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151.
Nanostructure development in nylon 6-Cloisite® 30B composites. Effects of the preparation conditions
Massimo Paci 《European Polymer Journal》2010,46(5):838-853
PA6 composites with Cloisite® 30B (30B), prepared by different procedures, i.e., melt compounding, static annealing and solution blending, have been characterized by X-ray diffraction and microscopic analyses (TEM, SEM, POM) in order to shed more light on the mechanism of nanostructure development. It has been demonstrated that intercalation of the PA6 chains within the 30B galleries takes place very rapidly, in the absence of applied stresses, even when the size of the clay particles is relatively large (tens of microns) and the clay loading is very high (even 50 wt.%). It has also been shown that, if, conversely, the filler content is low (∼10 wt.% or less) and the particles are tiny (e.g., as for polymer/clay mixtures prepared by precipitation from a common solution), intercalation continues, under quiescent conditions, and leads in reasonable times to complete destruction of the silicate platelets stacking order. The composites with higher filler contents display a mixed exfoliated/intercalated morphology, with the intercalated silicate stacks characterized by an interlayer distance of about 3.7 nm. Contrary to statically annealed composites, the melt kneaded ones are characterized by a homogeneous dispersion of the filler particles and a local parallel orientation of the silicate platelets that induces, during polymer crystallization, an orientation of the polymer crystallites parallel to the faces of the compression molded specimens. Experiments carried out using 30B samples previously treated at 250 °C for 4 h under vacuum (30Bdegr) indicate that this treatment, probably due to the collapsed interlayer spaces, lowers the extent of PA6 chains intercalation. Thus, the relevant PA6/30Bdegr composites are characterized by the coexistence of unintercalated clay tactoids/agglomerates and individual silicate layers formed as result of intercalation on the edges of the filler particles. 相似文献
152.
Massimo Bietti Alessandra Calcagni Daniel Oscar Cicero Roberto Martella Michela Salamone 《Tetrahedron letters》2010,51(31):4129-1537
A product study on the reactivity of a 1,1-diarylalkoxyl radical bearing 2,2-diphenylcyclopropyl groups in the para-positions has been carried out. The exclusive formation of a product deriving from cyclopropyl ring-opening has been observed, indicating that 1,1-diarylalkoxyl radicals exist in equilibrium with a bridged 1-oxaspiro[2,5]octadienyl radical. This represents the first experimental evidence in support of the stepwise nature of the O-neophyl rearrangement of 1,1-diarylalkoxyl radicals. 相似文献
153.
This short review describes a breakthrough provided by the synthesis of d6 metal complexes containing the intact molecules P4 and P4S3. The coordinated cage molecules acquire unexpected reactivity and undergo dismutation reactions in mild conditions in the presence of water. The outcomings are obtained either in form of free or coordinated molecules; the former are hypophosphorous and phosphorous acids, the latter comprise, besides phosphine, PH3, such species as thiophosphinous acid, PH2SH, diphosphane, P2H4, 1-hydroxytriphosphane, PH(OH)PHPH2 and 1,1,4-tris-hydroxytetraphosphane, P(OH)2PHPHPH(OH), which are either unknown or extremely reactive as free molecules. The formation of the above molecules provides a clue to the hydrolytic activation of the P4 and P4S3 cage molecules. 相似文献
154.
Agnes Voitl Alessandro Butté Massimo Morbidelli 《Journal of chromatography. A》2010,1217(34):5492-5500
Experiments with human serum albumin on a strong cation exchange resin exhibit a peculiar elution pattern: the protein elutes with two peaks in a modifier gradient. This behavior is modeled with a general rate model, where the two elution peaks are modeled with two binding conformations, one of which is at equilibrium conditions, while for the other, the adsorption process is rate limited. Isocratic experiments under nonadsorbing conditions were used to characterize the mass transfer process. The isotherm of both adsorption conformations as well as the kinetic of adsorption and desorption for the second conformation are functions of the modifier concentration. They are evaluated with linear modifier gradient experiments and step experiments with various adsorption times. All experimental features are well reproduced by the proposed modified general rate model. 相似文献
155.
Ayers J Apetre N Ruzzene M Sabra K 《The Journal of the Acoustical Society of America》2011,129(2):585-588
A single head scanning laser Doppler vibrometer is used for the estimation of the polarization of the first symmetric (S(0)) and antisymmetric (A(0)) Lamb wave modes. The measurements at two known incidence angles are performed in order to resolve the two components of motion. Filtering in the frequency/wavenumber domain of the response recorded along a scan line separates the contributions from each mode and allows the evaluation of the corresponding elliptical trajectories of particle motion. Comparison between measured and analytically estimated trajectories validates the measurement technique and suggests its application for the development of material characterization and diagnostics tools. 相似文献
156.
A frequency selective acoustic transducer (FSAT) is proposed for directional sensing of guided waves. The considered FSAT design is characterized by a spiral configuration in wavenumber domain, which leads to a spatial arrangement of the sensing material producing output signals whose dominant frequency component is uniquely associated with the direction of incoming waves. The resulting spiral FSAT can be employed both for directional sensing and generation of guided waves, without relying on phasing and control of a large number of channels. The analytical expression of the shape of the spiral FSAT is obtained through the theoretical formulation for continuously distributed active material as part of a shaped piezoelectric device. Testing is performed by forming a discrete array through the points of the measurement grid of a scanning laser Doppler vibrometer. The discrete array approximates the continuous spiral FSAT geometry, and provides the flexibility to test several configurations. The experimental results demonstrate the strong frequency dependent directionality of the spiral FSAT and suggest its application for frequency selective acoustic sensors, to be employed for the localization of broadband acoustic events, or for the directional generation of Lamb waves for active interrogation of structural health. 相似文献
157.
The thermal conductivity of disordered silicon-germanium alloys is computed from density-functional perturbation theory and with relaxation times that include both harmonic and anharmonic scattering terms. We show that this approach yields an excellent agreement at all compositions with experimental results and provides clear design rules for the engineering of nanostructured thermoelectrics. For Si(x)Ge(1-x), more than 50% of the heat is carried at room temperature by phonons of mean free path greater than 1 μm, and an addition of as little as 12% Ge is sufficient to reduce the thermal conductivity to the minimum value achievable through alloying. Intriguingly, mass disorder is found to increase the anharmonic scattering of phonons through a modification of their vibration eigenmodes, resulting in an increase of 15% in thermal resistivity. 相似文献
158.
We discuss the problem of localization of 4D massless states in Randall-Sundrum 2 (one-brane) models. A Randall-Sundrum 2 construction starting from N=8 gauged supergravity in 5D anti-de Sitter space gives rise to an N=4 supergravity-matter system. We explicitly show that only the modes of the N=4 graviton supermultiplet localize on the 4D brane, streamlining and generalizing previous works. We also point out that while charged 1/4 BPS black holes do exist in the 4D theory, they are always produced in sets of total charge zero. This zero-charge configuration uplifts to a 5D metric without naked singularities, thus avoiding the curvature singularity of the 5D uplift of an isolated charged BPS black hole. Finally, we resolve a puzzle with localization of massless high spin fields on a (putative) Randall-Sundrum 2 construction based on Vasiliev?s high spin theories. We show that while high spin fields do localize, the gauge symmetry that ensures decoupling of their unphysical polarizations is anomalous. This implies that the high spin fields must acquire a mass. 相似文献
159.
Using five alternative data sets and a range of specifications concerning the underlying linear predictability models, we study whether long-run dynamic optimizing portfolio strategies may actually outperform simpler benchmarks in out-of-sample tests. The dynamic portfolio problems are solved using a combination of dynamic programming and Monte Carlo methods. The benchmarks are represented by two typical fixed mix strategies: the celebrated equally-weighted portfolio and a myopic, Markowitz-style strategy that fails to account for any predictability in asset returns. Within a framework in which the investor maximizes expected HARA (constant relative risk aversion) utility in a frictionless market, our key finding is that there are enormous difference in optimal long-horizon (in-sample) weights between the mean–variance benchmark and the optimal dynamic weights. In out-of-sample comparisons, there is however no clear-cut, systematic, evidence that long-horizon dynamic strategies outperform naively diversified portfolios. 相似文献
160.
This article is the first in a series devoted to Kato’s Euler system arising from p-adic families of Beilinson elements in the K-theory of modular curves. It proves a p-adic Beilinson formula relating the syntomic regulator (in the sense of Coleman-de Shalit and Besser) of certain distinguished elements in the K-theory of modular curves to the special values at integer points ≥ 2 of the Mazur-Swinnerton-Dyer p-adic L-function attached to cusp forms of weight 2. When combined with the explicit relation between syntomic regulators and p-adic étale cohomology, this leads to an alternate proof of the main results of [Br2] and [Ge] which is independent of Kato’s explicit reciprocity law. 相似文献