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The shear-induced crystallization behavior of isotactic polypropylene (iPP) nanocomposite melt containing modified carbon nanofibers (MCNFs) was investigated by rheo-SAXS (small-angle X-ray scattering) and rheo-WAXD (wide-angle X-ray diffraction) techniques using synchrotron radiation. Under quiescent conditions, the nucleating effect of MCNFs on crystallization of iPP was pronounced and the system exhibited a remarkably low saturation point (ca. 0.05 wt% of MCNF). In-situ SAXS and WAXD results showed the development of shear-induced crystalline structures and lamellar morphology in nanocomposite melts. Under the same shear conditions, the filled system exhibited notably faster kinetics compared with the unfilled system. The oriented crystalline fraction was found to decrease with the MCNF loading, indicating the competition between oriented crystals (induced by shear) and unoriented crystals (due to the nucleating effect of MCNF). At the early stages of crystallization, the amount of the oriented crystals increased with the MCNF concentration, suggesting that the nanofiller hindered the motion of polymer chains after the cessation of flow resulting in the delayed relaxation of stretched polymer segments. [ABSTRACT FROM AUTHOR]

A variety of semicrystalline isotactic polypropylene composites containing carbon nanotubes and nanofibers were produced by melt and solution techniques. The effect of the nanofillers on the crystallization process was investigated by transmission electron microscopy, scanning electron microscopy, and differential scanning calorimetry. Under the processing conditions applied in this study, the surfaces of the carbon nanomaterials act as nucleation sites in bulk samples and highly oriented composite films. This conclusion is confirmed by the calculation of Avrami exponents and, in particular, by direct microscopy evidence. [ABSTRACT FROM AUTHOR]