Investigation on the Morphology and Tensile Behavior of β-Nucleated Isotactic Polypropylene with Different Stereo-Defect Distribution

摘要

Large amount of work has been published on the tacticity-properties relationship of isotactic polypropylene (iPP). However, the stereo-defect distribution dependence of morphology and mechanical properties of β-nucleated iPP (β-iPP) is still not clear. In this study, two different iPP resins (PP-A and PP-B) with similar average isotacticity but different uniformities of stereo-defect distribution were selected, their β-iPP injection molding specimens were prepared, and the morphology evolution and tensile behaviors were studied by means of differential scanning calorimetry (DSC), 2D wide-angle X-ray diffraction (2D-WAXD) and scanning electron microscope (SEM). DSC results showed that with the same concentration of β-nucleating agent (0.3 wt % WBG-II), PP-B with more uniform stereo-defect distribution exhibited more amount of β-phase than that of PP-A with less uniform stereo-defect distribution, indicating that PP-B is more favorable for the formation of β-phase. SEM results showed that PP-B formed more amount of β-crystals with relatively high structural perfection, while in PP-A a mixed morphology of α- and β-phase with obviously higher amount of structural imperfection emerges. The results of room-temperature tensile test indicated that the yield peak width of PP-B was obviously wider, and the elongation at break of PP-B was higher than that of PP-A, showing a better ductile of PP-B. The morphology evolution results of SEM, 2D-WAXD and DSC suggest that, a combination of lamellar deformation and amorphous deformation occurred in PP-A, while only amorphous deformation mainly took place in PP-B, which was thought to be the reason for the different tensile behaviors of the samples. In the production of β-PP products via injection molding, the uniformity of stereo-defect distribution was found to be an important factor. PP with more uniform distribution of stereo-defect favors the formation of large amount of β-phase with high perfection, which exhibit superior ductile property. The related mechanism was discussed.