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