Incubation period in the 2,2,4,4‐tetramethyl‐1‐piperidinyloxy‐mediated thermal autopolymerization of styrene: Kinetics and simulations

Mechanisms and simulations of the induction period and the initial polymerization stages in the nitroxide‐mediated autopolymerization of styrene are discussed. At 120–125 °C and moderate 2,2,4,4‐tetramethyl‐1‐piperidinyloxy (TEMPO) concentrations (0.02–0.08 M), the main source of radicals is the hydrogen abstraction of the Mayo dimer by TEMPO [with the kinetic constant of hydrogen abstraction (k_h)]. At higher TEMPO concentrations ([N?] > 0.1 M), this reaction is still dominant, but radical generation by the direct attack against styrene by TEMPO, with kinetic constant of addition k_ad, also becomes relevant. From previous experimental data and simulations, initial estimates of k_h ≈ 1 and k_ad ≈ 6 × 10^?7 L mol^?1 s^?1 are obtained at 125 °C. From the induction period to the polymerization regime, there is an abrupt change in the dominant mechanism generating radicals because of the sudden decrease in the nitroxide radicals. Under induction‐period conditions, the simulations confirm the validity of the quasi‐steady‐state assumption (QSSA) for the Mayo dimer in this regime; however, after the induction period, the QSSA for the dimer is not valid, and this brings into question the scientific basis of the well‐known expression k_th[M]³ (where [M] is the monomer concentration and k_th is the kinetic constant of autoinitiation) for the autoinitiation rate in styrene polymerization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6962‐6979, 2006     

Differential thermal analysis of the Al+20% (Fe-50%B) system

In the present study, aluminium and mechanically alloyed (36 h) Fe/B (50 wt%) are mixed. Al+20 (wt%) Fe/B mixture has been studied by differential thermal analysis to determine the aluminium quantity that is supposed to melt and afterwards does not solidify as it reacts with Fe/B powder. The different areas between endothermic reaction (melting peak) and exothermic reaction (solidification peak) allow in knowing the quantity of aluminium that reacts with Fe/B and the amount of intermetallic phases formed at high temperature. In order to follow the process, compacts were sintered at different temperatures (700, 800, 900, 1000 and 1200 °C), in N₂/10H₂/0.1CH₄ atmosphere. Microstructure was evaluated by image analysis and the results obtained by both techniques are compared.     

Preparation of Fe/B powders by mechanical alloying

Steels with a high boron content are valuable as a neutron shield in waste containers and as control absorbers in nuclear reactors. The purpose of this study was to obtain by mechanical alloying an iron powder with 50% boron (by weight) and then powder-metallurgy materials. The elementary powders were mixed in a high-energy mill for 36 h in an inert atmosphere. Samples were withdrawn at intervals, and the powder was characterized by differential thermal analysis, X-ray diffraction and electron microscopy. The Fe/B powders withdrawn at different intervals of milling were diluted with further additions of iron up to a final content of 10% boron. The mixtures were uniaxially compacted at 500 MPa; their green density was verified, and they were sintered in argon at 1150°C. Their physical properties (density and dimensional change) and bending strength were evaluated and microstructural studies and fracture tests were performed.     

Effect of particle geometry on phase transitions in two-dimensional liquid crystals

Using a version of density-functional theory which combines Onsager approximation and fundamental-measure theory for spatially nonuniform phases, we have studied the phase diagram of freely rotating hard rectangles and hard discorectangles. We find profound differences in the phase behavior of these models, which can be attributed to their different packing properties. Interestingly, bimodal orientational distribution functions are found in the nematic phase of hard rectangles, which cause a certain degree of biaxial order, albeit metastable with respect to spatially ordered phases. This feature is absent in discorectangles, which always show unimodal behavior. This result may be relevant in the light of recent experimental results which have confirmed the existence of biaxial phases. We expect that some perturbation of the particle shapes (either a certain degree of polydispersity or even bimodal dispersity in the aspect ratios) may actually destabilize spatially ordered phases thereby stabilizing the biaxial phase.     

Kalman filtering of data from first- and second-order kinetics

The Kalman filter algorithm was used to process data obtained in the simultaneous determination of species following first- and second-order kinetics. The performance of the algorithm in the quantification of chemical components from simulated data was assessed, and the influence of various parameters involved was estimated. The algorithm was applied to the resolution of cysteine-ascorbic acid and glutathione-ascorbic acid mixtures where the ascorbic acid followed pseudo first-order kinetics and the amino acids second-order kinetics in the reaction with the copper(II)-neocuproine system. Some features of the determinations (namely, afforded concentration ratios, accuracy and precision) are discussed.     

Study of hydromagnesite and magnesium hydroxide based fire retardant systems for ethylene-vinyl acetate containing organo-modified montmorillonite

A new flame retardant (FR) system for ethylene-vinyl acetate, mainly based on the combination of hydromagnesite (HM, obtained from an industrial by-product) and organo-modified montmorillonite (oMMT) has been compared with a magnesium hydroxide (MDH) and oMMT flame retardant system. The presence of oMMT in association with both hydrated minerals gave a strong decrease of heat release rate in cone calorimeter tests. Moreover, the HM/oMMT combination leads to a better improvement of resistance to ignition and self-extinguishability in comparison with the MDH/oMMT one. The study of residues formed during thermal decomposition revealed the formation of forsterite (Mg₂SiO₄) when either MDH or HM was used in combination with oMMT. SEM observations of residues showed sintering of the mineral particles at high temperature particularly in the case of HM/oMMT composition.     

On the mechanism of electroluminescence emission by porous silicon layers

The analytical treatment of a model considering the electrooxidation of p-porous silicon layers under galvanostatic conditions is able to give account of experimental facts such as the shape and location of the electroluminescence peak as well as of the spectral shift of the electroluminescence peak produced by oxidation. The proposed model considers electroluminescence to be the result of electron injection into the conduction band by an adsorbed intermediate produced by electrooxidation of the surface coverage with hydrogen or siloxene of the silicon nanocrystallites. The access of holes to the surface is made possible by low accumulation layer conditions and is the rate determining step in the electroluminescence mechanism. In this way it is possible to give a satisfactory explanation to the shift towards the blue experimented by the electroluminiscence emission maximum as a consequence of electrooxidation.     

On the universal behavior of some thermodynamic properties along the whole liquid-vapor coexistence curve

When thermodynamic properties of a pure substance are transformed to reduced form by using both critical- and triple-point values, the corresponding experimental data along the whole liquid-vapor coexistence curve can be correlated with a very simple analytical expression that interpolates between the behavior near the triple and the critical points. The leading terms of this expression contain only two parameters: the critical exponent and the slope at the triple point. For a given thermodynamic property, the critical exponent has a universal character but the slope at the triple point can vary significantly from one substance to another. However, for certain thermodynamic properties including the difference of coexisting densities, the enthalpy of vaporization, and the surface tension of the saturated liquid, one finds that the slope at the triple point also has a nearly universal value for a wide class of fluids. These thermodynamic properties thus show a corresponding apparently universal behavior along the whole coexistence curve.