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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]

Although observations of molecular processes in the formation of primary nuclei prior to actual crystallization are beyond the detection limits of current instrumentation, we attempted to probe the nature of primary nuclei in sheared isotactic polypropylene (iPP) polymer melt. In situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide angle X-ray diffraction) experiments using synchrotron radiation were carried out to evaluate the effects of an addition of a high molecular weight atactic polypropylene (aPP) (5 wt%), which is compatible with the iPP matrix but does not crystallize, on the evolution of oriented structures in the sheared iPP melt and its crystallization kinetics. It is unlikely that the aPP chain segments can be incorporated into iPP nuclei or crystal; hence, its addition effects, if any, would be seen only in the amorphous melt prior to crystallization. The results showed stonger orientation and improved crystallization kinetics in the iPP/aPP blend compared to pure iPP. Observations that the presence of long chains of an amorphous polymer aid in nucleation and crystallization kinetics of iPP, combined with our previous synchrotron results of sheared iPP melts at high temperature (165°C), lead us to conclude that primary nuclei in iPP most likely consist of liquid-crystalline or mesomorphic bundles of aligned chain segments prior to the formation of crystals. [ABSTRACT FROM AUTHOR]

Temperature scanning techniques, including synchrotron small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction(WAXD), and temperature-modulated differential scanning calorimetry (TMDSC), were used to study melting recrystallization in semicrystalline polyoxymethylene (POM). The isothermal crystallization of POM was also studied by time-resolved SAXS. From SAXS profiles, several morphological variables were calculated, including the long period L, lamellar thickness lc, interlamellar amorphous thickness la, and scattering invariant Q. The lamellar parameters were also obtained using tapping atomic force microscopy (AFM) for two thermal histories, and some lamellar-scale and larger-scale morphological changes were characterized before and after partial melting. These real-space images also provided support to interpretation of SAXS analysis in the interpretation of first- and second-order intensity maxima. During melting at constant heating rates, la from SAXS increased slowly starting at about 100°C, suggesting melting of thin inserted lamellae, and at about 150°C, lc began to increase combined with a more rapid increase in la due to further melting of inserted lamellae and some recrystallization into separate stacks of lamellae. The end of melting was about 182°C. TMDSC data also characterized the level of melting and recrystallization starting at low temperatures for a quenched sample. The DSC data provided the total extent of melting, and this was contrasted with the drop in SAXS and WAXD intensities. SAXS and TMDSC temperature scans on a quenched, but still highly crystalline, POM sample were compared with the data from a high-temperature (145°C) isothermally crystallized POM sample with a higher degree of crystal perfection. [ABSTRACT FROM AUTHOR]

Three commercial metallocene-catalyst synthesized short-chain branched polyethylene (SCBPE) samples with similar molecular weights and molecular weight distributions were investigated in terms of their molecular structural (comonomer sequences and compositions) inhomogeneity and crystal morphology. Two of these samples [SCBPE(B1) and SCBPE(B2)] contained different ratios of a butene comonomer (20.7 and 26.8 SCB/1000 carbons, respectively), while the third sample, SCBPE(H), contained a branched hexane comonomer (7.8 SCB/1000 carbons). A linear PE fraction was also investigated for comparison. Differential scanning calorimetry (DSC) results indicate that multiple-step isothermal (thermal segregation) experiments lead to multiple endothermic melting processes in these SCBPEs during heating, a phenomenon that was not observed in the linear PE. Wide-angle X-ray diffraction (WAXD) experiments show that all of these SCBPEs possess an orthorhombic crystal lattice with different crystallinities. Linear coefficients of thermal expansion along both the a- and b-axes of the PE crystals were also determined using WAXD at different temperatures. This lateral lattice expansion is critically associated with the comonomer size and composition ratio of the SCB series. Small-angle X-ray (SAXS) scattering results of the thermal segregated samples obtained during heating required a differential scattering data treatment. It was found that the long period for each isothermal crystallization step was different and increased with increasing temperature. By increasing the comonomer composition at a constant temperature, the long period was also increased, although the thickness of the crystal lamellae decreased. This was due to an increase of the noncrystalline layer thickness between two neighboring lamellae. The crystalline morphology was observed under transmission electron microscopy (TEM). During multiple-step isothermal crystallization, the crystalline morphology exhibited a... [ABSTRACT FROM AUTHOR]