Plastic deformation behavior of β-phase isotactic polypropylene in plane-strain compression at room temperature

Abstract

Isotactic polypropylene (iPP) rich in β crystal modification (constituting 92% of crystalline phase) was deformed by the plane-strain compression with constant true strain rate, at room temperature. The evolution of phase structure, morphology and orientation was studied by DSC, X-ray and SEM. The deformation sequence and the active deformation mechanisms were found out. The most important mechanisms were interlamellar slip operating in the amorphous layers, resulting in numerous fine deformation bands due to localization of deformation and the crystallographic slip systems, including the (110)[001] chain slip and (110)[1 1 ¯ 0] transverse slip. Shear within deformation bands leads to β → smectic and β → α solid state phase transformations. At room temperature the β → smectic transformation appeared to be the primary transformation, yielding the oriented smectic phase with high concentration of 19 wt.% at the true strain of e = 1.49. The β → α yields only about 4 wt.% of new α-phase at the same strain. As a result of the deformation and phase transformation within numerous fine deformation bands β-lamellae are locally destroyed and fragmented into smaller crystals. Another deformation mechanism is the cooperative kinking of lamellae, leading to their reorientation and formation of a chevron-like lamellar arrangement. At high strains, above e = 1, an advanced crystallographic slip and high stretch of amorphous material due to interlamellar shear bring further heavy fragmentation of lamellar crystals, earlier fragmented partially by deformation bands. This fragmentation is followed by fast rotation of small unconstrained crystallites with chain axis towards the direction of flow, FD. This process leads to development of the final texture of the highly deformed β-iPP with molecular axis of both crystalline and smectic phases oriented along FD.