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 C 294 H 590 and cooling experiments on C 194 H 390, C 198 H 398, C 210 H 422, C 246 H 494, and C 258 H 518 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 C 246 H 494 and C 198 H 398, and also for solution-crystallization of C 198 H 398 . 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.