New Understanding in Tuning Toughness of β-Polypropylene: The Role of β-Nucleated Crystalline Morphology

It is widely believed that the trigonal β-form is favorable and effective for toughening isotactic polypropylene (iPP). Therefore, β-form content should be achieved as high as possible to realize excellent toughness in iPP. However, in this study, we demonstrate that the connection between crystallites might mainly determine the toughness of iPP instead of the β-crystal content. A new rare earth nucleator (WBG) was used to generate the rich β-crystalline structure in the compression-molded bars that were fabricated upon different molten temperatures (Tf). Interestingly, the increase in tensile elongation can be as large as 8 times for increased Tf. The polymorphic composition and overall crystallinity of β-nucleated iPP are almost independent of Tf. Nevertheless, the β-nucleated crystalline morphology has completely changed. Three types of β-crystalline morphology, namely, β-spherulite, β-transcrystalline entity, and “flower”-like agglomerate of β-crystallites, are sequentially obtained with increasing Tf. From the morphological point of view, the connection between the crystallites in “flower”-like agglomerate is significantly better than that for the crystallites generated under lower Tf. Therefore, it is concluded that the formation of β-nucleated iPP provides very good toughness only with sufficient connection between the crystallites. The result of this study clearly verifies the importance of crystal morphology on tuning the toughness of iPP. It provides important information for potential industrial applications.