Crystal morphology and structure of the β‐form of isotactic polypropylene under supercooled extrusion

A supercooled melt of isotactic polypropylene (iPP) was extruded through a capillary die. Polarized light microscopy (PLM), wide‐angle X‐ray diffraction (WAXD), and differential scanning calorimetry (DSC) were used to investigate the effects of the relatively weak wall shear stress (σ w ), extrusion temperature ( T e ), and crystallization temperature ( T c ) on the structure and morphology of β‐form isotactic polypropylene (β‐iPP). β‐cylindrites crystals could be observed by PLM in the extruded specimen even at a lower σ w 's (0.020 MPa), and the β‐iPP content increased with decreasing T e . Under a given T e of 150°C, the increase in σ w positively influenced the β‐iPP content. The DSC and WAXD results indicate that the total crystallinity and β‐iPP content increased when T c was set from 105 to 125°C; the other experimental parameters were kept on the same level. Although T c was above 125°C, the β‐iPP content obviously decreased, and the total crystallinity continued to increase. On the basis of the influences of σ w , T e , and T c on the β‐iPP crystal morphology and structure, a modified model is proposed to explain the growing of shear‐induced β‐iPP nucleation. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011