Polymer crystallization studies under processing-relevant conditions at the SAXS/WAXS DUBBLE beamline at the ESRF

Recent developments on the experimental infrastructure and the acquisition of new detectors on the Dutch–Belgian beamline BM26B at the ESRF offer novel and promising possibilities for synchrotron X-ray experiments in the field of polymer crystallization under processing-relevant conditions. In this contribution, some of the most recent experiments mimicking conditions similar to those relevant for the plastics processing industry are discussed. Simultaneous thermal analysis and wide-angle X-ray scattering (WAXS) experiments, at the millisecond time-frame level, on β-nucleated isotactic polypropylene (i-PP) samples subjected to ballistic cooling up to 230 K s−1, show that the efficiency of the nucleating agent can be suppressed when quenched cooling rates higher than 130 K s−1are used.In situWAXS experiments using small-scale industrial equipment during a real film blowing process reveal the dependence of the onset of crystallinity (the so-called freeze line) and the crystal orientation as a function of different take-up and blow-up ratios.In situsmall-angle X-ray scattering (SAXS) experiments during high-flow fields reveal the formation of shish and kebab structures in i-PP as a function of the imposed stress. Quantitative analysis of i-PP flow-induced structures is presented. The beamline specifications required to obtain high quality and industrially relevant results are also briefly reported.

A Design to Study Flow Induced Crystallization in a Multipass Rheometer

The design and performance of a flow geometry for the multipass rheometer (MPR) is described, creating an experimental setup to study in-situ and ex-situ structure and morphology development with a proper control over the processing conditions and shear history. The slit used is equipped with diamond windows, to combine flow with different experimental techniques, such as optical microscopy (OM), birefringence and X-ray scattering. In this paper we present preliminary results, obtained on isotactic polypropylene, that demonstrate the possibilities of this device for more extended future research. The focus is on the in-situ birefringence measurements of crystallization and the relation with the final morphology.

Mesophase-Mediated Crystallization of Poly(butylene-2,6-naphthalate): An Example of Ostwald's Rule of Stages

The investigation of poly(butylene-2,6-naphthalate) crystallization by means of chip-calorimetry and ultrafast wide-angle X-ray diffraction (WAXD) revealed the existence of two possible mechanisms. The formation of the stable triclinic α-phase occurs directly from the undercooled melt at low cooling rates/high crystallization temperature. At higher cooling rates a two-stage route is observed: crystallization is preceded by the formation of a mesomorphic phase from the isotropic melt. The monotropic behavior of poly(butylene-2,6-naphthalate), becoming apparent only under severe cooling conditions, obeys the well-known Ostwald's rule of stages.

Oriented Gamma Phase in Isotactic Polypropylene Homopolymer

The presence of γ-phase in isotactic polypropylene is well-known but, up until now, could only be induced by specific processing conditions or material modifications. Typically, for Ziegler-Natta (ZN) iPPs pressures of 2000 bar are required, otherwise, metallocene (M) iPPs and copolymerization using olefin-type counits should be used. Here we show that crystallization under the unique combination of moderate pressure (p ≥ 900 bar) and strong shear flow oriented specimens with high contents of γ-phase are created in ZN-iPPs. The oriented morphology is qualified as a shish-kebab structure that templates densely branched γ-lamellae on parent α-lamellae as well as directly to the shish backbone.

Crystallization and dissolution of flow-induced precursors

We make use of a specially synthesized linear high density polyethylene with a bimodal molecular weight distribution (MWD) to demonstrate that it is possible to produce a suspension of extended-chain (shish) crystals only. Such a suspension can be generated at high temperatures, above but close to the equilibrium melting temperature of the unconstrained extended-chain crystals (T(m)(0)=141.2 degrees C) and requires stretch of the longest chains of the MWD. After the application of a shear flow of 120 s(-1) for 1 s at 142 degrees C, x-ray scattering suggests the presence of a large number of metastable needlelike precursors with limited or no crystallinity. Precursors that are too small dissolve on a timescale that correlates perfectly with the reptation time of the longest polymer molecules. Whereas, precursors that exceed a critical size crystallize forming extended-chain shishes.

Raman characterization of single-walled nanotubes of various diameters obtained by catalytic disproportionation of CO

Single-walled carbon nanotubes prepared by disproportionation of CO over Co-Mo/SiO2 catalysts have been characterized by Raman spectroscopy, using several excitation energies. By varying the reaction temperature, different ranges of nanotube diameter were obtained. The average diameter of a single-walled nanotube produced at 750 degrees C was 0.9 nm, while it increased up to about 1.5 nm when the synthesis was conducted at 950 degrees C. The analysis of the Raman spectra obtained with a range of laser excitation energies not only gives a definite description of the single-walled nanotubes diameters but also helps differentiate the metallic or semiconducting character of the samples. This analysis can be done by comparing the experimental data with calculated gap energies as a function of nanotube diameter as well as comparing the relative intensity of bands centered at 50-60 cm-1 lower than the tangential G mode. The analysis of this feature, which can be fitted with a Breit-Wigner-Fano line, offers a method for distinguishing between metallic and semiconducting single-walled carbon nanotubes.