Tailored Structure and Properties of Injection-Molded Atactic Polypropylene/Isotactic Polypropylene Blend

In the present work, atactic polypropylene (aPP)/isotactic polypropylene (iPP) with different aPP content was prepared through an injection-molding process to improve the toughness of iPP and make large scale use of aPP. The hierarchic structure of the injection-molded parts was characterized through differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), and scanning electron microscopy (SEM). It was found that the network of iPP crystals was still integral with shish-kebab structures in the skin layer and the relatively high crystallinity of iPP in injection-molded parts with low aPP content utilizing suitable process conditions. Therefore, the toughness of iPP was enhanced from 31.8 to 42.6 MJ/m³ due to the addition of a 20 wt % aPP component, meanwhile the tensile strength only decreased from 45.9 to 40.5 MPa. Furthermore, when aPP content reached 50 wt %, the toughness of the aPP/iPP blends increased to 52.0 MJ/m³ with the tensile strength staying at the level of 20 MPa, indicating that the A50 sample also has good toughness with reasonable strength. The results demonstrated that the aPP/iPP blends with high mechanical properties owing to the optimized inner structure can be obtained through the suitable processing method. Our results set up a new method to make large scale use of aPP. Moreover, with an increase in aPP content, the mechanical properties of the injection-molded part can be divided into three evolution stages with distinct differences. At high and low aPP content, the mechanical properties were not sensitive to aPP content. However, when aPP content fell between 20 and 50 wt %, the mechanical properties of the injection-molded part, especially the elongation at the break, changed dramatically acting like percolation phenomenon in electro-conductive polymer composites, which may be the result of phase inversion of aPP/iPP blends. The percolation in mechanical properties is meaningful to prepare the blend of crystallizable/noncrystallizable blends with different properties.