Effect of vibration in processes of structure formation in polymers

It was shown that the strength characteristics of the samples produced from thermoplastics (low density polyethylene, polyamide 6, Armamid, and polyethylene terephthalate) increase as a result of short vibration treating with a frequency close to that of natural vibrations of the structural elements.     

茶叶中高氯酸盐检测方法研究进展

茶叶中新型污染物高氯酸盐在近年来受到越来越多的关注,相应的检测技术也在不断加强.参考国内外文献,综述了茶叶中高氯酸盐的检测方法.目前,茶叶中高氯酸盐的检测方法主要有离子色谱法（IC）、离子色谱-质谱法（IC-MS）和高效液相色谱-串联质谱法（LC-MS-MS）.比较了不同检测方法的局限性和优越性,重点比较了高效液相色谱-串联质谱法不同前处理方法、净化小柱和检测条件的优劣,对茶叶高氯酸盐检测技术的发展和研究进行了展望,为检测茶叶中高氯酸盐的新材料研发和检测新标准的建立提供理论依据.     

Use of NAA in marine environment and in archaeology in Greece

Neutron activation analysis (NAA) is a very sensitive and accurate multielement analytical method that is widely applied to the investigation of environmental and archaeological problems. The first part of this paper is a review of pollution studies of toxic trace elements in sediments, seawater and marine organisms of Saronikos Gulf, Greece by NAA. The second part of this paper is a review of provenance studies based on minor and trace element research in ancient ceramics, obsidian, flint, limestone, marble and lead by Instrumental NAA, performed at the NCSR Demokritos.     

Role of orientation in kinetics of nucleation and growth of crystals in polymers

A theoretical framework is provided for generalizing the inferences drawn from the results of earlier experimental studies of kinetics of crystallization in oriented poly(ethylene terephthalate). The framework is obtained by combining extensions of classical nucleation theories in polymers^3,4 and a theory of crystal growth with anisotropic incorporation of segments into growing crystals.¹⁵ It is shown that, while a very strong dependence of rate of primary nucleation on orientation does exist, there is a only a much weaker dependence of rate of crystal growth on the orientation of the crystallizing polymer. The theoretical formulation provided here would allow qualitative estimates for comparison with experiments.     

Study of the electrochemical reduction of dioxygen in acetonitrile in the presence of weak acids

The electrochemical reduction of dioxygen has been studied in acetonitrile at glassy-carbon electrodes. The initial step is the reversible one-electron reduction to form superoxide. In the presence of hydrogen-bond donors (water, methanol, 2-propanol), the superoxide forms a complex with the donor resulting in a positive shift in the potential that can be analyzed to obtain formation constants for these complexes. Stronger acids result in protonation of the superoxide followed by reduction to produce HO2-. In the absence of hydrogen-bond donors, the reduction of superoxide occurs at very negative potentials, and this second reduction peak is very much drawn-out along the potential axis, indicating a small value of the transfer coefficient, alpha. The addition of hydrogen-bond donors, HA, brings about a positive shift in this peak, without a noticeable change in shape. The reaction occurring at the second peak is a concerted proton and electron transfer (CPET) in which the electron is transferred to superoxide and a proton is transferred from HA to the superoxide, forming HO2- and A- in a concerted process. An estimation of the standard potential for this reaction shows that the second reduction always occurs at a high driving force, which explains the small value of alpha that is observed. Consistent with a CPET, a kinetic isotope effect, HA versus DA, was detected for the three hydrogen-bond donors. The increasing positive shift of the second peak with increasing water concentration has been interpreted as being a consequence of the change in the formal potential, as water is both a reactant in the process and a participant through the hydrogen-bond stabilization of the anions.     

锅炉水总碱度测量不确定度的评估

对锅炉水中总碱度测量不确定度的来源进行分析,评定了总碱度测定过程中测量重复性、天平、标准物质纯度、滴定管和移液管等因素对总碱度测量不确定度的影响,计算得到炉水水样总碱度为26．00mmol／L时,扩展不确定度为0．12mmol／L。在总碱度的测量过程中,滴定管和移液管的准确度是影响总碱度测量不确定度的主要因素。     

Significance of different modes in dynamics of chains in a dense medium

Singular Value Decomposition technique is used to decompose the trajectory of the fluctuations of atom positions into their modes for a chain of fifty bonds embedded in a fluctuating environment. The temperature of the environment is changed according to a Monte Carlo Metropolis scheme. Results show that at low temperatures only a few modes are significant, these being the slowest ones. For high temperatures, on the other hand, higher modes corresponding to fast fluctuations of atom positions are significant as well.     

Assessment of performance of laboratories in determining acrylamide in crispbread

A proficiency testing round was undertaken to assess the performance of laboratories to measure acrylamide in a sample of crispbread. Retail samples of crispbread were ground to a fine powder and after thorough mixing were packed in 40 g units for distribution. Ten samples were selected at random and analyzed in duplicate for acrylamide by liquid chromatography/mass spectrometry (LC/MS). Standard statistical tests showed that the material was homogeneous for the purposes of proficiency testing. Test samples were distributed to 55 laboratories in 16 countries in Europe, North America, Australia, and the Middle East. The results were analyzed by standard proficiency testing statistical procedures, and laboratories were awarded z-scores on the basis of their reported results. Based on a target standard deviation (sigmap value) taken from the Horwitz equation, for a robust mean value of 1.2 mg/kg acrylamide, satisfactory results (z-score within +/- 2 for those between 0.8 and 1.6 mg/kg) were obtained by 86% of the 37 laboratories that returned results. Only 1 laboratory was unsatisfactory and 4 had questionable results. About equal numbers of laboratories used gas chromatography (GC)/MS and LC/MS procedures with about 25% using MS/MS and one using GC with electron capture detection. There was no evident trend in performance or bias in results. GC/MS and LC/MS data were evenly distributed across the population of laboratories reporting results.     

Degradation of chitosan in solutions in the presence of a redox system

The degradation of chitosan in aqueous solutions of acetic and hydrochloric acids initiated by the hydrogen peroxide-ascorbic acid redox system has been studied. The nature of acid strongly affects the degree of degradation and the molecular mass of oligochitosan—the reaction product. The relationship between the molecular mass of oligochitosan and the concentration of acetic acid in the solvent is described by a curve with a minimum. If acetic acid is present in a 5-fold molar excess with respect to chitosan units, the starting solution behaves as a Newtonian liquid. It has been demonstrated that, exactly in this solution, oligochitosan with the minimum amount of units is formed.     

Study of the early deactivation in pyrolysis of polymers in the presence of catalysts

In this work, the early degradation step of the pyrolysis of some polymers in the presence of certain catalysts has been studied using thermogravimetric analysis (TGA). Three commercial polymers (PE, PP and EVA) and three catalysts were studied (ZSM-5, MCM-41a, and MCM-41b), and the MCM-41a catalyst has been selected for the analysis of the earlier steps of the pyrolysis process carried out in the presence of catalysts. Several cycles of heating–cooling were performed using a thermobalance, in order to analyze the influence of the first stages of decomposition on the activity of the more accessible active sites involved. In this way, the behaviour of the polymer–catalyst mixtures (20% (w/w) of catalyst) was studied and compared with that observed in the corresponding thermal degradation as well as in the pyrolysis in the presence of catalysts, in a single heating cycle.The results obtained clearly show the existence of an early degradation step. For a polymer–catalyst system with low steric hindrances such as PE-MCM-41, this early degradation step causes a noticeable decrease of the catalyst activity for the main decomposition step (i.e., cracking of the chain). The decrease of the catalytic activity is lower for a polymer–catalyst system with higher steric restrictions, as occurs in the EVA-MCM-41 degradation process. However, in this case, the catalyst activity in the first decomposition step (i.e., the loss of the acetoxi groups) is noticeable decreased after one pyrolysis run, thus reflecting that the active sites involved are mainly the most accessible ones.     

Dynamic polarization of solvent nuclei in solutions of diphenylpicrylhydrazyl in organic liquids

Measurements have been made of the dynamic polarization of nuclei of solvent in concentrated solutions of diphenylpicrylhydrazyl (DPPG) in a weak magnetic field of approximately 21 Oe. In proton-containing solutions, a negative dynamic polarization was observed, corresponding to dipole-dipole interaction of electronic and nuclear spins. In a hexafluorobenzene solution a considerable contribution of scalar interaction to the relaxation of the fluorine nucleus was observed. The temperature dependence of the dynamic polarization of the fluorine nucleus has been investigated, and an activation energy of 3.3 kcal/mole has been found for the bond between the molecules of hexafluorobenzene and the solvate shell of DPPH molecules.     

Diffraction methods in the spectroscopy of thin films in the vicinity of resonances

This review is devoted to a theoretical discussion of the diffraction of volume and surface electromagnetic waves from the edge of a thin dielectric film on a metallic substrate. The film can be either semi-infinite in extent or of finite width. Of particular interest is the case in which the film possesses a dipole-active vibrational mode that contributes a pole to its dielectric susceptibility tensor at a frequency that falls within the range of frequencies allowed the surface polaritons at the metal-vacuum interface. An effect of such a film on the dispersion curve of the surface polaritons is to produce a gap in it, with the result that at frequencies below the gap two surface polaritons can propagate with the same frequency. Experimental results showing this gap are described. The anomalies in the diffraction of volume and surface electromagnetic waves from the edge of such a film due to the resonance of surface polaritons in the substrate with the dipole-active excitation in the film are determined analytically, and experimental results bearing on them are discussed. Effects of spatial dispersion and of optical nonlinearities in the dielectric tensor of the film on these diffraction processes are investigated. Some implications of the results presented here for surface polariton spectroscopy as a tool for the experimental study of the properties of surfaces and thin films are discussed.     

Role of water in atomic resolution AFM in solutions

We use computer modelling to investigate the mechanism of atomic-scale corrugation in frequency modulation atomic force microscopy imaging of inorganic surfaces in solution. Molecular dynamics simulations demonstrate that the forces acting on a model microscope tip result from the direct interaction between a tip and a surface, and forces entirely due to the water structure around both tip and surface. The observed force is a balance between largely repulsive potential energy changes as the tip approaches and the entropic gain when water is sterically prevented from occupying sites near the tip and surface. Only extremely sharp tips are likely to measure direct tip-surface interactions. An investigation into the dynamics of water confined between tip and surface shows that water diffusion can be slowed by at least two orders of magnitude compared to its rate in bulk solution.     

Effect of Pressure in the Gamma Radiolysis of Cyclohexane in the Vapor Phase

The vapor phase γ-radiolysis of cyclohexane has been investigated as a function of pressure, from 100 to 0.5 Torr, in vessels of different dimensions. With decreasing pressure, the yield of hydrogen and substituted cyclohexanes is decreased and the formation of molecules having less than six carbon atoms is enhanced. The curves of yield vs. pressure have a sigmoïd shape from which we define a transition pressure depending only on the dimensions of the vessels, the variation of the yield of a given product being the same for all the vessels. We propose and discuss a simple model of competition between the neutralization of the ions in the volume and their diffusion to the surface where they are neutralized. From the slope of the curve of the inverse of the transition pressure vs. the square of the characteristic diffusion length, we evaluate a maximum life-time of 68 ms for the ions, using a calculated value of the cyclohexane ions diffusion coefficient.     

Excitation of vibrations in microtubules in living cells

Microtubules, which are thought to be the primary organizers of the cytoskeleton, are electrical polar structures with extraordinary elastic deformability at low stress and with energy supply from hydrolysis of guanosine triphosphate (GTP) to guanosine diphosphate (GDP). At least a part of the energy supplied from hydrolysis can excite vibrations. Energy is mainly lost by viscous damping of the surrounding cytosol. Viscous damping is diminished by a slip layer which is formed by an attracted ionic charge layer and by a thin surface layer of the microtubule. Relaxation time caused by viscous damping may be several orders of magnitude greater than period of vibrations at 10 MHz. Energy supplied to the microtubule is of the order of magnitude of 10(-14) W cm(-1) (per unit length of the microtubule).     

Analysis of interlaboratory performance in the determination of total selenium in water

This study explored the performance of experienced laboratories in the analysis for total selenium in water by a variety of analytical methods. The goal of the study was to examine intra- and interlaboratory variability. Replicates (n = 7) of 7 sample types that included a reference material of known Se concentration, natural waters, and treated wastewaters were submitted to 7 laboratories with prequalified Se analytical experience. Results of the study indicated wide ranges in minimum and maximum results, distinct differences in laboratory precision, and routine reporting of numerical results below statistical limits of quantitation. Hydride generation as a sample introduction technique demonstrated superior performance. In general, the study supports a caution advisory about using low-level Se data, especially results lower than about 10 micrograms Se/L, without quantitating the statistical uncertainty of the data. Because this study used data from samples that were submitted in bulk to participating laboratories prequalified for Se analytical expertise and experience, it can be considered a best-case demonstration of performance.     

Electrochemical reduction of camphorquinone in DMF in the presence of a proton donor

The electroreduction of camphorquinone in DMF, at mercury electrodes, was investigated by a variety of techniques. In DMF, in the absence of proton donor, camphorquinone exhibits two one-electron waves: the first, a one-electron reversible wave to be due to a reversible charge transfer without a coupled chemical reaction. After the first charge transfer, the semidione anion radical is reduced to the dianion. The irreversibility of the second wave derives from a fast irreversible protonation of the dianion. A wide variety of changes in behaviour is observed in the reduction of camphorquinone as increasing amounts of benzoic acid are added: a new two-electron irreversible wave appears at a potential less negative than the original wave. A proton donor to substrate ratio of 2 is required to completely suppress the two original waves. A mechanism for the electroreduction of camphorquinone is proposed and discussed on the basis that the prewave current is controlled by the diffusion of the undissociated acid species and that the undissociated acid, rather than the solvated proton, takes part in the protonation, prior to the charge transfer.     

Advances in the Elegance of Chemistry in Designing Dendrimers

Highly functional and monodisperse macromolecules with tailored architecture constitute the key to designing efficient and smart nanomaterials. Dendrimers offer real potential to achieve this goal, and one of the earlier challenges faced by this novel class of polymers has been addressed by the evolution of synthetic methodologies. This review provides an evaluation of the role played by chemistry in taking these macromolecules of academic relevance to practical industrial and biological applications, in a relatively short period. One can now construct dendrimers in a ‘made‐to‐order’ fashion, for numerous applications in a variety of disciplines.

     

Photodimerisation of glycothymidines in solution and in micelles

Glycothymidines were designed and synthesized as a new class of functional glycomimetics in which a photochemical [2+2] cycloaddition of the thymine moiety induces structural changes of carbohydrate presentation. To test if photodimerisation of these glycothymidines is feasible within an array of molecules, the photochemical reaction was investigated using NMR and NMR diffusion experiments in solution as well as in the supramolecular context of detergent micelles that mimic cellular membranes.     

Formation of gold nanoparticles in microreactor composed of helical peptide assembly in water

A novel microreactor was prepared by self-assembly of an amphiphilic block copolymer composed of a hydrophobic helical peptide unit with a naphthyl group at the C terminal and a hydrophilic poly(ethylene glycol) unit. The copolymer formed a self-assembly in water, taking a vesicular structure. Noticeably, when the copolymer was dispersed in an Au(3+) aqueous solution, gold nanoparticles were formed without addition of any reducing reagent. The naphthyl groups, which are located at the inner surface of the vesicular assembly, promoted the reduction of Au(3+) ions with accompanying pH decrease.