In Situ Synchrotron X-ray Scattering Study on Isotactic Polypropylene Crystallization under the Coexistence of Shear Flow and Carbon Nanotubes

The crystallization of isotactic polypropylene (iPP) under the coexistence of shear flow and carbon nanotubes (CNTs) was investigated by means of in situ synchrotron X-ray scattering techniques, i.e. wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). Compared to sheared pure iPP, the combined effect of shear flow and CNTs endowed [PP crystals with weak degree of orientation at the early stage of crystallization but high degree of orientation in the later period. This was because the initial orientation of molecular chains induced by shear was suppressed as a result of the increased viscoelasticity of iPP melt in the presence of CNTs, but subsequently oriented molecular chains were stabilized by CNT surface absorption. The crystallization kinetics of sheared CNTs/iPP nanocomposites was synergistically promoted, where the crystallization rate was increased about 40 times in comparison to that of quiescently crystallized pure iPP. The Avrami exponent of CNTs/iPP nanocomposites and sheared iPP was around 2, indicating two-dimensional lamellar growth. The Avrami exponent of sheared CNTs/iPP nanocomposites surprisingly appeared to be 2.52, suggestive of mixed twodimensional lamellar growth and three-dimensional sphrtilitic growth geometries. Moreover, fl-crystals were absent in sheared CNTs/iPP nanocomposites in contrast to the normal observation that a-row nuclei induced by shear generated beta-crystals. The synergistic crystallization rate, the mixed crystal growth geometry as well as the absence of beta-crystals in sheared CNTs/iPP nanocomposites were in close relation with intense interaction between shear flow and CNTs, which gave rise to extra nuclei in sheared CNTs/iPP melt. Apart from heterogeneous nucleating sites originated from CNTs and homogeneous nucleating sites (row-nuclei) initiated by shear, extra nuclei were taken into account to contribute to the further accelerated crystallization kinetics. The extra nuclei became active growth points of branching sites on the two-dimensional lamellae to generate three-dimensional spherulitic growth, thus leading to mixed crystal growth geometry of sheared CNTs/iPP nanocomposites. Besides, extra nuclei as well as a-nuclei derived from CNTs remarkably encouraged the formation of alpha-crystals, responsible for inexistence of beta-crystals in sheared CNTs/iPP nanocomposites.