Semicontinuous seeded cationic emulsion polymerization of styrene: The effects of the concentration and type of cationic surfactant

The objective of this work was to analyze the effects of the concentration and type of cationic surfactant on the kinetic features (instantaneous and overall conversions) and colloidal characteristics [mean particle diameter, particle size distribution (PSD), and surface charge density] in the semicontinuous seeded cationic emulsion polymerization of styrene. 2,2′‐Azobis(N,N′‐dimethyleneisobutyramidine)dihydrochloride was used as an initiator. The surfactants were dodecyltrimethylammonium bromide (DTAB) and hexadecyltrimethylammonium bromide (HDTAB). So that the evolution of some polymeric and colloidal characteristics of the synthesized latices could be followed, the overall and instantaneous conversions were defined and determined gravimetrically. The PSDs and average particle diameters were determined by transmission electron microscopy and photon correlation spectroscopy. The surface charge density was determined by conductimetric titration. The evolution of the instantaneous conversions, the total number of particles, and the PSDs of the different reactions were related to the nucleation, growth, and coagulation processes taking place in the semicontinuous seeded emulsion polymerizations. The PSDs obtained from the reactions carried out with the emulsifier DTAB, at a concentration equal to its critical micelle concentration (cmc) and at a concentration twice its cmc, presented more and smaller particles than those obtained by the addition of HDTAB to the polymerization recipe. At lower emulsifier concentrations equal to half of the cmc, the system had lower colloidal stability with DTAB. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2322–2334, 2003     

Sequential Automated Focused Microwave-Assisted Soxhlet Extraction of Compounds with Different Polarity from Marine Sediments Prior to Gas Chromatography Mass Spectrometry Detection

An extraction method based on the use of a focused microwave-assisted Soxhlet extractor with subsequent gas chromatography separation and mass spectrometry detection is proposed for the analysis of environmental pollutants from marine sediments collected at the outflow of an urban wastewater treatment plant to the sea. For the extraction of compounds with different polarity, sequential extraction with dichloromethane and water was performed on each sample. The experimental variables were optimized by the experimental design methodology for both the organic and aqueous extractant. The total time required for quantitative extraction of triclosan, bisphenol A, estrone, estradiol, oxyfluorofen and permethrin was 75 min, a short time as compared with the 24 h Soxhlet extraction (diethylstilbestrol, 4-octylphenol, procymidone and 2,7/2,8-dichlorodibenzo-p-dioxin isomer pair).Revised: 10 March and 18 April 2005     

Fitting a Set of Points by a Circle

Given a set of points S={p ₁ ,. . ., p _n } in Euclidean d -dimensional space, we address the problem of computing the d -dimensional annulus of smallest width containing the set. We give a complete characterization of the centers of annuli which are locally minimal in arbitrary dimension and we show that, for d=2 , a locally minimal annulus has two points on the inner circle and two points on the outer circle that interlace anglewise as seen from the center of the annulus. Using this characterization, we show that, given a circular order of the points, there is at most one locally minimal annulus consistent with that order and it can be computed in time O(n log n) using a simple algorithm. Furthermore, when points are in convex position, the problem can be solved in optimal Θ(n) time. Received June 25, 1997, and in revised form March 5, 1998.     

Annular liquid jets in zero gravity

The equations describing the steady-state behavior of long annular liquid jets and liquid membranes in zero gravity are solved analytically as a function of the pressure difference across the jet or membrane, Weber number, and nozzle exit angle. The ranges of the parameters for which the analytical solutions are valid are determined, and analytical solutions of the mass absorption rate are obtained as a function of the Peclet and Weber numbers, nozzle exit angle, pressure difference, and thickness of the annular liquid jet. It is shown that the convergence length of annular liquid jets and liquid membranes increases as the Weber number, nozzle exit angle, and pressure coefficient are increased. It is also shown that the mass absoption rate increases as the nozzle exit angle, pressure coefficient, and Weber number are increased; however, the mass absorption rate decreases as the Peclet number and annular jet initial thickness-to-radius ratio are increased.     

Nash Equilibria for the Multiobjective Control of Linear Partial Differential Equations

This article is concerned with the numerical solution of multiobjective control problems associated with linear partial differential equations. More precisely, for such problems, we look for the Nash equilibrium, which is the solution to a noncooperative game. First, we study the continuous case. Then, to compute the solution of the problem, we combine finite-difference methods for the time discretization, finite-element methods for the space discretization, and conjugate-gradient algorithms for the iterative solution of the discrete control problems. Finally, we apply the above methodology to the solution of several tests problems.     

The order of chaos on a Bianchi-IX cosmological model

The purpose of this paper is to analyze the chaotic behavior that can arise on a type-IX cosmological model using methods from dynamic systems theory and symbolic dynamics. Specifically, instead of the Belinski-Khalatnikov-Lifschitz model, we use the iterates of a monotonously increasing map of the circle with a discontinuity, and for the Hamiltonean dynamics of Misner's Mixmaster model we introduce the iterates of a noninvertible map. An equivalence between these two models can easily be brought upon by translating them in symbolic-dynamical terms. The resulting symbolic orbits can be inserted in an ordered tree structure set, and so we can present an effective counting and referentation of all period orbits.     

Comparisons between thermodynamic and one-dimensional combustion models of spark-ignition engines

A one-dimensional combustion model, employing a constant eddy diffusivity and a one-step chemical reaction, has been developed and applied to study the flame propagation in a spark-ignition engine. Calculations have been made at 1600 and 4200 rev min⁻¹ under fuel rich conditions and compared with available engine pressure data. One- and two-zone thermodynamic models have also been developed and applied to study the combustion process in the engine. The thermodynamic models have been compared with the one-dimensional model results and comparisons include the average mixture temperature, the temperatures of the burned and unburned gases and the flame surface area. These comparisons indicate that the one-dimensional model predictions are very sensitive to the eddy diffusivity and reaction rate data. The two-zone thermodynamic model predicts, first, a monotonically increasing flame surface area with time and, then, a monotonically decreasing surface area, whereas the one-dimensional model always predicts a monotonically increasing flame surface area. The average mixture temperature predicted by the one-zone thermodynamic model is higher than those of the two-zone and one-dimensional models during the compression stroke, while that of the one-dimensional model is higher than the temperatures predicted by the one- and two-zone models during the expansion stroke. The one-dmensional model predicts an accelerating flame even when the front approaches the cold cylinder wall. This yields a faster fuel consumption rate than those predicted by the one- and two-zone thermodynamic models which predict smoother burned fuel mass profiles.     

Trace element concentrations in an urban area and source identification

Summary A winter sampling programme of measurements of 10 trace elements (SO ₄ ⁼ , Fe, Pb, Zn, K, Ca, Mg, Cd, Cr and Ni) was performed at four different locations (two samples points were located in the city centre, another one in a suburban area and the other one in the outskirts) in the city of Valladolid (Spain), with the purpose of identifying the main sources that contribute to their presence in the environment. In order to identify the specific sources of the trace elements analysed,R andQ clustering techniques have been applied to the concentrations of these trace elements. The comparison of the results obtained through these techniques, their ability for getting information as well as their advantages and disadvantages are discussed.

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Riassunto è stato eseguito un programma di campionamento invernale di misurazioni di 10 elementi in tracce (SO ₄ ⁼ , Fe, Pb, Zn, K, Ca, Mg, Cd, Cr e Ni) in quattro diverse località (due punti dei campionatori si trovavano nel centro della città, uno nell'area suburbana ed un altro in periferia) della città di Valladolid (Spagna), allo scopo d'identificare le principali fonti che contribuiscono alla loro presenza nell'ambiente. Per identificare le fonti specifiche degli elementi in tracce analizzati, sono state applicate le tecniche di accumulazione alle concentrazioni di questi elementi in tracce. Si discute il confronto tra i risultati ottenuti con queste tecniche, la loro capacità di ottenere informazioni, nonchè i loro vantaggi e svantaggi.

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Резюме Приводятся результаты измерений по зимней программе контроля концентрации 10 элементов (SO ₄ ⁼ , Fe, Pb, Zn, K, Ca, Mg, Cd, Cr, и Ni) в 4 различных точках города Валладолида (Испания) (2 точки расположены в центре города, одна в городской зоне и одна на окраине). Цель программы—идентификация главных источников, которые дают вклад в загрязнение окружающей среды. Чтобы идентифицировать источники проанализированных элементов, используется техникаR иQ кластеризации для определения концентраций этих элементов. Проводится сравнение полученных результатов. Обсуждаются возможности этих методик для получения информации, а также их преимущества и недостатки.

     

Study of low concentrations of dicumyl peroxide on the molecular structure modification of LLDPE by reactive extrusion

The effect of low concentrations of dicumyl peroxide (DCP) on the molecular weight of a linear low density polyethylene (LLDPE) during a reactive extrusion process was studied. The experiments were arranged in a two level factorial design in order to evaluate the effect of peroxide concentration (X₂), temperature zones (X₁) and screw rpm (X₃) on the crosslinking of LLDPE. The melt flow index (MFI) was used as a response variable. It was verified that the thermal properties, crystalline melting temperature (T_m), the heat of fusion (ΔH_melt) and the degree of crystallinity (X%) tend to decrease with increase of the peroxide concentration. The crystallization temperature (T_c) increased up to 0.5% w/w peroxide, whereafter the level stays almost constant. In this study, it was also verified through the dynamic rheological data that there was an increase of the complex viscosity and the complex modulus (G′ and G″). These results can be an indication that there was an increase of the polymer average molecular weight (M_w) and polydispersity (MWD). The gel content of the samples, however, indicated that a three-dimensional network was negligible in the experimental conditions used.