Investigation of structural changes of silica particles during dehydration and chemosorption

Investigations have been made of changes in the structures of ultradisperse pyrogenic silica particles by small-angle X-ray scattering during dehydration and chemosorption. It has been established that during dehydration there are abrupt changes in the geometrical dimensions of the particles, caused by breakdown of coordination complexes between surface silicon atoms and water molecules.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 6, pp. 745–748, November–December, 1985.     

Investigation of structural changes in semi-crystalline polymers during deformation by synchrotron X-ray scattering

The mechanical behavior of polymer materials is strongly dependent on polymer structure and morphology of the material. The latter is determined mainly by processing and thermal history. Temperature-dependent on-line X-ray scattering during deformation enables the investigation of deformation processes, fatigue and failure of polymers. As an example, investigations on polypropylene are presented. By on-line X-ray scattering with synchrotron radiation, a time resolution in the order of seconds and a spatial resolution in the order of microns can be achieved. The characterization of the crystalline and amorphous phases as well as the study of cavitation processes were performed by simultaneous SAXS and WAXS. The results of scattering experiments are complemented by DSC measurements and SEM investigations. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1574–1586, 2010     

Preparation of polyboronphenylsiloxanes by mechanochemical activation

Linear polyboronphenylsiloxanes containing fluorine atom at silicon and acetylacetonate group at boron have been synthesized by mechanochemical activation. Mechanochemical activation of polyphenylsilsesquioxane and boron difluoride acetylacetonate taken in molar ratio 1 : 1 was shown to lead to the formation of polyboronphenylsiloxane with the given Si/B ratio. The products are characterized by gel chromatography, IR, NMR spectroscopy, elemental analysis, and X-ray phase analysis. The increase of the starting ratio Si/B enhances side processes and formation of polydisperse products with the Si/B ratio different from the desired one. The mechanism of the reaction is suggested.     

Morphological and structural development of hardwood cellulose during mechanochemical pretreatment in solid state through pan-milling

Mechanochemical pretreatment of hardwood cellulose was conducted by our self-designed pan-mill equipment which has an unique and smart structure and can exert strong shear forces and pressure on materials in between and break them down. The structure transformations, including particle size, powder morphology, molecular structure, crystalline structure during milling were investigated by Laser Diffraction Particle Size Analyzer, SEM, FT-IR and WAXD, respectively. Compared with standard method of ball-milling, the pan-mill shows a much higher efficiency in mechanochemical pretreatment of hardwood cellulose. The average particle size reduced to 21 μm and the specific surface area increased to 0.8 m²/g after 40 milling cycles. Mechanical milling also led to collapse of hydrogen bonds and reduction of crystallinity. The crystallinity index of cellulose powder decreased from its original 65 to 22, after milling for 40 cycles. Thermal analysis and solubility testing illustrated that pan-milled cellulose has lower thermal stability and higher solubility in aqueous alkali.     

Study of the structural changes in urea-formaldehyde condensates during synthesis

Chemical constitution of urea-formaldehyde resins in synthesis and in storage was determined using ¹³C NMR spectroscopy. Methylolation, methylene and dimethylene ether bond-forming reactions were discussed. The changes in the content of structural elements with secondary and tertiary amino groups of urea at various stages were followed. The resin synthesis technology was used, which considers the requirement in low free formaldehyde content, connected with somewhat smaller storage stability of resins.     

Investigation of structural changes in wool fibers due to annealing

Structural changes in wool fibers after annealing, i.e., storage at 45°C and 80% RH, have been investigated by using the method of laser Raman spectroscopy and by measuring birefringence and density of wool. The results show that CH₂ groups in wool give parallel Raman dichroism which increases very significantly after annealing. No increase in birefringence and density occurred after annealing. A hypothesis has been put forward which states that for minimum energy considerations in wool fibers the existence of intrahelix crosslinks other than hydrogen bonds is important. It is highly probable that a large number of electrovalent intrahelix crosslinkages exist between the suitable side groups on the main polypeptide chain in the α-helix of annealed wool.     

Oxidation of metallic iron with oxygen via mechanochemical activation

The X-ray phase analysis and X-ray diffraction, as well as IR and Mössbauer spectroscopic techniques were employed to study mechanochemical oxidation of iron metal powder with oxygen. The phase composition of the products of mechanochemical activation of iron in a vibrating mill was determined. The extent of oxidation of iron was examined in relation to the particle-size distribution of the starting iron powder.     

The evolution of structural changes in ettringite during thermal decomposition

The thermal decomposition of ettringite, Ca₆[Al(OH)₆]₂(SO₄)₃·∼26H₂O, was studied with pulsed neutron time-of-flight diffraction combined with Rietveld structure refinement. Like prior investigations, transition from a crystalline to amorphous state occurred following the loss of ∼20 water molecules. In contrast to earlier investigations, which relied upon indirect measurements of water and hydroxyl occupancies, the present study inferred the occupancies directly from Rietveld crystal structure refinement of the diffraction data. The decomposition pathway was shown to be more complex than previously envisioned, involving the simultaneous loss of hydroxyl and water molecules. Nuclear magnetic resonance (NMR) spectroscopy studies of the rigid lattice lineshapes of fully and partially hydrated ettringite were performed and confirmed our decomposition model.     

Kinetics of kaolinite dehydration and its dependence on mechanochemical activation

For the mechanochemical activation of two processes can be considered: the structural disordering of laminar crystals, and the formation of molecular dense aggregates from small particles. The activation energy of the dehydration is shown to decrease if the first process predominates, and vice versa. Therefore, to study and compare the kinetics of dehydration of different kaolinites the whole complex of properties the initial samples should be taken into account: the structure defectness, the particle size, particle-packing into aggregates, and the size and density of the aggregates.

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Zusammenfassung Bei der mechanochemischen Aktivierung von Kaolinit sind zwei Prozesse zu betrachten: Die strukturelle Unordnung laminarer Kristalle und die Bildung molekular dichter Aggregate aus kleineren Teilchen. Es wird gezeigt, dass die Aktivierungsenergie der Entwässerung abnimmt, wenn der erste Prozess vorherrscht, und zunimmt, wenn der zweite Prozess überwiegt. Deshalb muss bei vergleichenden Untersuchungen zur Entwässerung verschiedener Kaolinite der Gesamtkomplex der Eigenschaften der Ausgangsproben berücksichtigt werden, also die strukturelle Perfektion, Teilchengrösse, Vereinigung von Teilchen zu Aggregaten sowie Grösse und Dichte der Aggregate.

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In situ monitoring of structural changes during colloidal self-assembly

Reflectance spectroscopy is utilized to monitor structural changes during the self-assembly of a monodisperse colloidal system at the meniscus of a sessile drop on an inert substrate. Treating the ordered colloidal structure as a photonic crystal is equivalent to monitoring the changes in the photonic band gap (PBG) as the colloidal system self-assembles heterogeneously into a crystal through solvent evaporation in ambient conditions. Using a modified Bragg's law model of the photonic crystal, we can trace the structural evolution of the self-assembling colloidal system. After a certain induction period, a face-centered cubic (FCC) structure emerges, albeit with a lattice parameter larger than that of a true close-packed structure. This FCC structure is maintained while the lattice parameter shrinks continuously with further increase in the colloidal concentration due to drying. When the structure reaches a lattice parameter 1.09 times the size of that of a true close-packed structure, it undergoes an abrupt decrease in lattice spacing, apparently similar to those reported for lattice-distortive martensitic transformations. This abrupt final lattice shrinkage agrees well with the estimated Debye screening length of the electric double layer of charged colloids and could be the fundamental reason behind the cracking commonly seen in colloidal crystals.     

Investigation on the structural variation of Co-Cu nanoparticles during the annealing process

This study uses molecular dynamics simulations to investigate the crystalline process of Co-Cu nanoparticles of high and low Co concentrations (5 and 25%) during the annealing process. The modified many-body tight-binding potential involving magnetic contribution is adopted to accurately model the Cu-Cu, Co-Co, and Co-Cu pair interactions. The Co-Co bond length increases, while the Co-Cu bond length decreases as the temperature gradually drops from 2000 K to the upper melting point. During that process, the Cu-Cu bond length remains constant and the value of the first peak of the radial distribution function (RDF) increases, which indicates that Cu atoms increase their short-range order by mutual rearrangement. At temperatures lower than the upper melting point, the bond length of each pair decreases while the value of the first peak increases as the temperature is continuously reduced. Because the kinetic energy of an individual atom is not enough for rearrangement, the variations of bond length and the first RDF peak can be attributed to the shrinking effect.     

Infrared technique for the measurement of structural changes during the orientation process in polymers

Polarized infrared measurements were made on polymer samples to obtain the structural changes occurring during the orientation process. The absorbances of the infrared bands were measured by determining the three components of the absorbance. Two components were obtained directly with plane-polarized light while the third is obtained by tilting the sample and extrapolating. Corrections were made for machine optics polarization, sample birefringence, polarizer inefficiency, anisotropy of the index of refraction, and scattering from the film surface. Data are reported for polyethylene obtained from cold-drawn specimens as a function of draw temperature. Polyethylene exhibits no strain-induced crystallization as a result of the chain-alignment process. Annealing of the drawn samples reperfects the distorted crystals.     

Changes in the composition and coagulation ability of humic acids after mechanochemical activation of peat

The coagulation ability of humic acids (HAs) obtained from mechanically activated high-moor peat has been investigated. The effects of the concentration and the methods of isolation of HAs from peat on the reversible aggregation of blood erythrocytes have been studied. The strength of aggregates of erythrocytes and the period of their aggregation have been found to depend on the concentration and composition of HAs.     

Modeling the effect of structural changes during dynamic separation processes on MOFs

A model able to describe the effect of structural changes in the adsorbent or adsorbed phase during the dynamic (breakthrough) separation of mixtures on metal-organic frameworks (MOFs) is presented. The methodology is exemplified for a few pertinent case studies: the separation of xylene isomers and ethylbenzene on the flexible MOF MIL-53 and the rigid MOF MIL-47. At low pressures, no preferential adsorption of any component occurs on both MOFs. Contrarily, at higher pressures separation of ethylbenzene (EB) from o-xylene (oX) occurs on MIL-53 as a result of the breathing phenomenon within the MIL-53 structure. The increase in selectivity, starting from the gate-opening pressure, could be modeled by using a pressure-dependent saturation capacity for the most strongly adsorbed component oX. In the separation of m-xylene (mX) from p-xylene (pX) on the rigid MOF MIL-47, separation at higher pressures is a result of preferential stacking of pX. Here, the selectivity increases once the adsorption of pX switches from a single to a double file adsorption. By implementing a loading dependent adsorption constant for pX, the different unconventional breakthrough profiles and the observed selectivity profile on MIL-47 can be simulated. A similar methodology was used for the separation of EB from pX on MIL-47, where the separation is a result from steric constraints imposed onto the adsorption of EB.     

Two-beam interference detection of the changes in fibre structural parameters during low drawing process

A Pluta polarizing interference microscope with a fibre stretching device attached was used to detect the changes in molecular structure that occur during the fibre cold drawing process. Fibres drawn with low draw ratios show different types of deformation mechanisms. The interference patterns recorded at different draw ratios are used to relate the deformation mechanisms with the measured structural parameters. Some optical parameters are measured such as; refractive indices nand n birefringence n, the polarizability per unit volume , the orientation distribution function f(θ), the angle θ between the stretching direction and the chain axis, and the angle θ_m which the transition dipole moment of the molecular species makes with the molecular axis (chain axis). Three polymeric fibres (Polyethylene terephthalate PET, Polypropylene PP and Polyamide PA) were used in this study and their interferograms are illustrated.     

The effect of mechanochemical activation upon the intercalation of a high-defect kaolinite with formamide

The effect of mechanochemical activation upon the intercalation of formamide into a high-defect kaolinite has been studied using a combination of X-ray diffraction, thermal analysis, and DRIFT spectroscopy. X-ray diffraction shows that the intensity of the d(001) spacing decreases with grinding time and that the intercalated high-defect kaolinite expands to 10.2 A. The intensity of the peak of the expanded phase of the formamide-intercalated kaolinite decreases with grinding time. Thermal analysis reveals that the evolution temperature of the adsorbed formamide and loss of the inserting molecule increases with increased grinding time. The temperature of the dehydroxylation of the formamide-intercalated high-defect kaolinite decreases from 495 to 470 degrees C with mechanochemical activation. Changes in the surface structure of the mechanochemically activated formamide-intercalated high-defect kaolinite were followed by DRIFT spectroscopy. Fundamentally the intensity of the high-defect kaolinite hydroxyl stretching bands decreases exponentially with grinding time and simultaneously the intensity of the bands attributed to the OH stretching vibrations of water increased. It is proposed that the mechanochemical activation of the high-defect kaolinite caused the conversion of the hydroxyls to water which coordinates the kaolinite surface. Significant changes in the infrared bands assigned to the hydroxyl deformation and amide stretching and bending modes were observed. The intensity decrease of these bands was exponentially related to the grinding time. The position of the amide C=O vibrational mode was found to be sensitive to grinding time. The effect of mechanochemical activation of the high-defect kaolinite reduces the capacity of the kaolinite to be intercalated with formamide.     

Effect of mechanochemical modification on the surfactant and structural properties of humic and himatomelanic acids

The structural properties of humic and himatomelanic acids are studied by means of ¹Н NMR spectroscopy and differential thermal analysis after mechanochemical modification of peat. The relationship between the structural modification of humic and himatomelanic acids and their surfactant properties in aqueous solutions is established. It is shown that the critical micelle concentration of transformed himatomelanic acids is halved in comparison to the initial sample, while the adsorption equilibrium constant grows by 9 times.