Exploring the Effects of Stereo-Defect Distribution on Nonisothermal Crystallization and Melting Behavior of β-Nucleated Isotactic Polypropylene/Graphene Oxide Composites

In this work, using two isotactic polypropylene (iPP) resins with similar average isotacticity and molecular weight but different uniformities of stereo-defect distribution, the beta-nucleated iPP/graphene oxide (beta-iPP/GO) composites (NPP-A and NPP-B) were prepared to investigate the effect of stereo-defect distribution on the nonisothermal crystallization kinetics and polymorphic melting behavior of the composites by means of scanning electron microscopy, wide-angle X-ray diffraction, and differential scanning calorimetry. The results showed that more uniform stereo-defect distribution led to a slight increase of the crystallization rate and decrease of the crystallization activation energy E-c. NPP-B with more uniform stereo-defect was more favorable for the formation of a large amount of beta-phase. Moreover, the role of the cooling rate was also discussed and it was found that the higher the cooling rate, the higher the beta-phase content and the smaller the crystalline sizes, meanwhile, the higher the amount of ss- phase with relatively lower thermal stability that will take part in beta-alpha recrystallization during the subsequent melting process. For beta-iPP/GO composites, although the cooling rate greatly influences the polymorphic behavior and crystalline structures of the composites, the uniformity of stereo-defect distribution was found to be the first factor determining the formation of the beta-phase.