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

Abstract

Formation of β-cylindrites of isotactic polypropylene under various wall shear stress ( σ w ), supercooled temperature of melt ( T e ) and crystallization temperature ( T c ) 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 σ w (0.020 MPa), and the number of β-cylindrites nuclei increases rapidly with the lowering of T e . The nucleation density of β-cylindrites increases with the raising of wall shear stress under a given T e of 160 °C. Furthermore, at lower supercooled temperature of melt (145 °C), the radius of β-cylindrites decreases with the increasing of σ w , and the number of β-cylindrites nuclei almost remain invariant. At relatively higher σ w (0.090 MPa), a saturation of β-cylindrites nuclei is observed with decreasing T c . 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.