Investigation on the crystallization behavior and polymorphic composition of isotactic polypropylene/multi-walled carbon nanotube composites nucleated with β-nucleating agentThe role of melt structures

The melt structure of isotactic polypropylene (iPP) nucleated with 1.0 mass% multi-walled carbon nanotubes (CNTs) and 0.05 mass% rare earth β-phase nucleating agent (WBG-II) was tuned by changing the fusion temperature Tf. In this way, the role of melt structure in the crystallization behavior and polymorphic composition of the sample was studied by calorimetry, X-ray diffraction, and scanning electronic microscopy. In this system, the sample cannot form any β-phase when Tf = 200 °C, since the α-nucleation efficiency (α-NE) of CNTs was obviously higher than the β-NE of WBG-II. By tuning Tf, the content of ordered structures in iPP melt was controlled and it was found that the polymorphic composition of the sample can be efficiently controlled. When Tf was in the range of 166–174 °C, an amount of ordered structures survived in iPP melt, resulting in a significant increase of β-phase proportion; moreover, study on the crystallization behaviors of single CNTs-filled iPP and WBG-II-nucleated iPP (iPP/CNTs and iPP/WBG-II, respectively) revealed that the α-NE of iPP/CNTs stayed almost constant as Tf varies from 200 to 166 °C; meanwhile, the β-NE of iPP/WBG-II increased gradually. Once Tf decreased to 174 °C and lower, the NE of iPP/WBG-II became higher than that of iPP/CNTs; meanwhile, WBG-II began to play determining role in the polymorphic composition and enhanced the β-phase proportion of iPP nucleated with CNTs/WBG-II. The findings of this study brought a new pathway to control the polymorphic composition and mechanical properties of iPP/CNTs composites.