Rheological behavior of precursor PPV monolayers

The rheological behavior of different precursor poly(p-phenylene vinylene) (prec-PPV) monolayers at the air-water interface was investigated using an interfacial stress rheometer (ISR). This device nicely reveals a transition of the precursor poly(2,5-dimethoxy-1,4 phenylene vinylene) (prec-DMePPV) monolayer from Newtonian to elastic behavior with increasing surface pressure. The transition is accompanied by an increase in the modulus. This behavior coincides with the coagulation of different 2D condensed domains as revealed by Brewster angle microscopy (BAM). However, partly converted prec-DMePPV monolayers show elastic behavior even at low surface pressures, although a sudden increase of the moduli does occur. This phenomenon is attributed to enhanced hydrophobic interactions between the conjugated moieties in the partly converted polymers. The latter also explains the stretching behavior of the partly converted prec-DMePPV upon transfer in Langmuir-Blodgett-type vertical dipping. The increase of the moduli which is observed is much more gradual in the precursor poly(2,5-dibutoxy-1,4-phenylene vinylene), prec-DBuPPV, a monolayer which is in agreement with the expected expanded state of the latter monolayer.     

Rheological behavior of surfactant-based precursors of silica mesoporous materials

The linear and non-linear viscoelastic behaviors of polymer-like micellar solutions of cetyltrimethylammonium tosilate (CTAT) with added NaOH and tetraethyl orthosilicate (TEOS) to produce precursors of mesoporous materials are studied. The effect of TEOS/CTAT (T/C) ratio at fixed CTAT concentration, CTAT concentration at fixed T/C and aging time are reported. The systems show increasingly larger deviations from near-Maxwell behavior upon increasing T/C ratio, CTAT concentration and aging. Moreover, in steady and unsteady shear-flow, shear banding develops between two critical shear rates, which tend to fade as the T/C ratio and aging increase. The Granek-Cates model is employed to analyze linear viscoelastic behavior. The Bautista-Manero-Puig (BMP) model is used here to reproduce the steady and transient nonlinear rheology of these systems. We explain these results in terms of the changes in inter-macromolecular interactions that arise out of the presence of colloidal additives in the viscoelastic gel. The ordered mesoporous materials were identified by X-ray diffractometry (XRD) and high-resolution transmission electron microscopy.     

Methyltriskaidecazirconates, molecular forms of zirconia

Repeated methanolysis of [Zr(3)O](OPr(n)(10) followed by extraction and crystallization from toluene yields material that is X-ray crystallographically indistinguishable from the compound previously formulated as [Zr(13)O(8)](OMe)(36). Elemental analysis and (1)H solution NMR spectroscopy strongly suggest that this material is a mixture of methyltriskaidecazirconates (MTZ) [Zr(13)O(8)](OMe)(x)(OH)(36)(-)(x), x(av) approximately 20, that readily cocrystallize from hydrocarbon solution. These species have the metal-oxygen framework structure reported for [Zr(13)O(8)](OMe)(36), where the 13 zirconium and 32 bridging oxygen atoms comprise a fragment of the fluorite structure adopted by ZrO(2) at elevated temperatures. Ethanolysis of [Zr(3)O](OPr(n)(10) yields its ethyl analogue, [Zr(3)O](OEt)(10). Both trizirconates display temperature-dependent (1)H solution NMR spectra that are interpreted mechanistically in terms of rearrangement mechanisms involving trigonal twists at the octahedral zirconium centers.     

Experimental and theoretical investigation on the correlation between aqueous precursors structure and crystalline phases of zirconia

Nanometer zirconia powders were prepared by the precipitation method at different pHs and different reaction temperatures. X-ray results show that monoclinic zirconia is favored at pH 4 while tetragonal zirconia is favored at pH 9.5 at room temperature, and monoclinic zirconia is also favored at pH 9.5 and 70 °C reaction temperature, with the slow addition of alkali. Four models of zirconium complexes were applied to simulate the structural monomers in different pH solutions. Geometric parameters and Mulliken charge population were calculated by optimizing these complexes using the density functional theory (DFT/B3LYP). Theoretical analyses show that if Model I ([Zr(OH)₂(H₂O)₄]²⁺ monomers) is favored in the aqueous precursor solution, it will be preferentially polymerized into monoclinic precursor structure irrespective of slow or quick alkali addition. Contrarily, if Model IV ([Zr(OH)₇]³⁻ monomers) is major in the aqueous precursor solution, tetragonal precursor structures are favored irrespective of slow or quick alkali addition. When Model II ([Zr(OH)₄(H₂O)₂]⁰ monomers) and Model III ([Zr(OH)₆]²⁻ monomers), respectively, predominate in the aqueous precursor solution, they will be preferentially polymerized into tetragonal precursor structure at slow alkali addition, however, for quick alkali addition, they will be preferentially polymerized into monoclinic precursor structure. Our theoretical models well explain the present experimental results as well as previous experimental results, and allow building up a correlation between aqueous precursor structures and crystalline phases of zirconia.     

Enhanced adsorption of arsenate on the aminated fibers: sorption behavior and uptake mechanism

Novel aminated polyacrylonitrile fibers (APANFs) were prepared through the reaction of polyacrylonitrile fibers (PANFs) with four multinitrogen-containing aminating reagents, and the best adsorbent was obtained after the optimization of preparation experiments. The APANFs were effective for arsenate removal from aqueous solution, and the sorption behaviors including kinetics, isotherms, effect of pH, and competitive anions were investigated. Experimental results show that the equilibrium of arsenate sorption on the fibers was achieved within 1 h, and Langmuir equation described the sorption isotherms well with a high sorption capacity of 256.1 mg/g obtained. The thermodynamic parameters calculated show that the sorption was spontaneous and exothermic under the condition applied. The zero point of zeta potential of the APANFs was at about pH = 8.2, in contrast with that of the PANFs at pH = 3.6. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) for the APANFs before and after arsenate adsorption revealed that the amine groups on the fiber surface played an important role in the removal of arsenate from water, attributed to the electrostatic interaction between the positive protonated amine groups and negative arsenate ions.     

Synthesis and physicochemical properties of biomorphic zirconia fibers

Biomorphic zirconia fibers were prepared by successive carbonization and/or calcining of sawdust impregnated by a solution of zirconium oxynitrate. Pyrolysis was performed in nitrogen (500°C), and calcining, in air (600°C). The physicochemical characteristics of samples were studied by adsorption measurements, electron microscopy, and X-ray diffraction. The biomorphic fibers were composed of zirconia nanoparticles not larger than 12 nm. The samples had a uniform phase composition dominated by the tetragonal ZrO₂ phase; their specific surface area was 13–38 m²/g depending on the salt content in sawdust. It was assumed that the stabilization of the tetragonal phase could be related to the incorporation of mineral components (calcium, magnesium, and potassium compounds) of sawdust into zirconia; carbonization had no substantial effect on the properties of the resulting oxide. The method developed could be used to obtain tetragonal zirconia (without expensive reagents and water consumption) and utilize wood industry wastes.     

Controlling the growth of zirconia needles precursor from a liquid crystal template

This work reports the directional growth of macroscopic ceramic needles by combining a thermo-responsive sulfated hydroxy zirconyl (SHZ) hydrosol with a swollen hexagonal liquid crystal (SHLC). The effect of the molar ratio R_s = [Zr⁴⁺]/[SO₄^2?] on the thermo-stability of both the SHZ hydrosol and the SHLC, and also on the nucleation and 1D-growth of ceramic particles was investigated. The dynamics of the thermo-induced aggregation growth process was monitored in situ by rheological property measurements. The results show that by increasing the R_s, the kinetic stability of the system in the sol state increases, leading to the formation of a less branched framework during the thermo-induced aggregation process. Furthermore, the small-angle X-ray diffraction (XRD) results show that the thermo-stability of the SHLC employed as soft template is improved by increasing the R_s. Both effects allow us to induce the particle's nucleation by heating the SHLC template to a moderate temperature (≈50 °C), while the kinetics of the needle's growth is enhanced by increasing the [Zr⁴⁺]/[SO₄^2?] molar ratio.     

Superoxide radical anions on the surface of zirconia and sulfated zirconia: formation mechanisms, properties and structure

In situ ESR spectroscopy has been used for direct comparison of different thermal and light-induced processes leading to generation of superoxide radical anions on the surface of various zirconia and sulfated zirconia materials. For materials of both types the magnetic resonance parameters of the radical anions were found to be practically independent of the generation method, except for oxygen coadsorption with NO that yields radicals with somewhat smaller gz values. The parameters appear to depend mostly on the state of the surface zirconia cations stabilizing the radical anions, so that the g tensor anisotropy is significantly smaller over sulfated zirconia. It is shown that light-induced formation of superoxide radical anions in the presence of coadsorbed hydrocarbons can be initiated with visible light. Original SIET reaction mechanisms are suggested for the formation of superoxide radical anions by coadsorption with hydrocarbons and illumination after such coadsorption to extend the previously known ones to account for the observed phenomena. Cluster model DFT calculations of magnetic resonance parameters of O2- radical anions stabilized on the surface of zirconium dioxide showed that the adsorption complexes have a -shape rather than linear structure. The magnetic resonance parameters obtained by calculations practically match experimental data and adequately describe their changes after the surface modification with sulfates.     

Applications of titania and zirconia hollow fibers in sorptive microextraction of N,N-dimethylacetamide from water sample

The convenient fabrications of titania and zirconia hollow fiber with three-dimensional porous structure using polypropylene hollow fibers as templates were developed. And an analytical method based on enrichment and extraction of analytes in the water sample, hollow fiber sorptive microextraction in combined with gas chromatography has been developed for the rapid analysis of N,N-dimethylacetamide (DMA) in the environmental samples. The results showed that zirconia hollow fiber gave higher extraction performance of DMA than that of titania hollow fiber. The method validations, including linearity, limit of detection, limit of qualification, precision, and repeatability were investigated. Linearity for six-point calibration curve was excellent with zirconia hollow fiber having r² value greater than 0.9993 at the linearity range of 0.001-1.0 mg mL⁻¹. In addition, it seems that hollow fiber sorptive extraction is a promising technique for the enrichment and purification of analytes extracted directly from liquid samples without any other pretreatment.     

Anodic zirconia nanotubes: composition and growth mechanism

The compositions of porous anodic films formed on zirconium in glycerol/fluoride electrolytes are examined prior to and following ageing in the electrolyte. The initial films are shown to contain significant amounts of fluorine, with an F:O atomic ratio between 1.0 and 2.5, dependent on the water content of the electrolyte. A three-layered film morphology is indicated, which is related to the migration rates of film species, incorporating a fluoride-rich layer at cell boundaries and bases, and outer layers compositionally differentiated by the presence of carbon species. Preferential loss of fluorine during ageing suggests degradation of the fluoride-rich material.     

Deposition of refractory coatings on carbon fibers from volatile Hf-containing precursors

The possibility to modify the carbon fiber surface by refractory hafnium compounds via chemical vapor deposition of volatile metal-organic complexes is studied. Thermodynamic modeling is applied to calculate the CVD diagram of the Hf-C-O-F-H system within a 25–1200°C temperature range, 10⁻¹–10 Torr pressure range, and 0–2000 hydrogen/precursor vapor ratio. It is demonstrated that the deposition should be carried out at the maximum hydrogen flow and the minimum total pressure in the system. SEM and AFM techniques are used to examine the morphology and topography of the coatings on the fibers. Continuous coatings that are uniform in length and diameter of monofilaments and are tightly bound to them are obtained.     

季戊四醇四硝酸酯的分子结构和热解机理

运用SCF-AM1-MO方法，在RHF和UHF水平上分别全优化计算了季戊四醇四硝酸酯(太安)的分子结构和热解机理。计算所得三种构象的能量相差很小、且相互转化的能垒也较小。其均裂O-NO~2键(生成双自由基)的热解引发反应活化能较小，而通过α-H转移产生RCHO和HONO的环消除反应需要较高活化能。     

Diplatinum(ii)-coordinated polyoxotungstate: synthesis, molecular structure, and photocatalytic performance for hydrogen evolution from water under visible-light irradiation

The synthesis and molecular structure of a monomeric diplatinum(ii) complex composed of mono-lacunary α-Keggin polyoxometalate is described. The polyoxometalate, Cs(3)[α-PW(11)O(39){cis-Pt(NH(3))(2)}(2)]·8H(2)O (Cs-1), afforded by a stoichiometric reaction of mono-lacunary Keggin polyoxotungstate with cis-diamminedichloroplatinum(ii) in water, followed by crystallization from water, was obtained as analytically pure, homogeneous, yellow crystals. The compound Cs-1 was characterized by elemental analysis, thermogravimetric/differential thermal analysis (TG/DTA), Fourier transform infrared (FTIR) and UV-visible spectroscopy, solution (1)H and (31)P nuclear magnetic resonance (NMR), and X-ray crystallography. The single-crystal X-ray structure analysis revealed that the two cis-platinum(ii) moieties, [cis-Pt(NH(3))(2)](2+), were coordinated each to two oxygen atoms in a mono-vacant site of [α-PW(11)O(39)](7-) with asymmetric configuration, resulting in an overall C(1) symmetry. Furthermore, hydrogen evolution from an EDTA·2Na (ethylenediamine tetraacetic acid disodium salt) aqueous solution under visible-light irradiation (≥400 nm) was achieved by using polyoxoanion 1 and titanium dioxide.     

Influence of silver nano‐additive amount on the supramolecular structure,porosity, and properties of polyacrylonitrile precursor fibers

This work examines the influence of the amount of silver nanoparticles added to polyacrylonitrile spinning solutions on their rheological properties as well as the structure and properties of the fibers produced. The influence of the amount of silver nanoparticles on the supramolecular structure of nanocomposite polyacrylonitrile precursor fibers, their porosity, as well as thermal and tensile strength properties was determined. The distribution of the nano‐ additive in fiber cross‐sections and on the surface was estimated. It was found that the addition of silver nanoparticles to polyacrylonitrile precursor fibers in an amount of up to 1.5% does not cause a decrease in the susceptibility of the fiber matter to deformation at the drawing stage. The produced fibers were characterized by an increased total volume of pores of 0.35 cm³/g and tenacity of more than 34 cN/tex. Copyright © 2009 John Wiley & Sons, Ltd.     

Alkali treatment of lignocellulosic fibers extracted from sugarcane bagasse: Composition,structure, properties

Lignocellulosic fibers extracted from sugarcane bagasse were treated with NaOH solutions of different concentration (0-40 wt%) to study the effect of alkali treatment on the composition, structure and properties of the fibers. Composition was determined by the van Soest method, structure was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), while mechanical properties by tensile testing. Hemicellulose and lignin content decrease, while cellulose content goes through a maximum as a function of alkali concentration. Crystallinity changes only slightly and microfibril angle (MFA) remains constant thus structural effects and especially MFA are not the primary reasons for changing properties. The Young's modulus of the fibers shows a slight maximum at around 2-4 wt% NaOH content, while tensile strength goes through a much more pronounced one at around 5-8 wt%. Direct correlation between structure and mechanical properties was not found indicating that composition is more important in the determination of properties than structure. Regression analysis proved that the combination of several compositional variables determines mechanical properties in a non-linear manner. The improvement in fiber properties was explained with the dissolution of weak amorphous fractions and the relative increase of cellulose content.     

核酸前体及其修饰结构的电子转移机理研究

应用荧光淬灭和激光光解瞬态吸收光谱技术研究了一系列核酸前体(核酸碱基、核苷及其结构修饰物)、小牛胸腺ctDNA与各种荧光探针及蛋白酶之间的瞬态、稳态电子转移作用机理。测定结果表明,它们的稳态、静态荧光淬灭作用很强,很好地符合Stern-Volmer线性方程,淬灭速率常数,k~q(s)和k~q(d),达10^10.M^-^1S^-^1,属于扩散控制,表明核酸前体的基态可作为电子受体或给体而分别与含色氨酸残基的蛋白酶、受电子型荧光探针之间发生具有电子转移性质的相互作用。对鸟嘌呤的结构修饰物进行了激光光解的瞬态吸收光谱研究,检测了几类活性中间体,论证了激发态的光致电子转移和能量转移机理。     

Synthesis of nanoscale NbSe2 materials from molecular precursors

This communication describes a new synthetic approach to one- (1D) and two-dimensional (2D) NbSe2 nanoscale materials using soft chemical methods. Our one-pot synthesis provides a direct route to control the morphology of nanostructures that can exhibit complex electronic properties, and can produce layered, nanocrystalline materials in high yield.     

Rheological characterization, crystallization, and gelation behavior of monoglyceride gels

Saturated monoglycerides can form firm gels in water. These gels are networks of stiff plate-like beta-crystals of monoglycerides (a "cardhouse"), grown from a space-filling lamellar liquid-crystalline phase. The molecular mechanism of crystallization is discussed in the light of network formation. The concentration dependence of gel development of (shear-cooled) monoglyceride gels has been studied by rheology. A gelation mechanism has been proposed, consisting of two steps: (i) After formation of a nucleus, rapid crystallization in a lateral direction occurs (probably within one bilayer) by which the first space-filling network is formed. (ii) This is followed by reinforcement of the network by which stacks of crystalline bilayers are formed. The plate-like crystals are linked in connective domains or junction zones, probably containing all the material (cosurfactants, diglycerides, etc.) that does not fit in the crystalline array. Small deformation rheology shows that above about 2 wt% monoglyceride a percolating network is formed. The large deformation rheology is typical for a particle gel with a relatively small strain at failure (both in shear deformation and compression). The connective domains or junction zones already fail when relatively small deformations are put on the system.     

Thermal behavior of silicophosphate gels obtained from different precursors

Phosphosilicate glasses are of great interest in important fields, such as optical active systems, energy generating systems, humidity sensors, and as materials for biomedical applications. Many studies were accomplished to establish the influence of different reaction parameters on the evolution and final structure of sol–gel prepared phosphosilicate gels. In the present work, we studied the thermal behavior of the silicophosphate gels obtained starting with different phosphorous precursors, the influence of these precursors on the composition and structure of the resultant gels, and their evolution with thermal treatment. By Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and thermodifferential analysis (DTA/TG), and Differential Scanning calorimetry coupled with Mass Spectrometry (DSC-MS), it was established that the type of the precursors essentially influences the composition and structure and consequently the thermal behavior of the obtained gels. In the case of triethylphosphate precursor, all used methods of investigation have shown that the ester is trapped in the silica matrix and it is eliminated during the thermal treatment. Triethylphosphite partially hydrolizes and reacts with the silica network during post-preparation thermal treatment. Only in the case of H₃PO₄, an interaction with TEOS takes place and leads to Si–O–P bond formation. By thermal treatment, the gels with different composition and structure lead to materials with different properties.     

Effects of different ultrahigh molecular weight polyethylene contents on the formation and evolution of hierarchical crystal structure of high-density polyethylene/ultrahigh molecular weight polyethylene blend fibers

In this paper, the blend fibers of ultrahigh molecular weight polyethylene (UHMWPE) and high-density polyethylene (HDPE) were prepared by solution blending and gel spinning process. The uniformity of the blend fibers has been confirmed by rheological data and thermodynamic unimodal curve. They were further characterized by single fiber strength test, scanning electron microscopy, wide-angle X-ray diffraction, small-angle X-ray scattering, and so forth, to explore the structural evolution mechanism with the change of UHMWPE content. The results showed that when the molar content of UHMWPE was only 2.9 mol%, entanglement appeared in the structure of shish-kebab, and when the proportion reached 20 mol%, an interlocking structure could be observed. With the increase of UHMWPE content, kebab began to be networked, and when the content reached 33 mol%, kebab's orientation reached its peak. After that, the interlocking network structure gradually improved. When the content reached 50 mol%, the shish's orientation reached saturation, and the shish-kebab network became perfect. In addition, with the increase of UHMWPE content, stress-induced recrystallization occurred on the wafer, some kebab would be converted into shish crystals, and when the content exceeded 50 mol%, the microfibers began to merge, and the wafer became denser, but still had entanglements. Our work has proposed a quantitative explanation for the evolution of hierarchical crystal structure of HDPE/UHMWPE blend fibers.