Effect of Shear Stress on Crystallization of Isotactic Polypropylene from a Structured Melt

Abstract

Based on a control of the melt structure at temperatures near but below the equilibrium melting point, we investigated the role of shear stress imposed by the wall of the capillary die extruder on crystal morphology of isotactic polypropylene (iPP). The nanoscale partially ordered molecular bundles within the quiescent melt at temperatures between the nominal melting temperature and the equilibrium melting temperature allowed for the possibility of shear-induced or shear-assisted formation of β-cylindrites which were investigated by means of polarized optical microscopy and small/wide-angle X-ray scattering. For a given temperature, the number of cylindrites increased with shear stress, indicating a decrease of the critical size of the partially ordered molecular bundles with shear stress. Interestingly, this increase in number coincides with an increase in preferred orientation of the cylindrites, potentially related to a higher nucleation probability of oriented structural elements within the melt. Concomitantly, the nucleation density of α-iPP within a single cylindrite structure increased with shear stress at the expense of β-iPP nucleation density.