Structural Evolution and Toughening Mechanism of β-Transcrystallinity of Polypropylene Induced by the Two-Dimensional Layered Interface during Uniaxial Stretching

The structure and morphology of beta-crystals of isotactic polypropylene (iPP) are of great significance because beta-crystals can improve the toughness and ductility of iPP. Toughening of beta-spherulites, which was ascribed to phase transformation, has been extensively investigated. However, the toughening mechanism of other beta-crystals with special structures and morphologies is not clear. In this study, beta-transcrystallinity (beta-TC), which showed a greater toughening effect than that of beta-spherulite, was constructed through microlayered coextrusion. During uniaxial stretching, beta-TC preferred to transform into an alpha-crystal, whereas beta-spherulite preferred to transform to a smectic mesophase. The transformation degree of beta-TC was much higher than that of beta-spherulite. More importantly, the lamellar fragments from beta-TC gradually rearranged along the stretching direction, accompanied by continuous absorption of energy. The special beta-alpha phase transformation, high transformation rate, and rearrangement of lamellar fragments led to the highly improved toughness of the layered samples.