Polymorphic Modifications of Chitosan

This work describes the analysis of the crystal structures of chitosan, its main polymorphic modifications, and its characteristic mutual chain packing and hydrogen bond systems in the crystalline regions of conformers. The analysis takes into consideration the crystal structures of chitosan complexes (salts) with organic and inorganic acids and their structural transformations. Notably, the transformation of the hydrated form of chitosan into anhydrous is found to be irreversible and occurs either at high temperatures or through a less stable form of hydrated salts. The interaction with polyanions during the formation of multilayer films can be considered as a way to form the anhydrous crystalline form of chitosan.     

Separation of polystyrenes by means of open tubular capillary chromatography

Simulating polymer separation in flow-through channels of monolithic columns, separation of a mixture of polystyrene standards was investigated using open tubular capillary column of 2 μm inner diameter. High column efficiency was observed for polymers of molar mass ranged from few tens to few hundred kDas. Column efficiency significantly decreased for polymers with molar mass larger than 500 kDa nevertheless preserving value of few tens of thousands theoretical plates. Calibration curve observed for open capillary column is rather steep and can be well described by simple equation without quadratic term. In spite of low selectivity, capillary columns were able in separating wide range of polystyrene standards due to column high efficiency and in such a way supported an idea of hydrodynamic mechanism of polymer separation in flow-through channel of monolithic packings.     

Structure of porphyrin-fullerene dyad monolayer on the water surface and solid substrate

Monolayers of porphyrin-fullerene dyad TBD6a were formed on the surface of a water subphase and then transported on a solid substrate by the Langmuir-Schaefer method. A simulation was performed for the structure of a single molecule and for a molecular monolayer, according to the area per molecule in the monolayer formed, which was calculated based on an analysis of the π-A isotherm. A unit cell was chosen for the proposed molecular packing (a = 1.54 nm, b = 1.50 nm, c = 1.75 nm, α = 80.0°, β = 90.0°, and γ = 90.0°), and the atomic coordinates were calculated. A comparison of the interplanar spacings and diffraction peak intensities in the experimental and calculated (for the unit cell proposed) diffraction patterns indicates that a platelike texture is formed in the monolayer and that the crystal structure of the domains corresponds to the model chosen.     

Some peculiarities of the photocatalytic oxidation of hydrocarbons by hydrogen peroxide on a heterogeneous catalyst: Titanium silicalite

Data on the effect of the type and intensity of irradiation on the basic characteristics of the process and distribution of products of catalytic liquid-phase oxidation of hydrocarbons were obtained using as an example the oxidation of n-dodecane by hydrogen peroxide aqueous solutions on the heterogeneous catalyst: titanium silicalite.     

Effect of the synthesis conditions on the compositional heterogeneity of copolymers of higher N-alkylacrylamides and butyl (meth)acrylate

Copolymerization of higher N-alkylacrylamides of various structures with butyl acrylate and butyl methacrylate in organic solvents was studied.     

Preparation and analysis of multilayer composites based on polyelectrolyte complexes

A method for preparing multilayer film composites based on chitosan has been developed by the example of polymer pairs: chitosan–hyaluronic acid, chitosan–alginic acid, and chitosan–carrageenan. The structure of the composite films is characterized by X-ray diffractometry and scanning electron microscopy. It is shown that the deposition of a solution of hyaluronic acid, alginic acid, or carrageenan on a chitosan gel film leads to the formation of a polyelectrolyte complex layer at the interface, which is accompanied by the ordering of chitosan chains in the surface region; the microstructure of this layer depends on the nature of contacting polymer pairs.     

Mn<Subscript>4</Subscript>Si<Subscript>7</Subscript>-Si〈Mn〉-Mn<Subscript>4</Subscript>Si<Subscript>7</Subscript> and Mn<Subscript>4</Subscript>Si<Subscript>7</Subscript>-Si〈Mn〉-<Emphasis Type="Italic">M</Emphasis> photodiodes

The current-voltage characteristics of Mn₄Si₇-Si〈Mn〉-Mn₄Si₇ and Mn₄Si₇-Si〈Mn〉-M photodiodes are studied experimentally. The current passage mechanism under illumination with hν ≥ E _g is considered. The role of a contact to Mn₄Si₇ in the provision of high photosensitivity under illumination of the base by light with hν ≥ 1.14 eV at low temperatures, 77–220 K, is analyzed. From electrical measurements, electron microscopic data for the Mn₄Si₇-Si〈Mn〉 interface, and photocurrent-voltage characteristics, a band diagram under the conditions of photocurrent passage is constructed. The high low-temperature photosensitivity of the diodes (I _ph/I _d ≥ 10⁹) is explained by the impact-ionization-induced modulation of the base conductivity and injection amplification of holes in the transition layer.     

Structure of a laser field of various polarizations in the focal region of an ideal focusing lens. Calculation by methods of scalar diffraction theory

Within the framework of the scalar diffraction theory, we elaborate the method of calculating the electromagnetic-field structure in the ideal-lens focal plane. Using the method of quaternions for describing the rotation in three-dimensional space and diffraction integrals, we derive analytic formulas for the intensity distribution in the fields in the vicinity of the lens focal plane. We generalize the Debye method for an arbitrary polarization of the incident laser beam, in particular, for the linear, radial, and azimuthal polarizations. Such an approach, in view of the analytic formulas obtained, allows us to decrease the processing speed by an order. We show that the algorithm elaborated allows us to calculate the field amplitude?Cphase structure in the case of multimode laser radiation with a random phase.     

Phonon localization in Ge nanoislands and its manifestation in Raman spectra

Multilayer structures with germanium nanoislands that are formed on the silicon (111) surface upon submonolayer deposition by molecular-beam epitaxy have been investigated using Raman spectroscopy. To interpret the experimental Raman spectra, numerical calculations of the spectra have been performed for nanoislands containing from several to several hundred germanium atoms. The calculations demonstrate that the in-plane sizes of nanoislands (with sizes less than 2–3 nm) substantially affect the frequencies of phonons localized in these nanoislands. The experimental Raman spectra confirm the occurrence of the quantum size effect.