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.     

Study of Inelastic <Emphasis Type="Italic">A</Emphasis>(<Emphasis Type="Italic">p,p′</Emphasis>)<Emphasis Type="Italic">X</Emphasis> Reaction with Nuclei at 1 GeV

The secondary-proton polarization and differential cross sections for the (p, p') inelastic reaction on ²⁸Si and ⁵⁶Fe nuclei at the initial proton energy of 1 GeV were measured over a wide range of the scattered-proton momenta at a laboratory angle of Θ = 21?. Scattered protons were detected by means of a magnetic spectrometer equipped with a polarimeter based on multiwire proportional chambers and a carbon analyzer. A structure in the polarization and cross section data, which is probably related to quasielastic scattering off nucleon correlations in the ²⁸Si and ⁵⁶Fe nuclei, was observed as earlier in the analogous data for ¹²C and ⁴⁰Ca nuclei. Momentum intervals within which cross-section ratios for nuclei did not depend on the scattered-proton momentum were found.     

New electrostatic cylindrical energy analyzer

The focusing properties of an ideal cylindrical field in full relativistic field have been investigated for charged particles originating from an extended source and an analytical solution for the equations of motion has been provided. This focusing system can be used for production of electrostatic energy analyzer provided surface analysis from an extended area.     

Promising Neutron Detector with Anticoincidence Protection

Physics of Particles and Nuclei Letters - The method of remote neutron and gamma spectrometry of bodies in the solar system (the Moon, Mars, and Mercury) has been used for several decades to...     

Methods for obtaining solid dispersions of drugs and their properties

The possibility of improving the solubility of drugs by obtaining solid dispersions (SDs) with polymers is demonstrated. The solubility of these SDs is dependent both on the nature of drug and polymer and on the conditions of their preparation. IR spectroscopy of the obtained SDs and the analysis of results found in literature indicate that inclusion complexes are formed within them.

     

Rheological peculiarities of concentrated suspensions

The rheological properties of concentrated suspensions of metal oxides dispersed in transformer oil, which are used as electrorheological fluids, are systematically studied. Colloidal particles have intermediate sizes between nano- and microsized scales. Low-amplitude dynamic measurements show that the storage moduli of the examined suspensions are independent of frequency and these materials should be considered as solidlike elastic media. The storage modulus is proportional to the five-powdered particle volume concentration. At the same time, a transition through an apparent yield stress with a reduction in the viscosity by approximately six orders of magnitude is distinctly seen upon shear deformation. The character of the rheological behavior depends on the regime of suspension deformation. At very low shear rates, a steady flow is possible; however, upon an increase in the rate, an unsteady regime is realized with development of self-oscillations. When constant shear stresses are preset, in some range of stresses, thickening of the medium takes place, which can also be accompanied by self-oscillations. In order to gain insight into the nature of this effect, measurements are performed for samples with different volume/surface ratios, which show that, in some deformation regimes, suspension is separated into layers and slipping occurs along a low-viscosity layer with a thickness of several dozen microns. Direct observations show a distinct structural inhomogeneity of the flow. The separation and motion of layers with different compositions explain the transition to the flow with the lowest apparent Newtonian viscosity. Thus, the deformation of concentrated suspensions is associated with self-oscillations of stresses and slipping along a low-viscosity interlayer.     

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.     

Linear Second-Order Partial Differential Equations of the Parabolic Type

The article is a review of the theory of linear partial differential equations of the parabolic type, where the classical facts of this theory are described in detail. Bibliography: 128 titles.