Effect of Flowing Preformed Spherulites on Shear-Induced Melt Crystallization Behaviors of Isotactic Polypropylene

The effects of flowing preformed spherulites on shear-induced melt crystallization behaviors of isotactic polypropylene (iPP) had been investigated by using a Linkam shearing cell coupled with an optical microscope, atomic force microscope, and rheometer. By quenching iPP melt to 130 °C for a short time, preformed spherulites were intentionally obtained. While iPP films were heated up to temperatures below the nominal melting point of iPP (164 °C), various shear conditions were applied, and then the crystalline morphologies during crystallization at the isothermal crystallization temperature of 135 °C were traced by alternately using phase contrast optical microscopy and polarized optical microscopy modes. Two distinct highly orientated thread- and cylinder-like crystalline morphologies were induced by shearing with preformed spherulites present, which were dependent on the applied shear rate and temperature. The distinct crystalline morphologies were attributed to relative movements between shear flowing melt and preformed spherulites. Atomic force microscopy was used to reveal the crystal aggregates of the two typical morphologies, indicating that the former was composed of highly packed orientated crystal nuclei, while the latter was composed of loosed packed random crystal nuclei, for which the thin crystal lamellae grew perpendicularly to crystal nuclei, leading to large widths for the cylinders as seen by an optical microscope. Rheological measurements indicated that preformed spherulites were isolated in the iPP films, which could obviously retard the stress relaxation of iPP films at the lower shear temperature.