Unexpected Strength and Toughness Reinforcement of the Injection-Molded Isotactic Polypropylene Parts with Oriented β-Crystals

In this work, three profiles of injection temperatures were adopted to tune the morphology of a β-nucleating agent and further affect the resulting β-crystal morphology of isotactic polypropylene (iPP) during an injection molding process. When the middle injection temperature profile was adopted (the maximum temperature was 220 °C), abundant oriented β-crystals with the c-axis perpendicular to the injection direction were presented throughout the whole injection-molded part, and their existence and distribution were confirmed by synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). These oriented β-crystals give rise to simultaneously enhanced tensile and impact strength of the β-nucleated iPP part. In a novel outcome, this work transformed the usually mechanically weak and soft component (i.e., the β-crystals) of iPP into one that added to its self-reinforcement and overall strength. This finding provides a new idea that a soft structure can be utilized to reinforce itself.