Saturation of pointlike nuclei and the transition to oriented structures in flow induced crystallization of isotactic polypropylene

The influence of molecular weight and processing conditions on the crystallization kinetics of isotactic polypropylene is studied using rheometry. Flow-induced crystallization experiments are performed with shear rates at which molecular stretch of the longest chains is expected. Depending on the molecular weight, a saturation of pointlike nuclei is observed with increasing shear time. In most cases, the process accelerates after sufficient flow time, and this change in kinetics is due to the occurrence of fibrillar nucleation resulting in the formation of row structures and/or shishes. The number of pointlike nuclei is derived from the rheometry experiments by modeling the system as a suspension. This method has some important advantages, i.e., (1) it is applicable to systems where optical microscopy does not work (i.e., colored systems) and (2) it is much easier, faster, and more accurate then optical methods.