Self-Acceleration of Nucleationand Formation of Shish in Extension-Induced Crystallization with StrainBeyond Fracture.

Extension flow induced crystallization of isotatic polypropylene(iPP) has been studied with a combination of extension rheologicaland in situ small-angle X-ray scattering (SAXS) measurements at 140°C. Rheological data of step extension on iPP melt are dividedinto before and beyond fracture strain zones in strain–strainrate space, where intermediate strains between them lead to fractureof samples. Coincidently, weak and strong accelerations of nucleationare observed in the before and beyond fracture strain zones respectively,where distinctly different features of crystallization kinetics andnucleation form occur in these two zones. The microrheological modelexplains the acceleration of nucleation in the “before fracturestrain zone” well, while a “ghost nucleation”mechanism is proposed to interpret the strong acceleration of nucleationin the “beyond fracture strain zone”. The “ghostnucleation” is due to the displacement of initial parent pointnuclei, where daughter nuclei are induced along the trails. This newmechanism explains well the acceleration of nucleation in orders ofmagnitude and the formation of shish in iPP melt. [ABSTRACT FROM AUTHOR]