Cyclobis[n]helicenes (n=3 or 5) are chiral D2-symmetric π-conjugated macrocycles with stable lemniscular, or figure-eight, shapes. The conformational analysis of five different cyclobis[n]helicenes revealed that these molecules can only exist as their lemniscular conformers with high barriers to enantiomerization (>200 kJ mol−1). The enantiomers of a cyclobis[5]helicene were resolved by HPLC and their unusual chiroptical properties were attributed to the inherent chirality of their macrocyclic figure-eight. 相似文献
Ionic electro-active polymer is an active material consisting in a polyelectrolyte (for example Nafion). Such material is usually used as thin film sandwiched between two platinum electrodes. The polymer undergoes large bending motions when an electric field is applied across the thickness. Conversely, a voltage can be detected between both electrodes when the polymer is suddenly bent. The solvent-saturated polymer is fully dissociated, releasing cations of small size. We used a continuous medium approach. The material is modelled by the coexistence of two phases; it can be considered as a porous medium where the deformable solid phase is the polymer backbone with fixed anions; the electrolyte phase is made of a solvent (usually water) with free cations. The microscale conservation laws of mass, linear momentum and energy and the Maxwell’s equations are first written for each phase. The physical quantities linked to the interfaces are deduced. The use of an average technique applied to the two-phase medium finally leads to an Eulerian formulation of the conservation laws of the complete material. Macroscale equations relative to each phase provide exchanges through the interfaces. An analysis of the balance equations of kinetic, potential and internal energy highlights the phenomena responsible of the conversion of one kind of energy into another, especially the dissipative ones : viscous frictions and Joule effect. 相似文献
A high performance liquid chromatographic (HPLC) method for the determination of metronidazole in new gel formulations was developed and validated according to the recommendations of the International Harmonization Conference. An adaptation of the method described for metronidazole determination in plasma has been used, and the step of extraction has been developed. The method was demonstrated to be linear over a range of 60–140% of nominal label claim, accurate (100.08±0.63%) and precise (0.67% for intra-assay and 1.68% for inter-assay). Single point calibration was chosen for analysis because of its simplicity and wide use in the pharmaceutical industry for content uniformity analysis. 相似文献
In this paper we review the salient features of the different instabilities exhibited by the quasi-1D system BaVS3 and show that there is a subtle interplay between the different phases stabilized. The analysis of the Peierls instability shows that the mobile dz² electrons are more localized than calculated because of their strong correlation with the localized e(t2g) electrons. The complex AF magnetic structure of BaVS3 incorporates the magnetization of the e(t2g) electrons with the Peierls pairing of the dz² electrons into magnetic singlets. Finally, we propose that the zig-zag disorder remaining after an incomplete orthorhombic phase transition could change the sign of the magnetic coupling and thus help to stabilize the canted ferromagnetism observed in non stoichiometric BaVS3−δ and Sr and Ba substituted compounds. 相似文献
The study of the influence of an external electric field on the motion from rest of a domain wall in ferroelectric crystals is presented. The solution of the problem representing the translational and rotational motions of the chain is sought in terms of solitary waves. Both electric and mechanical state of the structure in domains can be determined. The evolution of the velocity of the wall altered by the applied field is determined by means of energy arguments accounting for electromechanical couplings. A numerical simulation is given which illustrates the transient motion from rest of a wall separating two ferroelectric domains. 相似文献
The Lewis acid B(C6F5)3 in combination with hydrosilanes exhibits remarkable activity in the oligomerization of sulfone‐ and phosphonate‐based monomers. This process opens new routes to high‐tech silicone‐based materials, i.e., thermoplastic elastomers and heat‐resistant polysiloxanes.
Summary The paper presents an efficient two-dimensional approach to piezoelectric plates in the framework of linear theory of piezoelectricity.
The approximation of the through-the-thickness variations accounts for the shear effects and a refinement of the electric
potential. Using a variational formalism, electromechanically coupled plate equations are obtained for the generalized stress
resultants as well as for the generalized electric inductions. The latter are deduced from the conservative electric charge
equation, which plays a crucial role in the present model. Emphasis is placed on the boundary conditions at the plate faces.
The model is used to examine some problems for cylindrical bending of a single simply supported plate. Number of situations
are examined for a piezoelectric plate subject to (i) an applied electric potential, (ii) a surface density of force, and
(iii) a surface density of electric charge. The through-thickness distributions of electromechanical quantities (displacements,
stresses, electric potential and displacement) are obtained, and compared with results provided by finite element simulations
and by a simplified plate model without shear effects. A good agreement is observed between the results coming from the present
plate model and finite element computations, which ascertains the effectiveness of the proposed approach to piezoelectric
plates.
Received 17 July 2000; accepted for publication 26 September 2000 相似文献
Summary: Iodine transfer polymerization of vinyl acetate in aqueous miniemulsion, initiated by UV radiation in the presence of an α,ω-diiodo-poly(dimethylsiloxane) macrophotoiniferter has been performed. The formation of a triblock copolymer latex PVAc-b-PDMS-b-PVAc has been evidenced by 1H-NMR and size exclusion chromatography. The size of the PDMS and PVAc blocks were modulated thus opening the way to a wide range of copolymers with different properties. A detailed study of the reaction mechanism showed the importance of the aqueous dispersed medium to achieve a controlled polymerization. 相似文献