Constitutive modeling for characterizing the compressive behavior of PMMA open‐cell foams

A constitutive model for evaluating the compressive behavior of Poly(methyl‐methacrylate) (PMMA) open‐cell foams is herein proposed. Specifically, the study investigates the viscoelastic and viscoplastic behaviors of the PMMA open‐cell foams. The constitutive equation is expressed in terms of the following polymer and foam properties: elastic modulus, relative density, as well as the relaxation and densification constants. PMMA open‐cell foams are manufactured using a gas foaming/particulate leaching method and uniaxial compression tests are performed. The mechanical properties and compressive stress‐strain responses obtained from the experiments are compared with those predicted by the proposed constitutive model. The results suggest that the constitutive model is an apt one for assessing and evaluating the compressive behaviors of PMMA open‐cell foams. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 436–443, 2007     

The Biodiversity of the Genus Dictyota: Phytochemical and Pharmacological Natural Products Prospectives

Although a broad variety of classes of bioactive compounds have already been isolated from seaweeds of the genus Dictyota, most different species are still chemically and biologically unexplored. Dictyota species are well-known brown seaweeds belonging to the Dictyotaceae (Phaeophyta). The phytochemical composition within the genus Dictyota has recently received considerable interest, and a vast array of components, including diterpenes, sesquiterepenes, sterols, amino acids, as well as saturated and polyunsaturated fatty acids, have been characterized. The contribution of these valued metabolites to the biological potential, which includes anti-proliferative, anti-microbial, antiviral, antioxidant, anti-inflammatory, and anti-hyperpigmentation activities, of the genus Dictyota has also been explored. Therefore, this is the most comprehensive review, focusing on the published literature relevant to the chemically and pharmacologically diverse biopharmaceuticals isolated from different species of the genus Dictyota during the period from 1976 to now.     

Enhanced FPGA realization of the fractional-order derivative and application to a variable-order chaotic system

Nonlinear Dynamics - The efficiency of the hardware implementations of fractional-order systems heavily relies on the efficiency of realizing the fractional-order derivative operator. In this work,...     

An Accurate Solution of the Poisson Equation by the Finite Difference-Chebyshev-Tau Method

ＩｎｔｒｏｄｕｃｔｉｏｎＭａｎｙｓｃｈｏｌｏｒｓｈａｖｅｄｉｓｃｕｓｓｅｄｔｈｅｓｏｌｕｔｉｏｎｏｆｔｈｅｔｗｏ＿ｄｉｍｅｎｓｉｏｎａｌＰｏｉｓｓｏｎｅｑｕａｔｉｏｎ .ＡｍａｔｒｉｘｄｉａｇｏｎａｌｉｚａｔｉｏｎｍｅｔｈｏｄｗａｓｄｅｖｅｌｏｐｅｄｂｙＨａｉｄｖｏｇｅｌａｎｄＺａｎｇ[1]ｆｏｒｔｈｅｓｏｌｕｔｉｏｎｏｆｔｈｅｔｗｏ＿ｄｉｍｅｎｓｉｏｎａｌＰｏｉｓｓｏｎｅｑｕａｔｉｏｎ .Ｔｈｉｓｍｅｔｈｏｄｉｓｅｆｆｉｃｉｅｎｔｂｕｔｒｅｑｕｉｒｅｓａｐｒｅｐｒｏｃｅｓｓｉｎｇｃａｌｃｕｌａｔｉｏ…     

Finite element evaluation of stresses on closed leaflets of bioprosthetic heart valves with flexible stents

The purpose of this study is to evaluate the influence of stent flexibility on the magnitude and distribution of stresses on the closed leaflets of a porcine bioprosthetic valve. The finite element technique which incorporated large deformation theory has been used in the analysis. Nonlinearities due to geometry, material and pressure dependent boundary conditions are included in the model. An incremental method has been employed in determining the stresses. The pressure was incremented from 0 to 21.3 kPa. Three separate stent flexibilities were considered and this was achieved by modifying the Young's modulus of the stent material. The calculated radial deflections of the tip of the stent post were in agreement with the results of other investigators. The distribution of stresses in the leaflet of a valve mounted on a flexible stent were compared to the stresses in the leaflet of a valve mounted on a rigid stent. Stent flexibility markedly reduced the stresses in the main body of the leaflet in comparison to a rigid stent, but did not affect the magnitude of stresses near commissures.     

Autoxidation of NO in aqueous solution

The reaction of NO with O₂ has been investigated in aqueous solution. As demonstrated by ion chromatography, the sole product is NO₂^?. Kinetic studies of the reaction by stopped-flow methods with absorbance and conductivity detection are in agreement that the rate law is -d[O₂]/dt=k[NO]²[O₂] with k = 2.1 × 10⁶ M^?2 s^?1 at 25°C. This rate law is unaffected by pH over the range from pH 1 to 13, and it holds with either NO or O₂ in excess. By studying the reaction over the temperature range from 10 to 40°C, the following activation parameters were obtained: ΔH^≠ = 4.6 ± 2.1 kJ mol^?1 and ΔS^≠=?96 plusmn; 4 J K^?1 mol^?1. © 1993 John Wiley & Sons, Inc.     

Neighboring group participation by the pyrrole nucleus

Using proton and carbon-13 NMR spectroscopy, the transformation of 2-hydroxyethylpyrroles (5,15) into 2-haloethylpyrroles by treatment with thionyl chloride or carbon tetrabromide/triphenylphosphine is shown to proceed by randomization of the two side-chain carbon atoms. A spirocyclopropylpyrrolium ion (19) is postulated as an intermediate in this unexpected process.     

Efficient DNA binding by optically Pure Ruthenium Tris(bipyridyl) complexes incorporating carboxylic functionalities. Solution and structural analysis

Herein, we report on the binding of optically pure ruthenium complexes Delta- or Lambda-[Ru(bpy)(2)(L-L)][PF(6)](2) [L-L = Hcmbpy = 4-carboxy-4'-methyl-2,2'-bipyridine (1), L-L = H(2)dcbpy = 4,4'-dicarboxy-2,2'-bipyridine (2)] to DNA. The binding constants of the two enantiomeric Delta-1 and Lambda-1 complexes to DNA were estimated from titration monitored by (1)H NMR spectroscopy. 2D transferred NOESY (TRNOESY) experiments support the conclusion that Delta-1 and Lambda-1 bind to DNA and that an intermediate-to-fast exchange occurs between bound and free Ru(II) complex. Further, evidence for enantioselective DNA cleavage by Delta-2 is provided by means of gel electrophoresis performed in the presence and in the absence of light; in contrast, the Lambda-2 enantiomer does not. The IR spectrum of enantiomer Delta-2 (or Lambda-2) compared to that of the racemate (rac-2) gives evidence that, in the latter form, the enantiomers are strongly associated. Moreover the X-ray structure of rac-2 was also determined and exhibits as an outstanding feature the formation of a one-dimensional supramolecular species in which the cohesion of the system is maintained by strong hydrogen bonding between carboxylic acid groups of enantiomers Delta-2 and Lambda-2 (cationic parts) with d(O...O) = 2.6 A in agreement with the infrared results. The conclusion that can be drawn from IR and X-ray spectroscopies together is that the self-association in rac-2 is strong.     

Vrănceanu connections and foliations with bundle-like metrics

We show that the Vrănceanu connection which was initially introduced on non-holonomic manifolds can be used to study the geometry of foliated manifolds. We prove that a foliation is totally geodesic with bundle-like metric if and only if this connection is a metric one. We introduce the notion of a foliated Riemannian manifold of constant transversal Vrănceanu curvature and the notion of a transversal Einstein foliated Riemannian manifold. The geometry of these two classes of manifolds is studied and the relationship between them is determined.     

Nonlinear eigenvalue problems with symmetry

In this paper we use the conjugate gradient predictor corrector method (CGPCM) in the context of continuation methods. By exploiting symmetry in certain nonlinear eigenvalue problems, we can decompose the centered difference discretization matrices into small ones and reduce computational cost. We use the cyclic group of order two to divide the system into two smaller systems. We reduce the cost and the computational time by combining CGPCM with the idea of the exploiting symmetries. Theoretical and numerical results are presented. Conclusions are given.