Crystallization rate minima in a series of n-alkanes from C₁₉₄H₃₉₀ to C₂₉₄H₅₉₀

Melt-crystallization kinetics of long monodisperse n-alkanes with chain lengths in the region between 194 and 294 C atoms was investigated by differential scanning calorimetry (DSC) and real-time small-angle X-ray diffraction. Isothermal crystallization experiments on C₂₉₄H₅₉₀ and cooling experiments on C₁₉₄H₃₉₀, C₁₉₈H₃₉₈, C₂₁₀H₄₂₂, C₂₄₆H₄₉₄, and C₂₅₈H₅₁₈ confirmed that in all cases the crystallization rate reaches a maximum several degrees below the melting point of the extended-chain crystal form, followed by a minimum as crystallization temperature is lowered by a further few degrees. This anomalous rate effect has been observed previously for melt-crystallization of C₂₄₆H₄₉₄ and C₁₉₈H₃₉₈, and also for solution-crystallization of C₁₉₈H₃₉₈. The effect is interpreted in terms of "self-poisoning" of the crystal growth surface, i.e. obstruction to chain extension by frequent deposition of the nearly-stable chain-folded overgrowth. The present results confirm the generality of this process which, it is believed, also operates in crystallization of polydisperse polymers.