Abstract
In the field of metals, especially in magnesium alloys, a new concept has been reported that introducing a kink by applying compression or other deformation to a material with an LPSO structure, in which hard and soft layers are alternately stacked, results in higher strength. Because crystalline polymers are alternately layered with a crystalline phase, the hard layer, and an amorphous phase, the soft layer, it is expected that crystalline polymers can be made stronger if kinks can be introduced by applying compression or other deformation. In this study, the effects of a high-pressure press on the tensile properties and morphology of polypropylene (PP) were investigated. We found that a high-pressure press reduced the strain at break but increased the tensile modulus and the stress at break in the stress–strain curves. Thus, we succeeded in developing high-strength PP using a high-pressure press. In addition, it is found that the tensile properties were isotropic with no directional dependence after press. This implies that the tensile strength can be increased isotropically. Observing the morphology parallel to the press direction by small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS), it was found that the crystal lamellae spread isotropically. Conversely, observation of the morphology perpendicular to the press direction by optical microscopy (OM) and transmission electron microscopy (TEM) revealed the formation of a shear band where deformation was concentrated owing to pressure. In the shear band, it was found that lamella fragmentation occurred and a kinked structure was formed. In this region, the molecular chains may be constrained by pressure, and become a tension state, which leads to the improvement of the mechanical properties.
