Changes in the morphology and orientation of bulk spherulitic polypropylene due to plane-strain compression

Abstract

Studies on the morphology and the development of texture in isotactic polypropylene (iPP) subjected to plane-strain compression are reported. The iPP samples were compressed in a channel-die at 110°C up to the true strain of 1.89 (compression ratio, CR=6.6). The structure of deformed specimens was investigated by means of light microscopy, differential scanning calorimetry, density measurements, small- and wide-angle X-ray diffraction techniques and dynamic mechanical analysis. A scheme of morphology changes on all structural levels was proposed. It was found that initial spherulitic morphology was destroyed and was transformed into stacks of crystalline lamellae with their normals rotating towards loading direction, while chain axis tending towards the flow direction at the true strain near 1.1 (CR≈3). The main active deformation mechanisms found were the crystallographic slips along the chain direction: (010)[001], (110)[001] and (100)[001] slip systems, supported by the deformation of the amorphous component by interlamellar shear. No evidence of the twinning modes was found. The intense chain slip caused the fragmentation of the lamellae into smaller crystalline blocks due to slip instabilities. That transformation occurred above true strain of 1.39 (CR=4). Further slips in these fragmented crystallites led to formation of a sharp orientation of the chains along the flow direction. The final texture of the compressed iPP found at the true strain of 1.89 (CR=6.6) was the multi-component texture with two main components of (010)[001] and (110)[001]. Mechanical properties of deformed samples follow the evolution of their structure through successive increase of storage modulus and a decrease of mechanical loss, ascribed to the glass–rubber transition, with increasing strain. The behavior of mechanical loss evidences substantial stiffening of the amorphous component with increasing strain.