X-ray investigations concerning the physical structure of crosslinking in urethane elastomers. III. Common structure principles for extensions with aliphatic diamines and diols

Physical crosslinking of urethane elastomers is essentially based on statistical systems of hydrogen bonds within the hard segment domains. Two dimensional models of these systems, given in previous papers, are now replaced by three-dimensional models in the case of butane diol and pentane diol extenders, respectively. A suggestion is made for the transverse crosslinking mechanism with diamine extenders. Taking into account the conditions for three-dimensional hydrogen bonded systems a correlation between the cross-linking structure and heat-distortion temperature of diamine and diol extended elastomers is obtained.     

The level of cross-linking and the structure of anisotropic magnetorheological elastomers

The influence of the concentration of the magnetic powder on the level of cross-linking of magnetorheological elastomers (MREs) has been studied. Afterwards the structural characterisation of manufactured MREs has been performed by using non-destructive method, specifically the computed tomography. The correlation between internal structures of MREs and the developing of its cross-linking level during the curing was found. It was shown that changes in the concentration of the powder significantly affect morphologies of the sample.     

Oxygen sites in the zeolite stilbite: a comparison of static, MAS, VAS, DAS and triple quantum MAS NMR techniques

Static, magic angle spinning (MAS), variable angle spinning (VAS), dynamic angle spinning (DAS) and triple quantum magic angle spinning (3QMAS) NMR techniques were applied to separate and quantify oxygen signals from Al–O–Si and Si–O–Si sites of ¹⁷O-enriched samples of the mineral stilbite, a natural zeolite. DAS experiments showed that there was a distribution of quadrupolar coupling constants, asymmetry parameters and isotropic chemical shifts. Two methods were used to study the quantification problem of DAS and 3QMAS. Our results showed that DAS was quantitative. In 3QMAS, signal intensity from sites with larger quadrupolar coupling constants was reduced because of less efficient excitation. All techniques have shown a clear difference in rates of exchange between the two types of sites with interchannel H₂O molecules.     

First-principles investigations of the structure and physical properties for new TcN crystal structure

Using a newly developed particle swarm optimisation technique on crystal structural prediction, we have predicted an orthorhombic Imm2 structure for TcN crystal, which is energetically much superior to the previously proposed NbO-type and R-3m structures. The new phase is stable against decomposition into the mixture of Tc and N at ambient condition. The stability is confirmed by the subsequent calculations on the phonon dispersion curves and elastic constants. An analysis of the mechanical properties indicates that Imm2-TcN is incompressible and common hard material. The evidence of strong covalent bonding of Tc-N, which plays a leading role to form a hard material, is manifested by the partial densities of state analysis. In addition, the thermodynamic properties, such as Debye temperature, heat capacity, thermal expansion coefficient, and Grüneisen parameter of TcN are investigated by the quasi-harmonic Debye model.     

The effect of polystyrene-block-poly(4-vinylpyridine) prepared by a RAFT method in the dispersion polymerization of styrene

The dispersion polymerization of styrene has been carried out using polystyrene-block-poly(4-vinylpyridine) copolymer [P(S-b-4VP)], prepared by a reversible addition-fragmentation chain transfer (RAFT) method, as a steric stabilizer in alcohol media. These block copolymer contains a long poly(4-vinylpyridine) block and a short polystyrene block. The stable spherical particles were obtained when the block copolymer concentrations increased from 2 to 20 wt.% relative to the monomer and the average particle sizes decreased from 340 to 200 nm with increasing concentration of the block copolymer. Alcoholic solvents, from methanol to n-hexanol, are responsible for the particle size. These results indicate that the poly(S-b-4VP) block copolymer is effective for providing polystyrene nano-sized particles with a low content of it working as a good stabilizer in any kind of alcoholic medium.     

NMR relaxation and NMR imaging of elastomers in the course of thermal aging

A complex aging regime occurs in the course of thermal aging of elastomers. Depending on the type and the content of the rubber filler materials, temperature, chemical environment (normally air), and time, a different aging process can be observed also by nuclear magnetic resonance (NMR) [1–6]. The methods used are the common spin-echo ¹H-NMR, including variable echo times and parameter-selective NMR-¹H-imaging (material properties imaging). The decay of the echo-magnetization is discussed on the basis of a single-chain model with a distribution of dipolar interactions. This model is based on the influence of a very fast, but anisotropic, local motion, as well as larger and slower motions, which are able to diminish the residual dipolar interaction. Carbon-black-filled natural rubber, as well as silica and carbon-black-filled E-SBR (emulsion-polymerized styrene butadiene rubber) and S-SBR (solution-polymerized SBR) are the systems under investigation, with the results showing some characteristic features of the course of aging observable by NMR.     

Theoretical investigations of the hydrogen bond in a tetraamido/diamino quaternized macrocycle

ABSTRACT

We present theoretical investigations on the nature and persistence of the strong central hydrogen bond of a tetraamido/diamino quaternized macrocycle. Our theoretical study of the NMR properties of the central hydrogen bond proved difficult to reconcile with the available experimental results, suggesting the possibility of the non-persistence of the central hydrogen bond of the macrocycle in solution. We demonstrate alternative scenarios, in which a tautomerization of the macrocycle in solution gives rise to the experimentally observed NMR shifts.     

14N NMR relaxation times of several protein amino acids in aqueous solution--comparison with 17O NMR data and estimation of the relative hydration numbers in the cationic and zwitterionic forms

The 14N nuclear magnetic resonance (NMR) linewidths of the alpha-amino groups of several protein amino acids were measured in aqueous solution, with and without composite proton decoupling, to estimate the effect of proton exchange and molecular weight on the linewidths. It is shown that, contrary to earlier claims, the increase in the linewidth at low pH is not exclusively due to the effect of proton exchange broadening. The 14N linewidths, under composite proton decoupling, increase with the bulk of the amino acid, and increase at low pH. Statistical treatment of the experimental 14N and literature 17O NMR data was performed assuming two models: (i) an isotropic molecular reorientation of a rigid sphere in a medium of viscosity eta, (ii) a stochastic diffusion of the amino and carboxyl groups comprising contributions from internal (tauint) and overall (taumol) motions. Assuming a single correlation time from overall molecular reorientation (taumol), then, a linear correlation was found between the linewidths and the molecular weights of the protein amino acids at the pH values 0.5 and 6.0, which are characteristic of the cationic and zwitterionic forms, respectively. The slopes of the straight-lines were found to be dependent of pH for 14N, contrary to the 17O linear correlations whose slopes were found to be independent of pH. Assuming effective correlation times of the amino and carboxyl groups, which comprise contributions from the internal (tauint) and overall (taumol) motions, then, a significant improvement of the statistics of the regression analysis was observed. The 14N relaxation data, in conjunction with 17O NMR linewidths, can be interpreted by assuming that the 14N quadrupole coupling constants (NQCCs) are influenced by the protonation state of the carboxyl group, the 17O NQCCs remain constant, and the cationic form of the amino acids is hydrated by an excess of 1-3 molecules of water relative to the zwitterionic state.     

MAS NMR spectra of quadrupolar nuclei in disordered solids: the Czjzek model

Structural disorder at the scale of two to three atomic positions around the probe nucleus results in variations of the EFG and thus in a distribution of the quadrupolar interaction. This distribution is at the origin of the lineshape tailing toward high fields which is often observed in the MAS NMR spectra of quadrupolar nuclei in disordered solids. The Czjzek model provides an analytical expression for the joint distribution of the NMR quadrupolar parameters upsilon(Q) and eta from which a lineshape can be predicted. This model is derived from the Central Limit Theorem and the statistical isotropy inherent to disorder. It is thus applicable to a wide range of materials as we have illustrated for 27Al spectra on selected examples of glasses (slag), spinels (alumina), and hydrates (cement aluminum hydrates). In particular, when relevant, the use of the Czjzek model allows a quantitative decomposition of the spectra and an accurate extraction of the second moment of the quadrupolar product. In this respect, it is important to realize that only rotational invariants such as the quadrupolar product can make sense to describe the quadrupolar interaction in disordered solids.     

PMRD investigations of molecular dynamics and phase transitions in the liquid crystal 6CHBT

Proton Magnetic Relaxation Dispersion (PMRD) measurements were carried out over a wide Larmor frequency range near the isotropic-nematic transition temperature (T _NI) and in the mid nematic phase of a low viscous liquid crystal 4-(trans-4′-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT), with a view to examining the pre-transitional effects just above isotropic-nematic transition (T > T _NI) and the role of director fluctuations (DF) on the nematic phase stability of this system. The results near the transition indicate critical slowing down of the short-range nematic order modes just above T _NI with a mean field critical exponent characterizing the coherence length (ξ). The observed temperature independence of the correlation time associated with the reorientations (τ_R) and the extension of DF mode spectrum, in the mid-nematic phase, to relatively higher resonance frequencies seem to suggest fairly low hindering barriers for the tumbling of the molecules about their short axes.     

内标法在土壤重金属镍元素X荧光分析中的应用研究

利用能量色散型XRF重金属实验平台,在实验室自然大气环境下获取土壤的X射线荧光,通过X射线荧光光谱法定量分析了国家标准土壤样品中元素Ni的含量。实验研究了土壤中Ni元素的X荧光特征分析谱线,采用加入内标法测定了Ni元素定标曲线,并依据实验结果分析了内标元素选取方法。实验结果表明,对土壤样品进行内标法分析时,可选取基体中适宜元素作为内标元素;使用Pb的Lα线,Cu和Fe的Kα线作内标线时相对标准偏差(RSD)分别为6.24%,5.24%和5.22%,说明选择合适基体主量元素的特征谱线作为内标线,可有效提高测量结果的准确性。     

Polymer-templated mesoporous carbons synthesized in the presence of nickel nanoparticles, nickel oxide nanoparticles, and nickel nitrate

Mesoporous carbon composites, containing nickel and nickel oxide nanoparticles, were obtained by soft-templating method. Samples were synthesized under acidic conditions using resorcinol and formaldehyde as carbon precursors, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock co-polymer Lutrol F127 as a soft template and nickel and nickel oxide nanoparticles, and nickel nitrate as metal precursors. In addition, a one set of samples was obtained by impregnation of mesoporous carbons with a nickel nitrate solution followed by further annealing at 400 °C. Wide angle X-ray powder diffraction along with thermogravimetric analysis proved the presence of nickel nanoparticles in the final composites obtained using nickel and nickel oxide nanoparticles, and Ni(NO₃)₂ solution. Whereas, the impregnation of carbons with a nickel nitrate solution followed by annealing at 400 °C resulted in needle-like nickel oxide nanoparticles present inside the composites’ pores. Low-temperature (−196 °C) nitrogen physisorption, X-ray powder diffraction, and thermogravimetric analysis confirmed good adsorption and structural properties of the synthesized nickel-carbon composites, in particular, the samples possessed high surface areas (>600 m²/g), large total pore volumes (>0.50 cm³/g), and maxima of pore size distribution functions at circa 7 nm. It was found that the composites were partially graphitized during carbonization process at 850 °C. The samples are stable in an air environment below temperature of 500 °C. All these features make the synthesized nickel-carbon composites attractive materials for adsorption, catalysis, energy storage, and environmental applications.     

New applications of the magnetic X-ray circular dichroism method for surface-magnetism investigations in a photoemission electron microscope

It is shown that magnetic X-ray circular dichroism (MXCD) can be exploited in photoemission electron microscopy not only to visualize the domain structure of ferromagnets, but also to perform quantitative measurements of the stray magnetic fields at the domain boundaries. In the general situation, two MXCD images obtained at different extractor voltages are required. In specific cases, however, it suffices to consider a single image, if it is deformed by the stray magnetic fields compared to a known object geometry. The object geometry means its real shape, scratches or other defects. It is also possible to deposit a paramagnetic film structured in the form of stripes or a grid as a reference on the ferromagnetic sample being investigated. Received: 2 August 2001 / Accepted: 6 September 2001 / Published online: 20 December 2001     

ICP-MS在水库水源地重金属污染健康风险评价中的应用

对辽河流域水库饮用水源地水样中重金属元素(Cr, As, Cd, Cu, Zn和Pb)浓度进行了调查研究,并采用美国环保署推荐的健康风险评价模型对水库中重金属污染做了初步的健康风险评价。结果表明,辽河流域5个水库中6种重金属元素Cr,Cu,Zn,As,Cd和Pb的平均浓度分别为3.36, 1.03, 2.70, 1.23, 0.02, 0.03 μg·L-1,均未超过国家生活饮用水卫生标准(GB 5749—2006)。健康风险评价结果表明,研究区域的金属致癌风险较高,致癌重金属的健康风险大小依次为Cr>As>Cd。其中,水库中Cr元素致癌风险最高,成人和儿童分别为4.50×10-5~7.53×10-5和6.29×10-5~1.05×10-4 a-1。非致癌重金属引起的健康风险值为10-13~10-10 a-1,均在国际辐射防护委员会(ICRP)所规定的可接受范围内,健康风险大小为Cu>Zn>Pb。重金属总健康风险成人为1.07×10-4~1.72×10-4 a-1,儿童为1.49×10-4~2.40×10-4 a-1,均超过ICRP推荐的5×10-5 a-1。辽河流域饮用水源地致癌重金属的风险明显高于非致癌重金属。     

NMR spectroscopy of organolithium compounds,part XXXIII: trans‐2,3‐dimethylcyclopropyllithium aggregation in diethylether: An equilibrium between three different complexes of comparable energy and the influence of LiBr on aggregate structure

The aggregate formation of trans‐2,3‐dimethylcyclopropyllithium ( 2 ) was studied in diethylether (DEE). With the help of the isotopic fingerprint method three clusters, a monomer, a dimer, and a fluxional tetramer in the ratio 1.00:0.53:0.22 were identified at 187 K. In the presence of 1 equivalent of LiBr cyclopropyllithium ( 1 ) forms in DEE at 163 K a mixed dimer and a mixed tetramer, while in THF a mixed dimer dominates. Under the same conditions 2 and 2,2,3,3‐tetramethylcyclopropyllithium ( 3 ) form in the solvent mixture DEE/THF (1:1) mixed dimers, as does 2 also in THF.     

Theoretical investigations of the EPR parameters and local structure for Cu2+ in BeO

The electron paramagnetic resonance (EPR) parameters (the anisotropic g factors, the hyperfine structure parameters and the quadrupole coupling constant Q) and local structure for Cu²⁺ in BeO are theoretically investigated from the perturbation formulas of these parameters for a 3d⁹ ion under trigonally distorted tetrahedra. The ligand orbital and spin-orbit coupling contributions are included in the basis of the cluster approach, in view of the strong covalency of the [CuO₄]^6? cluster. From the calculations, the impurity Cu²⁺ is suggested not to occupy exactly the ideal Be²⁺ site but to suffer a slight inward displacement (≈0.024 Å) toward the ligand triangle along the C₃ axis. The theoretical EPR parameters show good agreement with the experimental data.     

T1-weighting of Hahn echo-trains in the stray-field for deuterium: prospects for imaging using long echo-train summation

Hahn-echoes have been observed in stray-field gradients of 37.5 T/m and 74.5 T/m for deuterium in heavy water in a number of forms: liquid, ice, and coordinated in solid copper sulfate and silica gel. In the diamagnetic samples, the application of many pulses (>1000 or more) in long pulse-trains produced many sustained echoes. This lengthening of T_echo is caused by T₁-weighting of the components of the Hahn echo (even in the absence of spin-locking). In contrast, for the paramagnetic complex, the shortening of T₁ greatly reduces T_echo and only few echoes are obtained. A new protocol for stray field imaging is suggested when T₁ is long: long echo-train summation (LETS). The observed magnetisation is much greater for heavy ice than for heavy water because the line-width, and hence the thickness of the excited slice, is greater.     

X-ray solution structure of the native neuronal porosome-synaptic vesicle complex: Implication in neurotransmitter release

Nanoportals at the cell plasma membrane called porosomes, mediate secretion from cells. In neurons porosomes are 15 nm cup-shaped lipoprotein structure composed of nearly 40 proteins. The size and complexity of the porosome has precluded determination of its atomic structure. Here we report at nanometer resolution the native 3D structure of the neuronal porosome-synaptic vesicle complex within isolated nerve terminals using small-angle X-ray solution scattering. In addition to furthering our understanding of the porosome structure, results from the study suggests the molecular mechanism involved in neurotransmitter release at the nerve terminal.     

Application of some physical organic chemistry models to the study of oil spills residues in Patagonian soils

The present study focuses on the application of some physical organic chemistry (POC) models for a better understanding of the interactions between oil spills and soil. By studying the sorption behavior of pure compounds, it was possible to examine the application of the multilayer model to the prediction of crude oil isotherms. The difference between the predicted and experimental oil isotherms was only 13%, which indicates the reliability of the model. The distribution coefficients, K_d, of soil samples containing oil residuals of different ages, were also determined using methanol as a co‐solvent. Desorption experiments showed that they are a function of the polarity of the liquid phases; the K_ds, calculated by applying a model derived from the solvophobic theory, increase with increasing age. On the other hand, the natural attenuation of oil spills was studied by using GC and ¹H NMR techniques. Signals for four types of aliphatic and for the aromatic protons were clearly assigned; signals for alcohol (OH) and carboxylic (COOH) protons were also observed; compounds exhibiting these polar groups are usually not detected in the GC–FID conventional analysis. The principal component analysis (PCA) of K_d, and the parameters determined by GC and NMR, showed that the first and second PC, accounted for more than 95 and 81% of variance, for NMR and GC parameters, respectively. The detailed results suggest that the ¹H NMR data would be more useful than GCs to evaluate the environmental transformations that oil spills undergo in Patagonian soils. Nevertheless, direct extrapolation of the present results to other environments is not possible because the changes depend strongly on the original chemical composition of the crude oil and the variable exposure conditions along the time. Copyright © 2008 John Wiley & Sons, Ltd.