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

Abstract

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 (σw), extrusion temperature (Te), and crystallization temperature (Tc) 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 σw's (0.020 MPa), and the β-iPP content increased with decreasing Te. Under a given Te of 150°C, the increase in σw positively influenced the β-iPP content. The DSC and WAXD results indicate that the total crystallinity and β-iPP content increased when Tc was set from 105 to 125°C; the other experimental parameters were kept on the same level. Although Tc was above 125°C, the β-iPP content obviously decreased, and the total crystallinity continued to increase. On the basis of the influences of σw, Te, and Tc 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