700 research outputs found

    Rubber Toughened and Nanoparticle Reinforced Epoxy Composites

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    Epoxy resins have achieved acceptance as adhesives, coatings, and potting compounds, but their main application is as matrix to produce reinforced composites. However, their usefulness in this field still limited due to their brittle nature. Some studies have been done to increase the toughness of epoxy composites, of which the most successful one is the modification of the polymer matrix with a second toughening phase. Resin Transfer Molding (RTM) is one of the most important technologies to manufacture fiber reinforced composites. In the last decade it has experimented new impulse, due to its favorable application to produce large surface composites with good technical properties and at relative low cost. This research work focuses on the development of novel modified epoxy matrices, with enhanced mechanical and thermal properties, suitable to be processed by resin transfer molding technology, to manufacture Glass Fiber Reinforced Composites (GFRC’s) with improved performance in comparison to the commercially available ones. In the first stage of the project, a neat epoxy resin (EP) was modified using two different nano-sized ceramics: silicium dioxide (SiO2) and zirconium dioxide (ZrO2); and micro-sized particles of silicone rubber (SR) as second filler. Series of nanocomposites and hybrid modified epoxy resins were obtained by systematic variation of filler contents. The rheology and curing process of the modified epoxy resins were determined in order to define their aptness to be processed by RTM. The resulting matrices were extensively characterized qualitatively and quantitatively to precise the effect of each filler on the polymer properties. It was shown that the nanoparticles confer better mechanical properties to the epoxy resin, including modulus and toughness. It was possible to improve simultaneously the tensile modulus and toughness of the epoxy matrix in more than 30 % and 50 % respectively, only by using 8 vol.-% nano-SiO2 as filler. A similar performance was obtained by nanocomposites containing zirconia. The epoxy matrix modified with 8 vol.-% ZrO2 recorded tensile modulus and toughness improved up to 36% and 45% respectively regarding EP. On the other hand, the addition of silicone rubber to EP and nanocomposites results in a superior toughness but has a slightly negative effect on modulus and strength. The addition of 3 vol.-% SR to the neat epoxy and nanocomposites increases their toughness between 1.5 and 2.5 fold; but implies also a reduction in their tensile modulus and strength in range 5-10%. Therefore, when the right proportion of nanoceramic and rubber were added to the epoxy resin, hybrid epoxy matrices with fracture toughness 3 fold higher than EP but also with up to 20% improved modulus were obtained. Widespread investigations were carried out to define the structural mechanisms responsible for these improvements. It was stated, that each type of filler induces specific energy dissipating mechanisms during the mechanical loading and fracture processes, which are closely related to their nature, morphology and of course to their bonding with the epoxy matrix. When both nanoceramic and silicone rubber are involved in the epoxy formulation, a superposition of their corresponding energy release mechanisms is generated, which provides the matrix with an unusual properties balance. From the modified matrices glass fiber reinforced RTM-plates were produced. The structure of the obtained composites was microscopically analyzed to determine their impregnation quality. In all cases composites with no structural defects (i.e. voids, delaminations) and good superficial finish were reached. The composites were also properly characterized. As expected the final performance of the GFRCs is strongly determined by the matrix properties. Thus, the enhancement reached by epoxy matrices is translated into better GFRC´s macroscopical properties. Composites with up to 15% enhanced strength and toughness improved up to 50%, were obtained from the modified epoxy matrices

    Layer-by-Layer Engineered Superparamagnetic Polyelectrolyte Hybrid Hollow Microspheres With High Magnetic Content as Drug Delivery System

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    <div><p>Polyelectrolyte hybrid hollow microspheres with sandwich structure of about 450 nm have been accomplished by layer-by-layer self-assembling of two modified ferroferric oxide nanoparticles, lysine modified ferroferric oxide nanoparticles (Fe<sub>3</sub>O<sub>4</sub>-LYs) and citrate modified ferroferric oxide nanoparticles (Fe<sub>3</sub>O<sub>4</sub>-CA), as the main assembling materials via electrostatic interaction for the first time. They are superparamagnetic with saturation magnetization of 45.69 emu/g, revealing their high magnetic content of 70%. As drug delivery system, they also exhibited pH-stimuli responsive controlled release of an anticancer drug doxorubicin, following the Fickian diffusion model. Their unique structure and high magnetic content make them good candidate for targeted delivery.</p></div

    Novel Smart Yolk/Shell Polymer Microspheres as a Multiply Responsive Cargo Delivery System

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    An effective strategy was developed to fabricate the novel dually thermo- and pH-responsive yolk/shell polymer microspheres as a drug delivery system (DDS) for the controlled release of anticancer drugs via two-stage distillation precipitation polymerization and seed precipitation polymerization. Their pH-induced thermally responsive polymer shells act as a smart “valve” to adjust the diffusion of the loaded drugs in/out of the polymer containers according to the body environments, while the movable P­(MAA-<i>co</i>-EGDMA) cores enhance the drug loading capacity for the anticancer drug doxorubicin hydrochloride (DOX). The yolk/shell polymer microspheres show a low leakage at high pH values but significantly enhanced release at lower pH values equivalent to the tumor body fluid environments at human body temperature, exhibiting the apparent tumor-environment-responsive controlled “on–off” drug release characteristics. Meanwhile, the yolk/shell microspheres expressed very low in vitro cytotoxicity on HepG2 cells. Consequently, their precise tumor-environment-responsive drug delivery performance and high drug loading capacity offer promise for tumor therapy

    Overall and Disease-Specific Survival (N = 1207).

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    <p><b>Abbreviations</b>: NR, not reached.</p

    Mechanism and Origins of Ligand-Controlled Linear Versus Branched Selectivity of Iridium-Catalyzed Hydroarylation of Alkenes

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    The iridium-catalyzed carbonyl-directed hydroarylation of monosubstituted alkenes developed by Bower and co-workers [Crisenza, G. E. M.; McCreanor, N. G.; Bower, J. F. <i>J. Am. Chem. Soc</i>. <b>2014</b>, 136, 10258–10261] provides an efficient strategy for highly branched-selective hydroarylation of both aryl- and alkyl-substituted alkenes. Density functional theory calculations in the present study revealed that the unique regiochemical control in this reaction is due to an unconventional modified Chalk–Harrod-type mechanism. Instead of the commonly accepted Chalk–Harrod-type mechanism of transition metal-catalyzed hydroarylation that involves C–H oxidative addition, olefin migratory insertion into the Ir–H bond, and C–C reductive elimination, the Ir-catalyzed reaction occurs via migratory insertion of the olefin into the Ir–aryl bond and C–H reductive elimination. The experimentally observed ligand-controlled selectivity is attributed to a combination of electronic and steric effects in the selectivity-determining olefin migratory insertion step. Ligand steric contour maps show that, in reactions with large-bite-angle bisphosphine ligands, such as d<sup>F</sup>ppb, the steric repulsions between the substrate and the aryl substituents on the ligand lead to complete branched selectivity, and the linear selectivity in reactions with small-bite-angle ligands is due to electronic effects that favor 2,1-olefin migratory insertions

    Mechanism and Origins of Selectivities in the Copper-Catalyzed Dearomatization-Induced <i>ortho</i> C–H Cyanation of Vinylarenes

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    The mechanism of the copper-catalyzed regioselective <i>ortho</i> C–H cyanation of vinylarenes has been investigated using density functional theory calculations. This C–H cyanation is composed of two discrete catalytic cycles (the copper-catalyzed electrophilic cyanative dearomatization and the subsequent base-catalyzed hydrogen transposition) that furnish the <i>ortho</i> C–H cyanated arenes. The electrophilic cyanation step features a unique six-membered transition state, leading to the formation of the dearomatized intermediate with a high level of site selectivity. Such dearomatization significantly increases the reactivity of the C–H bond, thereby enabling the base-assisted C–H activation in the following steps

    Dispersion Estimation and Its Effect on Test Performance in RNA-seq Data Analysis: A Simulation-Based Comparison of Methods

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    <div><p>A central goal of RNA sequencing (RNA-seq) experiments is to detect differentially expressed genes. In the ubiquitous negative binomial model for RNA-seq data, each gene is given a dispersion parameter, and correctly estimating these dispersion parameters is vital to detecting differential expression. Since the dispersions control the variances of the gene counts, underestimation may lead to false discovery, while overestimation may lower the rate of true detection. After briefly reviewing several popular dispersion estimation methods, this article describes a simulation study that compares them in terms of point estimation and the effect on the performance of tests for differential expression. The methods that maximize the test performance are the ones that use a moderate degree of dispersion shrinkage: the DSS, Tagwise wqCML, and Tagwise APL. In practical RNA-seq data analysis, we recommend using one of these moderate-shrinkage methods with the QLShrink test in QuasiSeq R package.</p></div

    Large series of patients with AITL reported in the literature.

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    <p><b>Abbreviations</b>: AITL, angioimmunoblastic T-cell lymphoma.</p><p>*Patients were treated with high-dose therapy followed by autologous stem-cell transplantation.</p>#<p>Age at stem-cell transplantation.</p>∞<p>Stages IV, %.</p>§<p>Patients were treated with allogeneic stem-cell transplantation.</p

    Reduction-Responsive Core–Shell–Corona Micelles Based on Triblock Copolymers: Novel Synthetic Strategy, Characterization, and Application As a Tumor Microenvironment-Responsive Drug Delivery System

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    A facile and effective approach was established for fabricating core–shell–corona micelles by self-assembly of poly­(ethylene glycol)-<i>b</i>-poly­(acrylic acid-<i>co</i>-<i>tert</i>-butyl acrylate)-poly­(ε-caprolactone) (PEG<sub>43</sub>-<i>b</i>-P­(AA<sub>30</sub>-<i>co</i>-<i>t</i>BA<sub>18</sub>)-<i>b</i>-PCL<sub>53</sub>) triblock copolymer, synthesized via a combination of ring-opening polymerization (ROP), atom transfer radical polymerization (ATRP), click chemistry, and hydrolyzation. The prenanovehicles with three different hydrolysis degrees from PEG<sub>43</sub>-<i>b</i>-P<i>t</i>BA<sub>48</sub>-<i>b</i>-PCL<sub>53</sub> were developed to evaluate the drug loading capacity (DLC) and drug encapsulation efficiency (DEE). After cross-linking with a disulfide bond to regulate the drug release kinetics, the spherical core–shell–corona micelles with average diameter of 52 ± 4 nm were obtained in aqueous solution. The reduction-responsive cross-linked micelles showed a slow sustained release in normal physiological conditions and a rapid release upon exposure to simulated tumor intracellular conditions. In addition, the cytotoxic analysis and HepG2 cell growth inhibition assays demonstrated their remarkable biocompatibility and similar excellent anticancer activity as the free doxorubicin (DOX), which has also been revealed by the confocal laser scanning microscope (CLSM) analysis. So the reduction-sensitive core–shell–corona micelles are expected to be promising tumor microenvironment-responsive nanovehicles for hydrophobic drugs by glutathione (GSH) triggering

    Hydrophobic-Polymer-Grafted Graphene Oxide Nanosheets as an Easily Separable Adsorbent for the Removal of Tetrabromobisphenol A

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    Hydrophobic polymer brushes have been grafted from graphene oxide nanosheets (GO) via the facile surface-initiated redox radical polymerization of <i>tert</i>-butyl acrylate (tBA) from the GO with cerium ammonium nitrate (CAN) as an oxidant. After the hydrophobic surface modification, the poly­(<i>tert</i>-butyl acrylate) (PtBA)-grafted graphene oxide nanosheets (GO-PtBA) could still be dispersed in water because of the remaining oxygen-containing groups but deposited within 40 min. The feature makes it an easily separable adsorbent for environmental pollutants. For example, tetrabromobisphenol A (TBBPA) could be removed from aqueous solution via hydrogen bonds (between hydroxyl groups of TBBPA and hydroxyl and carboxyl groups of GO) and π–π interactions (between the benzene ring of TBBPA and GO), with an adsorption capacity of 22.2 mg g<sup>–1</sup> at pH 7.0. The TBBPA-adsorbed GO-PtBA could be deposited completely within 30 min, and the adsorbed TBBPA could be easily desorbed with ethanol, demonstrating its good recyclability
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