Formation of cylindrite structures in shear-induced crystallization of isotactic polypropylene at low shear rate

The shear-induced crystallization behavior of isotactic polypropylene (iPP) at low shear rate and low temperature was investigated by in situ optical microscopy, time-resolved small-angle light scattering (SALS) and ex situ atomic force microscopy (AFM). Some new details and insight of the cylindrites were observed, which are important to delineate the formation mechanism of the cylindrite structures. Optical microscopic measurements showed that the core of cylindrite was formed rapidly after shear cessation, at late stage the quantity of the newly formed crystals increased and these crystals impinged into each other rapidly due to the space limitation. Measurement of the length of cylindrite core from optical microscopic observation showed that the length of cylindrite core did not change or changed very slightly with crystallization time. SALS results indicated that immediately after shear flow, the highly oriented structure emerged and the scattering intensity increased with crystallization time, and eventually the scattering patterns became circularly symmetric and the anisotropic scattering decreased, indicating the impingement of different cylindrite structures. AFM observation showed that the crystalline lamellae grew both perpendicular to and along the cylindrite core. In addition, a much wider cylindrite core was observed. Compared with the radius of gyration of a molecule, the process of shear-induced crystallization may have involved a large number of entangled molecules under a low shear rate. These evidences clearly indicate that the cylindrite core comes from the stretched bundles of the entangled network strands but not from the extended crystals of stretched single chains. A model based on the above observations has been proposed to explain the mechanism of the cylindrite developing process at low shear rate and low temperature.