Fibrillar morphology formation in a sheared polypropylene melt

In common polymer processing operations such as injection molding, film blowing, and fiber spinning, the molten polymer is subjected to shear and/or elongational flow fields that are often so intense to induce the formation of highly oriented crystalline structures, namely a fibrillar morphology. This peculiar morphology highly affects the resulting mechanical and functional properties of semicrystalline polymers and thus the study of the conditions under which it forms are of high scientific and industrial interest. In this work, crystallization during step shear experiments at 140°C were carried out in a plate-plate geometry by means of a Linkam shearing cell. The evolution of crystalline structures was observed during the tests at a fixed radial position. After the shear step the samples were allowed to fully crystallize at the test temperature and then were cooled down to room temperature. The samples were then analyzed by optical microscopy and the radial position at which a transition between spherulitic and fibrillar morphology took place was measured. Since in a plate-plate geometry, each radial position means a different shear rate, the analysis of the samples allowed to detect the critical shear rate at which, after a given shearing time, the fibrillar morphology replaces the spherulitic structure. © 2014 American Institute of Physics