁₀₀ on {100} faces. With decreasing T_c, 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 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 C₂₄₆H₄₉₄-n-octacosane solvent exhibits classical behavior, reasonably well described by the Flory-Huggins theory.