SHEAR-INDUCED CRYSTALLIZATION OF ISOTACTIC POLYPROPYLENE MELTS - ISOTHERMAL WAXS EXPERIMENTS WITH SYNCHROTRON-RADIATION

It is well known that crystallization nuclei are created in the melt of semicrystalline themoplastic materials by shearing. Sheared polymer melts have a higher number of crystallization nuclei than quiescent melts at the same crystallization temperature. Two types of isotactic polypropylene with different molecular weight distributions were investigated using special shearing equipment and synchrotron radiation diffraction. The results of our time-resolved WAXS investigations are time-dependent functions of the degree of crystallinity, from which certain parameters such as relaxation time can be determined with the help of a given crystallization model. Even a very small shearing of the melt, after a few seconds, enhances the crystallization of both materials at the temperature of 130 and 135-degrees-C. At the same temperatures, quiescent polypropylene melts start to crystallize after approximately 1 h. Without shearing the alpha-modification prevails, whereas an important volume fraction of the beta-modification appears in sheared samples even at low shearing velocities of 4 s-1.