β-Crystal in injection moulded poly(ethylene terephthalate) fibre/isotactic polypropylene composite

The poly(ethylene terephthalate) (PET) fibre/isotactic polypropylene (iPP) composite was moulded by a mixing-injection moulding machine developed in our group. The molecular orientation and crystallinity of β-crystal were investigated by two-dimensional wide-angle X-ray diffraction (2D-WAXD), and shish-kebab structure was detected by two-dimensional small-angle X-ray scattering (2D-SAXS). In addition, crystalline morphology of β-crystal in different regions was observed by scanning electron microscope (SEM). As well known, PET fibre generally serves as heterogeneous nucleating agent for iPP to induce α-crystal. Unexpectedly, a remarkably increasing crystallinity of β-crystal from skin to core region was obtained in the present work, which is substantially different from other studies. Two origins of β-crystal developed in core region are proposed: (a) local shear caused by the different flow rates of molten iPP and solid PET fibre; (b) survived smectic ordering brought by the rotating screw during plasticization and melt mixing in barrel.