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