Sol–gel transition and rheological properties of silica nanoparticle dispersions

Possible variants of the rheological behavior of silica model dispersions have been analyzed. Different types of interaction between the particles and a dispersion medium make it possible to obtain different systems from low-viscosity sols to gels. Proton-donor (water) and aprotic (dimethyl sulfoxide) media have been used for comparison. Dispersions in the aprotic medium behave as non-Newtonian viscous fluids exhibiting shear thinning or shear thickening depending on deformation rate. Aqueous dispersions are viscoelastic and viscoplastic objects that exhibit the shear thickening at stresses higher than the yield stress. The introduction of small amounts of poly(ethylene oxide) into the organic dispersion medium initiates gelation. An increase in the polymer content in the dispersion medium above the concentration corresponding to the formation of a macromolecular network promotes an increase in stiffness and strength of the gels. The rheological behavior of gels is influenced by the polymer molecular mass and its affinity for a solvent.     

Statistical mechanics of the glass transition in one-component liquids with an anisotropic potential

We study a recently introduced model of one-component glass-forming liquids whose constituents interact with an anisotropic potential. This system is interesting per se and as a model of liquids such as glycerol (interacting via hydrogen bonds) which are excellent glass formers. We work out the statistical mechanics of this system, encoding the liquid and glass disorder using appropriate quasiparticles (36 of them). The theory provides a full explanation of the glass transition phenomenology, including the identification of a diverging length scale and a relation between the structural changes and the diverging relaxation times.     

The Effect of Tackifier on the Properties of Pressure-Sensitive Adhesives Based on Styrene–Butadiene–Styrene Rubber

Russian Journal of Applied Chemistry - Pressure-sensitive polymer adhesives based on styrene–butadiene–styrene rubbers and aromatic petroleum hydrocarbon resins making the block...     

The role of chain structure in the rheological behavior of vinyl acetate-vinyl alcohol copolymers

A comparative study of the viscoelastic properties of melts of vinyl acetate-vinyl alcohol copolymers with equimolar compositions characterized by different statistical distributions of chain units has been performed. It has been shown that the principle of temperature-frequency superposition is obeyed by copolymers close to a random copolymer, but is violated by copolymers with the block distribution of units. Unlike amorphous random copolymers, a multiblock copolymer is characterized by weak crystallinity, the absence of the relaxation flow state, and a more pronounced tendency to form interchain hydrogen bonds both between two hydroxyl groups and between hydroxyl and ester groups.     

Dark matter in galaxies and the growth of giant black holes

The relationship of dark matter to giant black holes (BHs) in galactic nuclei is investigated. The simultaneous evolution of dark and baryonic matter under the effect of an averaged self-consistent gravitational field is considered. The distribution of dark matter is shown to remain spherically symmetric even if there is an appreciable asymmetry in the distribution of baryonic matter in the galaxy. A kinetic equation that describes the evolution of the distribution function for dark matter with gravitational scattering by stars is derived. A significant flux of dark matter on a seed BHe at the galactic center is shown to arise under these conditions. The law of growth of the seed BH via the absorption of dark matter has been established. The seed BH is shown to grow significantly, up to 10⁷–10⁸ M⊙, in the lifetime of the galaxy. Observational data are briefly analyzed, and the presented theory has been found to be in reasonable agreement with experimental data.

     

Hypercross-linked polystyrene and its potentials for liquid chromatography: a mini-review

A recently introduced method [Biotechnol. Prog. 13 (1997) 429] for determining intraparticle mass transfer parameters in high speed liquid chromatography is considered in the present study for the case where the eluite adsorbs onto the stationary phase. The validity of the method was verified theoretically using simulated elution profiles and then applied to experimental data obtained using columns packed with either a macroporous or a gel-filled gigaporous stationary phase. For this purpose, experimental measurements were made using alpha-lactalbumin and bovine serum albumin as eluites at several retention factors. Apparent intraparticle diffusivities measured for the gel-filled gigaporous stationary phase were seen to increase with the retention factor, which indicates that for this material surface diffusion is a significant mechanism of mass transfer under retained conditions. Data obtained on the macroporous stationary phase revealed that the intraparticle diffusivity was independent of the retention factor, which suggests that pore diffusion remains the principal mass transfer mechanism even under conditions where proteins are adsorbed on the column packing.     

Photocatalytic hydrogen evolution over mesoporous TiO2/metal nanocomposites

Na⁺ complex with the dibenzo-18-crown-6 ester was used as a template to synthesize mesoporous titanium dioxide with the specific surface area 130–140 m²/g, pore diameter 5–9 nm and anatase content 70–90%. The mesoporous TiO₂ samples prepared were found to have photocatalytic activity in Cu^II, Ni^II and Ag^I reduction by aliphatic alcohols. The resulting metal–semiconductor nanostructures have remarkable photocatalytic activity in hydrogen evolution from water–alcohol mixtures, their efficiency being 50–60% greater than that of the metal-containing nano-composites based on TiO₂ Degussa P25.The effects of the thermal treatment of mesoporous TiO₂ upon its photocatalytic activity in hydrogen production were studied. The anatase content and pore size were found to be the basic parameters determining the photoreaction rate. The growth of the quantum yield of hydrogen evolution from TiO₂/Ag⁰ to TiO₂/Ni⁰ to TiO₂/Cu⁰ was interpreted in terms of differences in the electronic interaction between metal nanoparticles and the semiconductor surface. It was found that there is an optimal metal concentration range where the quantum yield of hydrogen production is maximal. A decrease in the photoreaction rate at further increment in the metal content was supposed to be connected with the enlargement of metal nanoparticles and deterioration of the intimate electron interaction between the components of the metal–semiconductor nanocomposites.