Formation of β-cylindrites under supercooled extrusion of isotactic polypropylene at low shear stress

Formation of β-cylindrites of isotactic polypropylene under various wall shear stress (σw), supercooled temperature of melt (Te) and crystallization temperature (Tc) has been investigated by polarized light microscopy (PLM), wide angle X-ray diffraction (WAXD), and differential scanning calorimeter (DSC). To have better control over the thermomechanical history, instead of a reciprocating screw, the samples were prepared by extruding supercooled melt through capillary die. β-cylindrites can be observed by PLM in the extruded specimen even at a lower σw (0.020 MPa), and the number of β-cylindrites nuclei increases rapidly with the lowering of Te. The nucleation density of β-cylindrites increases with the raising of wall shear stress under a given Te of 160 °C. Furthermore, at lower supercooled temperature of melt (145 °C), the radius of β-cylindrites decreases with the increasing of σw, and the number of β-cylindrites nuclei almost remain invariant. At relatively higher σw (0.090 MPa), a saturation of β-cylindrites nuclei is observed with decreasing Tc. A modified model based on above results has been proposed to explain the effect of the original structure of quiescent supercooled melt on the formation of β-cylindrites under low shear stress.