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

It is widely believed that the trigonal beta-form is favorable and effective for toughening isotactic polypropylene (iPP). Therefore, beta-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 beta-crystal content. A new rare earth nucleator (WBG) was used to generate the rich beta-crystalline structure in the compression-molded bars that were fabricated upon different molten temperatures (T-f). Interestingly, the increase in tensile elongation can be as large as 8 times for increased T-f. The polynnorphic composition and overall crystallinity of beta-nucleated iPP are almost independent of T-f. Nevertheless, the beta-nucleated crystalline morphology has completely changed. Three types of beta-crystalline morphology, namely, beta-spherulite, beta-transcrystalline entity, and flower-like agglomerate of beta-crystallites, are sequentially obtained with increasing T-f. 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 T-f. Therefore, it is concluded that the formation of beta-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.