Acyclic diene metathesis (ADMET) depolymerization: The synthesis of 1,4-polybutadiene telechelics

ADMET depolymerization is a viable route in the formation of tailored-molecular-weight polybutadiene telechelics. The bulk depolymerization requirement is met by initially forming macrocyclic butadiene, thereby decreasing the amount of macrocyclic formation. This synthetic scheme is capable of generating well-defined, tailored-molecular-weight polybutadiene telechelics (M̄_n = 2600).     

Modelling copolymerization kinetics

The composition and rate behavior of free radical copolymerizations is usually described by the Mayo-Lewis (ML) model and the associated reactivity ratios, r₁ and r₂. Particularly with respect to rate, a number of systems have been found to be poorly described by the simple ML model and the penultimate unit effect (PUE) model has been suggested as an explanation. A small but significant amount of work has been done with small model analogues of polymer chains and with ESR which has established the chemical feasibility of a PUE. This paper reviews recent work with pulsed laser polymerization kinetic measurements which are successfully described in terms of a modified PUE. It is concluded that the strength of the PUE correlates inversely with the monomer reactivity ratio product, r₁r₂. The effect is important for rate, but not for composition.     

Emission from electronically excited metal atoms during single-bubble Sonoluminescence

Variations in sonoluminescence (SL) from an acoustically driven but rapidly translating bubble in solutions of sulfuric acid with alkali-metal salts coincide with variations in translational bubble dynamics. At low acoustic pressures, emission from Ar excited states is observed and the bubble motion is smooth and elliptical. At elevated acoustic pressures, SL intensity decreases, emission from excited alkali-metal atoms is observed, and the bubble motion becomes increasingly erratic with frequent and abrupt changes in direction. These results provide a direct experimental link between single and multibubble SL and point toward the origins of sonochemical reactivity of nonvolatile species.     

超声无损检测中的数字匹配滤波技术

本文在超声无损检测技术中采用线性调频脉冲信号,并用数字化了的电激励信号自身作为数字匹配滤波器,在不同的时延带宽乘积的系统中得到了声脉冲的压缩信号。实验结果与理论预期值符合得很好.对于Hamming和Gaussian时间加权的信号,其旁瓣抑制可分别达到—41.9dB和—41.5dB.数字匹配滤波技术的引入可以有力地改善超声检测系统的性能。     

A theory of mixtures with microstructure

Zusammenfassung Eine Theorie der Mischungen von Flüssigkeiten mit Mikrostruktur wird von einem Energiepostulat hergeleitet, wobei das Prinzip der Indifferenz des Bezugssystems benutzt wird. Es werden Definitionen für die Wärmeproduktion der Mischung, innere Energie, Wärmeströmung und Spannungen gegeben, welche mit denjenigenTruesdells übereinstimmen.     

Highly Enantioselective,Organocatalytic, and Scalable Synthesis of a Rare cis,cis-Tricyclic Diterpenoid

A highly enantioselective, organocatalytic, and scalable synthesis of a very unusual cis-decalin-cis-hydrindane tricyclic diterpenoid system has been achieved. Despite the prevalent pharmacological space that the related trans,trans and trans,cis-systems occupy, there have been no reports of an asymmetric synthesis of the cis,cis systems in the literature until now. We demonstrate the flexibility of our approach not only through access to a diverse range of products, all of which are attained in exceptionally high selectivities, but also by showing their easy conversion to the corresponding trans,cis-system and other derivatives.     

Metastable states in the van der Waals-Maxwell theory

A slight modification of the recent Penrose and Lebowitz treatment of thermodynamic metastable states is presented. For the case of periodic boundary conditions, this modification allows the condition of metastability to be extended to all the metastable states in the van der Waals-Maxwell theory of the liquid-vapor phase transition, that is, for all states satisfyingf ₀()+1/2 ²>f(, 0+) andf₀()+x>0 wheref(, 0+) is the (stable) Helmholtz free energy density of the generalized van der Waals-Maxwell theory andf ₀() is the Helmholtz free energy density of a reference system with no long-range interaction, is a mean field-type term arising from a long-range Kac interaction, is the overall mean particle density, andx is any positive number. For the case of rigid-wall boundary conditions, a more restrictive condition is placed onx.     

Repulsive Casimir forces

We discuss repulsive Casimir forces between dielectric materials with nontrivial magnetic susceptibility. It is shown that considerations based on the naive pairwise summation of van der Waals and Casimir-Polder forces may not only give an incorrect estimate of the magnitude of the total Casimir force but even the wrong sign of the force when materials with high dielectric and magnetic responses are involved. Indeed repulsive Casimir forces may be found in a large range of parameters, and we suggest that the effect may be realized in known materials. The phenomenon of repulsive Casimir forces may be of importance both for experimental study and for nanomachinery applications.     

The incremental shear modulus in a compressible isotropic elastic material

A compressible isotropic elastic material is subjected to a finite pure homogeneous deformation. An infinitesimal simple shear is superposed on the deformation. Necessary and sufficient conditions are obtained for the shear modulus to be positive for all choices of the direction and plane of shear.

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Zusammenfassung Ein kompressibles isotropes elastisches Material wird einer endlichen homogenen Verformung ausgesetzt. Ein einfacher, infinitesimaler Schub wird dieser Verformung überlagert. Für alle Schubrichtungen und -ebenen werden notwendige und hinreichende Bedingungen für einen positiven Schubmodul hergeleitet.