Jing-wei Chen

Crystallization behavior of isotactic polypropylene induced by competition action of β nucleating agent and high pressure

Addition of β-form nucleating agent (β-NA) to isotactic polypropylene (iPP) can greatly induce the variation of the crystallization of matrix. Here, we introduce high pressure to the crystallization process of β-NA nucleated iPP. It is observed that there is a competition between the effect of high pressure and the nucleation effect of β-NA on crystallization of iPP. With the increasing of high pressure, both the contents of β-iPP and α-iPP decrease while the content of γ-iPP increases. A novel crystalline morphology of γ-iPP with the fragmentations of γ-spherulites regularly organized in a local region is observed. Namely, γ-spherulites grow on the lateral of β-NA needlelike structure. The dual nucleation effects of the special needlelike structure of β-NA towards β/α-iPP and the effect of high pressure which prevents the growth of β-iPP but promotes the growth of γ-iPP are the main mechanisms for the novel crystalline morphology of γ-iPP. Specifically, the mixed polymorphic β/γ-iPP can be achieved under a certain pressure. This possibly enlarges the application of iPP material.

Enhancing chain segments mobility to improve the fracture toughness of polypropylene

As a part of a serial work about the annealing inducing improvement of fracture toughness of polypropylene (PP) articles, in this work, a highly efficient mobilizer was introduced into PP and the injection-molded samples were annealed at different temperatures. The mobility of chain segments of PP was investigated by measuring the glass transition temperature. Differential scanning calorimetry (DSC) and wide angle X-ray diffraction (WAXD) were used to characterize the variation of crystalline structure of PP during the annealing process. The fracture behaviors including notched Izod impact fracture and universal tensile fracture were investigated to detect the mechanical properties in response to the variations of both chain segments mobility and crystalline structures. It was found that the mobilizer greatly improved the chain segments mobility. Further results showed that the mobilizer also induced apparent changes of the glass transition temperature and the degree of crystallinity of PP during the annealing process. Consequently, the annealed PP samples containing a few amount of mobilizer exhibited largely increased fracture toughness.

Annealing Induced Microstructure and Mechanical Property Changes of Impact Resistant Polypropylene Copolymer

The effects of annealing on microstructure and mechanical properties of an impact resistant polypropylene copolymer (IPC) were investigated. Different annealing temperatures ranging from 80 °C to 160 °C were selected. The phase reorganization of IPC during annealing process was studied through morphological characterization technologies, including scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The crystalline structure changes in the IPC sample, including the iPP matrix and PE component, were investigated using wide angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). Dynamic mechanical analysis (DMA) was used to analyze the relaxation extent of IPC before and after annealing. The results showed that annealing induced phase reorganization in IPC and the degree of phase reorganization depended on annealing temperature. The annealed IPC samples exhibited largely increased crystallinity compared with the unannealed one. Intensified damping peak with increased molecular chain mobility was achieved for the annealed IPC samples. At an appropriate annealing temperature (140 °C), largely enhanced impact strength was achieved for the annealed IPC sample. The toughening mechanisms were analyzed based on the phase reorganization and relaxation behavior.