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

Studies on the morphology and the development of texture in isotactic polyp ropylene (iPP) subjected to plane-strain compression are reported. The iPP samples were compressed in a channel-die at 110 degrees C up to the true st rain of 1.89 (compression ratio, CR = 6.6). The structure of deformed speci mens was investigated by means of light microscopy, differential scanning c alorimetry, density measurements, small- and wide-angle X-ray diffraction t echniques and dynamic mechanical analysis. A scheme of morphology changes o n all structural levels was proposed. It was found that initial spherulitic morphology was destroyed and was transformed into stacks of crystalline la mellae with their normals rotating towards loading direction, while chain a xis tending towards the how direction at the true strain near 1.1 (CR appro ximate to 3). The main active deformation mechanisms found were the crystal lographic 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 i ntense chain slip caused the fragmentation of the lamellae into smaller cry stalline blocks due to slip instabilities. That transformation occurred abo ve true strain of 1.39 (CR = 4). Further slips in these fragmented crystall ites led to formation of a sharp orientation of the chains along the flow d irection. 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 fol low the evolution of their structure through successive increase of storage modulus and a decrease of mechanical loss, ascribed to the glass-rubber tr ansition, with increasing strain. The behavior of mechanical loss evidences substantial stiffening of the amorphous component with increasing strain. (C) 1999 Elsevier Science Ltd. All rights reserved.