Plastic Deformation of the γ Phase Isotactic Polypropylene in Plane−Strain Compression at Elevated Temperatures

Abstract

Plastic deformation behavior of iPP homopolymer crystallized exclusively in the γ modification was studied. Samples of γ-iPP were obtained by isothermal crystallization under pressure of 200 MPa. Deformation experiments in plane−strain compression were performed in the temperature range of 55−100 °C. Samples of γ-iPP demonstrated higher modulus, higher yield stress, and flow stress, yet slightly lower ultimate strain comparing to α-iPP in the entire range of temperature studied. During plastic deformation numerous fine shear bands, initiated by the interlamellar shear of the amorphous layers, start to develop already at the yield point. Their propagation across the sample causes a limited destruction of γ lamellae oriented perpendicularly to the direction of the band. Destroyed fragments of crystallites partially reconstruct into either mesophase (smectic) domains or crystals of α phase, depending on the deformation temperature. Mesophase is produced upon deformation at room temperature, while at 55 °C and above the crystalline α phase is formed instead. With increasing strain shear bands multiply and tilt toward the flow direction. Fragmented lamellae undergo kinking and rotation, which results in formation of a chevron-like lamellar morphology. This leads also to the development of a weak crystalline texture. Both crystalline texture and lamellae orientation emerge due to the same deformation mechanism of interlamellar slip, produced by the shear within interlamellar amorphous layers. The activity of any crystallographic deformation mechanism within the crystalline component was not detected at any temperature. The interlamellar amorphous shear appears to be the primary deformation mechanism of γ-iPP. The other identified mechanisms, i.e., γ−smectic and γ−α transformations, play a supplementary role in the deformation sequence.