Biaxial orientation of polypropylene by hydrostatic solid state extrusion. Part I: Orientation mechanism and structural hierarchy

Abstract

The structure which results from solid state extrusion using biaxial orientation is analyzed for oriented polypropylene. Structural changes on the spherulitic, lamellar, and macromolecular level during orientation are investigated using optical microscopy (OM), small angle X‐ray scattering (SAXS), and wide angle X‐ray scattering (WAXS). The results show that polypropylene spherulites undergo stepwise biaxial affine deformation and deform homogeneously into a disc‐like morphology. During this spherulitic flattening process, lamellar rotation into the planar direction occurs prior to lamellar break‐up at a baxial draw ratio of about 1.5. On the macromolecular level, the crystalline c‐axis orients in the plane concurrently with the lamellar break‐up, while the crystalline b*‐axis gradually orients normal to the plane. Amorphous chains are also oriented preferentially in the plane of deformation. A hierarchical model is proposed to illustrate the nature of the orientation in the flattened spherulites. Copyright © 1986 Society of Plastics Engineers