Amphiphile polydimethylsiloxane‐based networks reinforced with in situ generated silica

Polydimethylsiloxanes, side functionalized in different degrees with chloromethyl groups, were reacted with 4,4′‐bipyridyl and crosslinking occurred by the formation of ionic (bipyridinium) groups. The reactions were carried out in a silica sol–gel system, and thus, two networks were generated simultaneously: amphiphile siloxane‐organic and silica networks. The samples, processed as films, were characterized by Fourier transform infrared spectroscopy to verify the occurrence of the crosslinking reactions. Different techniques were also used to evaluate the properties: differential scanning calorimetry for emphasizing the transitions, scanning electron microscopy, and atomic force microscopy for the evaluation of the morphology and surface topography. The modification of the film surface topography, which depends on the solvent polarity, was also emphasized. The sorption capacities of the water vapors and of liquid solvents (water and chloroform) were investigated, and the obtained values proved to be a function of the polar group's content. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers     

Influence of the substitution degrees on the optical properties of cellulose acetates

Cellulose acetates with different substitution degrees possess a set of properties that makes them highly suitable for optical applications. This article discusses some of these properties, namely refractivity, dielectric properties, and transmittance. The contribution of the ratio between the molar refraction, which is proportional to the induced dipole moment, and the molar volume for the different atoms present in the studied polymers to the optical properties was investigated. The study illustrates the importance of such structural and compositional characteristics in tailoring some specific optical applications. Furthermore, optical transmission has been investigated in the 200–1000 nm range. To obtain the optical parameters, the approach proposed by Tauc for amorphous semiconductors has been used because of the similarity of the absorption edges. The values of pseudogap energy ranged between 3.44 and 4.70 eV, whereas those of Urbach energy were modified in the 18–37 meV range. All parameters determined have been related to the influence of the substitution degrees and of the synthesis condition. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Influence of octyl substituted octakis(dimethylsiloxy)octasilsesquioxane on the morphology and thermal and mechanical properties of low density polyethylene

Low density polyethylene (LDPE) was melt blended with octyl substituted octakis(dimethylsiloxy)octasilsesquioxane (AS8) prepared in high yield by hydrosilylation of octene with octakis(hydrodimethylsiloxy)octasilsesquioxane. Two procedures were used to incorporate AS8 into LDPE at nanolevel and their efficiency was investigated by X‐ray diffraction (XRD), scanning electron microscopy, atomic force microscopy (AFM), differential scanning calorimetry, thermogravimetry and mechanical characterization. A slight increase of crystallite mean dimensions was observed by XRD in both types of LDPE‐AS8 composites compared with neat LDPE. Better AS8 dispersion was observed by AFM in the composite prepared by the first procedure and the increase of both tensile strength and elongation at break in this composite confirmed the nanolevel dispersion and interface improvement in this case. AS8 influence in the final stages of polyethylene fracture was illustrated by AFM of elongated tensile specimens and a new mechanism of AS8 action was suggested. An interesting behavior consisting of simultaneous improvement of elastic modulus, strength and ductility was detected in the low range of polyhedral oligomeric silsesquioxane concentration in LDPE (0.5 wt%). © 2013 Society of Chemical Industry     

Photochemistry of 2,6‐di(4′‐azidobenzylidene)‐methylcyclohexanone in polymer matrices

2,6‐Di(4′‐azidobenzylidene)‐methylcyclohexanone (ABC) was one of the most used photoinitiators in negative photoresists industry, rendering insolubility of the resist films at short UV exposure times (several minutes). Although the photodecomposition of aromatic azides is very well established, the peculiarities of the irradiation medium impose specific reaction pathways for arylnitrenes. In this study, photoreactions of arylnitrenes resulted from ABC photolysis were applied in the photoinitiated crosslinking of 1,2‐polybutadiene (1,2‐PB), under soft monochromatic UV irradiation (365 nm). To elucidate the crosslinking mechanism, studies on a model compound were performed. 3‐Methyl‐1‐butene was chosen to simulate the monomeric unit of 1,2‐PB. As a support in photoproducts identifying and with the purpose of a deeper investigation of the ABC photochemistry alone, ABC was photolysed in a rigid matrix of poly(methyl methacrylate). © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44694.     

Trichloroethene degradation by UV/H2O2 advanced oxidation process: product study and kinetic modeling

The broadband UV irradiation of 1.1 mM trichloroethene (TCE) aqueous solution in the presence of 10.4 mM H2O2 resulted in formic, oxalic, dichloroacetic (DCA), and monochloroacetic (MCA) acids, as organic byproducts. The organic chlorine was converted completely to chloride ion as a final product. TCE and its degradation products were completely mineralized in 30 min, under a volume-averaged UV-C irradiant power of 35.7 W/L from a 1 kW medium-pressure mercury vapor arc lamp. TCE degraded primarily through hydroxyl radical-induced reactions and onlyto a low extentthrough direct UV photolysis and chlorine atom-induced chain reactions. The experimental patterns of TCE, H2O2, and detected reaction products combined with the literature information on radical reactions in the aqueous phase were used to postulate a degradation mechanism and to develop a kinetic model to predict the TCE decay, formation and degradation of byproducts, and pH and oxygen profiles. The agreement between the model calculations and the experimental data is satisfactory.     

Modelling the Dynamic Dependence Structure in Multivariate Financial Time Series

Abstract. The dependence structure in multivariate financial time series is of great importance in portfolio management. By studying daily return histories of 17 exchange‐traded index funds, we identify important features of the data, and we propose two new models to capture these features. The first is an extension of the multivariate BEKK (Baba, Engle, Kraft, Kroner) model, which includes a multivariate t‐type error distribution with different degrees of freedom. We demonstrate that this error distribution is able to accommodate different levels of heavy‐tailed behaviour and thus provides a better fit than models based on a multivariate t‐with a common degree of freedom. The second model is copula based, and can be regarded as an extension of the standard and the generalized dynamic conditional correlation model [Engle, Journal of Business and Economics Statistics (2002) Vol. 17, 425–446; Cappiello et al. (2003) Working paper, UCSD] to a Student copula. Model comparison is carried out using criteria including the Akaike information criteria and Bayesian information criteria. We also evaluate the two models from an asset‐allocation perspective using a three‐asset portfolio as an example, constructing optimal portfolios based on the Markowitz theory. Our results indicate that, for our data, the proposed models both outperform the standard BEKK model, with the copula model performing better than the extension of the BEKK model.     

Synthesis and characterization of polysaccharide-maghemite composite nanoparticles and their antibacterial properties

The aim of this study was to obtain saccharide (dextran and sucrose)-coated maghemite nanoparticles with antibacterial activity. The polysaccharide-coated maghemite nanoparticles were synthesized by an adapted coprecipitation method. X-ray diffraction (XRD) studies demonstrate that the obtained polysaccharide-coated maghemite nanoparticles can be indexed into the spinel cubic lattice with a lattice parameter of 8.35 Å. The refinement of XRD spectra indicated that no other phases except the maghemite are detectable. The characterization of the polysaccharide-coated maghemite nanoparticles by various techniques is described. The antibacterial activity of these polysaccharide-coated maghemite nanoparticles (NPs) was tested against Pseudomonas aeruginosa 1397, Enterococcus faecalis ATCC 29212, Candida krusei 963, and Escherichia coli ATCC 25922 and was found to be dependent on the polysaccharide type. The antibacterial activity of dextran-coated maghemite was significantly higher than that of sucrose-coated maghemite. The antibacterial studies showed the potential of dextran-coated iron oxide NPs to be used in a wide range of medical infections.     

Spin-spin relaxation times in myocardial hypertrophy induced by endocrine agents in rat

Magnetic resonance techniques afford a significant advantage for noninvasive diagnosis of cardiovascular pathology. The purpose of our present study was to assay the proton nuclear magnetic resonance (¹H-NMR) sensitivity in the differential diagnosis of certain endocrine cardiovascular complications. In this context, we investigated the water state and content in the hypertrophied myocardium. Male and female Wistar rats were treated with different hormones (hydrocortisone acetate, testosterone, estradiol, thyroid hormones) in combination with isoproterenol (a synthetic catecholamine that induces myocardial ischemia and hypertrophy). The animals were sacrificed after 20 days of treatment and samples of integral myocardium and left ventricular myocardium were analyzed on a¹H-NMR AREMI spectrometer (0.6 T; proton resonance at 25 MHz). The estimation ofT ₂ was made by Carr-Purcell-Meiboom-Gill pulse sequence. The data were fitted to a bi-exponential curve, yielding short (T ₂₁) values forbound water and long (T ₂₂) values forfree water. In order to evaluate the myocardial hypertrophy, the following ratios were calculated: integral myocardium to body weight; left ventricle to body weight; left ventricle to integral myocardium. The first two ratios were also calculated for dried tissue, in order to estimate its contribution to myocardial hypertrophy. Our findings demonstrate that myocardial hypertrophy is associated with a decrease ofT ₂₂, as a consequence of the increase in the dried component (i.e. proteins) of the tissue, while the total tissue, while the total tissue water (H₂O_t%, measured by gravimetry) was not significantly modified. Nevertheless, it is reasonable that the increase in the protein content would be proportional with the increase in H₂O_t%. The decrease ofT ₂₁ seems to be proportional with the level of left ventricle hypertrophy in female groups. The¹H-NMR measurements were much sensitive for the differential diagnosis of myocardial hypertrophy in the case of left ventricle.     

On the Hopcroft’s minimization technique for DFA and DFCA

We show that the absolute worst case time complexity for Hopcroft’s minimization algorithm applied to unary languages is reached only for deterministic automata or cover automata following the structure of the de Bruijn words. A previous paper by Berstel and Carton gave the example of de Bruijn words as a language that requires $O(nlogn)$ steps in the case of deterministic automata by carefully choosing the splitting sets and processing these sets in a FIFO mode for the list of the splitting sets in the algorithm. We refine the previous result by showing that the Berstel/Carton example is actually the absolute worst case time complexity in the case of unary languages for deterministic automata. We show that the same result is valid also for the case of cover automata and an algorithm based on the Hopcroft’s method used for minimization of cover automata. We also show that a LIFO implementation for the splitting list will not achieve the same absolute worst time complexity for the case of unary languages both in the case of regular deterministic finite automata or in the case of the deterministic finite cover automata as defined by S. Yu.     

Improved sensor network lifetime with multiple mobile sinks

A critical issue for data gathering in wireless sensor networks is the formation of energy holes near the sinks. Sensors near the sinks have to participate in relaying data on behalf of other sensors and thus will deplete their energy very quickly, resulting in network partitioning and limitation of the network lifetime. The solution that we propose in this paper is to use mobile sinks that change their location when the nearby sensors’ energy becomes low. In this way the sensors located near sinks change over time. In deciding a new location, a sink searches for zones with richer sensor energy.First, we study the improvement in network lifetime when sinks move on a predetermined path, along the perimeter of a hexagonal tiling. Two cases are considered for data gathering when sinks stop in the hexagon’s corners and when the sinks stop on multiple locations on the hexagon perimeter. This study shows an improvement of up to 4.86 times in network lifetime. Second, we design a distributed and localized algorithm used by the sinks to decide their next movement location such that the virtual backbone formed by the sinks remains interconnected at all times. Two extensions of the distributed algorithm, coverage requirement and limitation of the time-delivery requirement, are also addressed. Simulation results are presented to verify our approaches.