Structuring of star-like multiarm polydimethylsiloxanes

The in situ structuring of multiarm star-like polydimethylsiloxanes (PDMS) synthesized on the base of the high-generation carbosilane dendrimers was studied. It was shown that, in contrast to the structures with 12, 16, and 48 arms, 128-arm star-like polydimethylsiloxanes form a stable mesophase formed by the PDMS arms. The data from polarized optical microscopy and differential scanning calorimetry revealed for the first time the formation of extended liquid-crystalline domains for the diorganosiloxane chains with the methyl substituents at the silicon atom. This effect can be attributed to the influence of close packing of the PDMS chains at the dendritic branching site in the case of actually multiarm structures.     

Structural, textural and adsorption characteristics of nanosilica mechanochemically activated in different media

The structural, textural, and adsorption characteristics of mechanochemically activated (MCA) fumed silica A-300 as dry or water, ethanol, or water/ethanol-wetted powders (0.5 g of a solvent per gram of silica) in a ball mill for 1-6 h were studied in comparison with those of the initial powder. The MCA treatment enhances bulk density (ρ(b)) of the powder (from 0.045 g/cm(3) for the initial silica to 0.4 g/cm(3) for 6 h-MCA-treated water-wetted silica) depending on medium type and MCA time (t(MCA)). Stronger effects are observed for the MCA treatment of water-wetted silica than of dry or ethanol- or water/ethanol-wetted samples. The MCA treatment weakly affects the specific surface area (S(BET)). However, void (pore) size distribution, porosity, particle aggregation and size distribution in aqueous suspension, behavior of interfacial water, properties of poly(vinyl alcohol)/silica composites and adsorption of gelatin depend more strongly on the t(MCA) and ρ(b) values. Some of the observed changes in the characteristics (e.g., gelatin adsorption) depend on the ρ(b) value but are independent of the medium type used on the MCA. Other characteristics are nonlinear functions of both t(MCA) and ρ(b) values.     

Stress state of laminar systems formed from shells of revolution with nonrigid layer contact

An approach is developed for determination of the strain state of elastic systems of layered orthotropic shells of revolution with elastic slip layers when the difference in the tangential displacements of the contact-surface layers is proportional to the tangential stress with a coefficient dependent on the arc length of the generatrix. A resolving system of differential equations and the conditions of the mating sections are obtained for the systems. Results are presented for the solution of specific problems. S. P. Timoshenko Institute of Mechanics, National Academy of Sciences of Ukraine, Kiev. Translated from Prikladnaya Mekhanika, Vol. 35, No. 9, pp. 51–57, September, 1999.     

A Facile Approach to Bis(isoxazoles), Promising Ligands of the AMPA Receptor

A convenient synthetic approach to novel functionalized bis(isoxazoles), the promising bivalent ligands of the AMPA receptor, was elaborated. It was based on the heterocyclization reactions of readily available electrophilic alkenes with the tetranitromethane-triethylamine complex. The structural diversity of the synthesized compounds was demonstrated. In the electrophysiological experiments using the patch clamp technique on Purkinje neurons, the compound 1,4-phenylenedi(methylene)bis(5-aminoisoxazole-3-carboxylate) was shown to be highly potent positive modulator of the AMPA receptor, potentiating kainate-induced currents up to 70% at 10⁻¹¹ M.