Infrared Spectroscopy and X-ray Diffraction Characterization of Dimorphic Crystalline Structures of Polyethylenes with Halogens Placed at Equal Distance along the Backbone.
Polyethylenes with halogens placed on each and every 21st, 15th, or ninth backbone carbon display crystallization patterns enabled by the size of the halogen and by changing crystallization kinetics. The different structures have been identified from X-ray patterns combined with a detailed analysis of the infrared spectra of series containing F, Cl, or Br atoms that were either fast or isothermally crystallized from the melt. Under both crystallization modes, all specimens develop layered crystallites that accommodate 5-9 repeating units along the chain's axis. The size of the halogen and intermolecular staggering to maximize packing symmetry are responsible for striking structural differences observed between the series and between the two modes of crystallization. While the small size of the F atom causes a small perturbation to the crystal lattice and the orthorhombic structure is maintained for all members of the series either fast or isothermally crystallized, each Cl or Br-containing system presents dimorphism. Under fast crystallization, Cl and Br containing samples adopt the all-trans conformation (planar Form I), while in slowly crystallized samples gauche conformers set for bonds of the backbone carbons adjacent to the carbon with the halogen due to a close intermolecular staggering of halogens (herringbone Form II). In both forms the methylene sequence between halogens maintains the all-trans conformation. The structural details are extracted from the analysis of the C-halogen stretching region of the IR spectra, and from adherence to the n-alkane behavior of CH2 rocking, CH2 wagging, and C-C stretching progression modes.
Differences in the IR Methylene Rocking Bands between the Crystalline Fatty Acids and n-Alkanes: Frequencies, Intensities, and Correlation Splitting
Detailed low-temperature infrared spectra in the methylene rocking-twisting progression band region from 700 to 1000 cm -1 are presented for the C-phase crystalline fatty acids with even numbers of carbons from 16 through 22. There are significant differences between these spectra and those of the corresponding crystalline n-alkanes. The differences in band frequencies, intensities, and splitting are relevant to the interpretation of infrared spectra of complex assemblies of chain molecules such as biomembranes. They are due to chain end and chain-packing differences and can be accounted for using simple models developed in earlier studies, particularly those on the n-alkanes. Band frequency differences result from shifts imposed on the unperturbed frequency by the end groups. Shifts associated specifically with the methyl and acid end groups were estimated from the observed frequencies of the fatty acids, fatty diacids, and n-alkanes and unperturbed frequencies obtained from the dispersion curve for the infinite polymethylene chain. The shifts observed for rocking band frequencies are found to be the sums of the shifts assigned to the end groups. The differences in intensity and intensity distribution can be explained using a model in which the contribution of the individual methylenes to the dipole moment derivative associated with a given band is assumed to be the same except for the terminal methylenes. These methylenes are distinguished by the adjoining chain end group. The intensity differences between a fatty acid and an n-alkane occur because the contribution from the acid-end methylene is much greater than that from the other methylenes. The methylene bands in the spectra of the orthorhombic and monoclinic n-alkanes and C-form fatty acids are split because of interchain vibrational coupling. Their splitting patterns depend on the chain tilt angle. The three different patterns can be accurately reproduced using a simple coupled oscillator model, the different tilts, and three methylene -methylene interchain interaction constants.
Effects of the mineral phase and valence of vanadium on vanadium extraction from stone coal
The influence of roasting on the leaching rate and valence of vanadium was evaluated during vanadium extraction from stone coal. Vanadium in stone coal is hard to be leached and the leaching rate is less than 10% when the raw ore is leached by 4 mol/L H2SO4 at 90°C for 2 h. After the sample is roasted at 900°C for 2 h, the leaching rate of vanadium reaches the maximum, and more than 70% of vanadium can be leached. The crystal of vanadium-bearing mica minerals decomposes and the content of V(V) increases with the rise of roasting temperature from 600 to 900°C, therefore the leaching rate of vanadium increases significantly with the decomposition of the mica minerals. Some new phases, anorthite for example, form when the roasting temperature reaches 1000°C. A part of vanadium may be enwrapped in the sintered materials and newly formed phases, which may impede the oxidation of low valent vanadium and make the leaching rate of vanadium drop dramatically. The leaching rate of vanadium is not only determined by the valence state of vanadium but also controlled by the decomposition of vanadium-bearing minerals and the existence state of vanadium to a large extent.