Thermal analysis of magnesium hydroxide

Natural brucite and two precipitated Mg(OH)₂ samples were analysed in a simultaneous TG-DTG-DSC analyser. The initial dehydroxylation temperature of natural brucite is lower than those of the precipitated samples, but the maximum and final temperatures of the former are higher than those of the latter. The maximum temperatures of individual samples obtained from DTG and DSC curves are almost the same. Heats of reaction derived from peak areas for the three samples are not exactly the same, as they are influenced by the specific surface area of the individual sample. Activation energies deduced by Freeman and Carroll's method are very different from one another. This is attributed to the difference in pressure when the sample is crimped. A linear relationship is observed between the deduced activation energy and the specific height of the DSC peak.

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Zusammenfassung Natürlicher Brucit und zwei gefällte Mg(OH)₂-Proben wurden in einem simultanen TG-DTG-DSC-Analysator geprüft. Die Anfangstemperatur der Dehydroxylierung des natürlichen Brucits ist niedriger als die der gefällten Proben, doch sind Maximal- und Endtemperaturen des ersteren höher als jene der letzteren. Die an Hand von DTG und DSC-Kurven erhaltenen Maximaltemperaturen der einzelnen Proben sind fast identisch. Die aus den Peakflächen der drei Proben errechneten Reaktionswärmen sind nicht genau dieselben, da sie von der spezifischen Peakfläche der einzelnen Proben beeinflußt werden. Die mittels der Methode von Freeman und Carroll abgeleiteten Aktivierungsenergien sind von einander sehr verschieden. Sie werden den Unterschieden im Druck bei dem Schrumpfen der Probe zugeschrieben. Ein linearer Zusammenhang zwischen der abgeleiteten Aktivierungsenergie und der spezifischen Höhe der DSC-Peaks wurde beobachtet.

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Résumé La brucite naturelle et deux échantillons de Mg(OH)₂ précipités ont été étudiés par TG-TGD et DSC simultanées. La température initiale de la déshydroxylation de la brucite naturelle est plus faible que celle des deux échantillons précipités, mais les températures maximale et finale de déshydroxylation de la brucite sont plus élevées que celles des échantillons précipités. Les chaleurs de réaction déduites de l'aire des pics DSC des trois échantillons ne sont pas exactement les mêmes, puisqu'elles sont influencées par l'état de surface des échantillons individuels. Les énergies d'activation trouvées en appliquant la méthode de Freeman et Carroll sont très différentes les unes des autres. Elles sont attribuées à la différence de pression au niveau de l'échantillon. On a observé une relation linéaire entre l'énergie d'activation déduite et la hauteur spécifique du pic DSC.

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Mg(OH)₂ -- . , , - , . - , , . , , , , . , —, , . - .

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The authors wish to thank Dr. T. C. W. Mak, Department of Chemistry, The Chinese University of Hong Kong, for taking the X-ray diffraction data.     

Evaluation of kinetic parameters from thermogravimetric curves: Part I. Combined numerical method

A new method is proposed to evaluate kinetic parameters from thermogravimetric traces. The method consists of two steps, α, T and dα/dT values are first employed to estimate kinetic parameters by linear least squares fitting, using the five types of mechanism for solid phase reactions suggested by ?esták. From the different sets of parameters thus obtained, the most probable mechanism type is decided. The resultant parameters for the chosen type of mechanism may further be improved by differential correction method, if necessary, using the more accurate data of α and T. The proposed method was tested with artificial data and the data for the thermal dehydration of gypsum by ?esták et al. The results were very satisfactory.     

A Survey of Strain-Promoted Azide–Alkyne Cycloaddition in Polymer Chemistry

Highly efficient reactions that enable the assembly of molecules into complex structures have driven extensive progress in synthetic chemistry. In particular, reactions that occur under mild conditions and in benign solvents, while producing no by-products and rapidly reach completion are attracting significant attention. Amongst these, the strain-promoted azide–alkyne cycloaddition, involving various cyclooctyne derivatives reacting with azide-bearing molecules, has gained extensive popularity in organic synthesis and bioorthogonal chemistry. This reaction has also recently gained momentum in polymer chemistry, where it has been used to decorate, link, crosslink, and even prepare polymer chains. This survey highlights key achievements in the use of this reaction to produce a variety of polymeric constructs for disparate applications.     

Characterization of deep sea sediments by INAA for radioactive waste management purposes

An instrumental neutron activation analysis method, using both reactor flux and epi-cadmium neutrons, has been developed for the determination of major, minor and trace elements in sub-seabed sediments. The method involves two different irradiations followed by three different counting periods using high-resolution Ge(Li) and low-energy photon detectors. The list of elements determined includes Ag, Al, As, Ba, Br, Ca, Ce, Cl, Co, Cr, Cs, Dy, Eu, Fe, Ga, Gd, Hf, Hg, I, In, K, La, Lu, Mg, Mn, Mo, Na, Nd, Ni, Pt, Rb, Re, Rh, Sb, Sc, Se, Si, Sm, Sn, Sr, Ta, Tb, Th, Ti, Tm, U, V. W, Yb, Zn, and Zr. The accuracy of the method has been evaluated by analyzing four different standard reference materials. The method has been applied to core samples collected from the Great Meteor East and the South Nares Abyssal Plain which are being evaluated as potential sites for the possible disposal of vitrified highly active waste.     

Hyperbranched aromatic poly(ether imide)s: Synthesis,characterization, and modification

The synthesis and characterization of hyperbranched aromatic poly(ether imide)s are described. An AB₂ monomer, which contained a pair of phenolic groups and an aryl fluoro moiety activated toward displacement by the attached imide heterocyclic ring, was prepared. The nucleophilic substitution of the fluoride with the phenolate groups led to the formation of an ether linkage and, subsequently, to the hyperbranched poly(ether imide), which contained terminal phenolic groups. A similar one‐step polymerization involving a monomer that contained silyl‐protected phenols yielded a hyperbranched poly(ether imide) with terminal silylated phenols. The degree of branching of these hyperbranched polymers was approximately 55%, as determined by a combination of model compound studies and ¹H NMR integration experiments. End‐capping reactions of the terminal phenolic groups were readily accomplished with a variety of acid chlorides and acid anhydrides. The nature of the chain‐end groups significantly influenced physical properties, such as the glass‐transition temperature and the solubility of the hyperbranched poly(ether imide)s. As the length of the acyl chain of the terminal ester groups increased, the glass‐transition temperature value for the polymer decreased, and the solubility of the polymer in polar solvents was reduced, becoming more soluble in nonpolar solvents. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2536–2546, 2001     

A tunable gas sensor using an acoustic waveguide

A tunable gas sensor using surface acoustic waves (SAW) is described. It is designed on the basis of a waveguide delay line fabricated on a piezoelectric substrate made of 128° Y-cut LiNbO₃. A voltage applied between the waveguide and two electrodes causes a local change in the properties of the substrate near the waveguide and differently affects the sensor’s response to the vapors of various analytes. Some results of the experimental study of the sensor, which show the change in the selectivity under the effect of voltage, are presented. The analytes used for testing include a number of alcohols and deionized water. The possibilities for employing such a sensor in the sensor arrays of gas analyzers of the electronic nose type are discussed.     

Danes' Drop Theorem in locally convex spaces

Danes' Drop Theorem is generalized to locally convex spaces.

     

Development of Ultrahigh Permeance Hollow Fiber Membranes via Simple Surface Coating for CO2/CH4 Separation

Most researchers focused on developing highly selective membranes for CO₂/CH₄ separation, but their developed membranes often suffered from low permeance. In this present work, we aimed to develop an ultrahigh permeance membrane using a simple coating technique to overcome the trade-off between membrane permeance and selectivity. A commercial silicone membrane with superior permeance but low CO₂/CH₄ selectivity (in the range of 2–3) was selected as the host for surface modification. Our results revealed that out of the three silane agents tested, only tetraethyl orthosilicate (TEOS) improved the control membrane’s permeance and selectivity. This can be due to its short structural chain and better compatibility with the silicone substrate. Further investigation revealed that higher CO₂ permeance and selectivity could be attained by coating the membrane with two layers of TEOS. The surface integrity of the TEOS-coated membrane was further improved when an additional polyether block amide (Pebax) layer was established atop the TEOS layer. This additional layer sealed the pin holes of the TEOS layer and enhanced the resultant membrane’s performance, achieving CO₂/CH₄ selectivity of ~19 at CO₂ permeance of ~2.3 × 10⁵ barrer. This performance placed our developed membrane to surpass the 2008 Robeson Upper Boundary.