Influence of thermal treatments on the polymorphism in stereoirregular isotacic polypropylene: Effect of stereo-defect distribution

The influence of cooling rate from the melt on the polymorphism and crystallinity is investigated as a function of isotacticity and stereo-defect distribution in polypropylenes. Detailed analysis of wide angle x-ray diffraction patterns shows that crystallinity in the materials used is nearly independent of the experimental cooling rates (0.5–408C/min). At high cooling rates, the materials exist mainly in the a-phase, whereas the amount of the g-phase increases at the lower cooling rates. With an increasing amount of stereo-defects, this cooling-rate dependence of the polymorphism is enhanced. The effect of different stereo-defect distributions, as observed in metallocene- (random) and Ziegler–Natta (blocklike) derived isotactic polypropylenes, was investigated. The formation of the g-phase is more prevalent in materials with a random defect distribution compared to the materials in which the stereodefects have a blocklike distribution. The crystallinity decreases more rapidly as a function of the tacticity in the random defect-distributed materials.