Collaboration of β-crystals and rubbery phases in polypropylene random copolymer for toughening

Polypropylene random copolymer (PPR) is in high demand for pipelines due to its comprehensively brittle-ductile balance. It is a multiphase polymer composed of crystalline phases and rubbery phases in which improving the quantities of ductile β-crystals can effectively toughen PPR. However, the influence of β-crystallization on the aggregation of rubbery phases and their collaboration for toughening are still not well elucidated. In this work, the toughening mechanism of PPR/iPP/β-NA composites were illustrated by manipulating the crystallization and phase separation behaviors using different molecular weight of isotactic polypropylene (iPP) and β-nucleating agent (β-NA). A high relative β-crystal content (K_β) was obtained in PPR/iPP/β-NA composites and the influence of molecular weight of iPP on crystal structures of PPR/iPP/β-NA composites was investigated via differential scanning calorimetry (DSC) and x-ray diffraction (XRD). The phase separation behaviors were then studied using rheological measurements and scanning electron microscope (SEM). The impact behaviors of PPR/iPP/β-NA composites with similar crystal structures but different size of rubbery particles were demonstrated. It was found that crystallization of PPR into β-crystal promoted the phase separation process and the resulting rubbery particle size. The deformation of large rubbery particles induced small wavy cracks at the fracture surface other than layered cracks for small rubbery particles. This work basically introduces the toughening mechanism from collaboration of β-crystals and rubbery phases in polypropylene random copolymers and provides a new method for fabricating high toughness PPR pipe.