The structure of star-branched copolymers. A Monte Carlo study

A coarse-grained model of a many-arm star-branched polymer chain has been studied. The chains were flexible heteropolymers built of two kinds of united atoms (segments) – hydrophobic and hydrophilic. The positions of segments were restricted to the vertices of a [310] hybrid lattice. The force field consisted of the softened excluded volume and the long-distance potential between a pair of non-bonded segments. The properties of these macromolecular models were studied in a wide range of temperatures from random coil to dense globule states. Monte Carlo simulations were performed using an algorithm based on the chain’s local changes of conformation and employing the Replica Exchange technique. The influence of the polymer length, the sequence of segments in the chain and the temperature on the dimension and the structure of chains have been studied. It has been shown that the process of the heteropolymer chain collapse to low-temperature structures is a complicated phenomenon and a possible explanation of this behavior has been discussed.     

Interface Shape Studies for Fluoride- and Silicon Rods Grown by the E.F.G. Method

The influence of the die geometry and of the thermal conductivity of materials on the solid-liquid interface shape in Edge-defined Film-fed Growth has been studied.     

Molecular ordering of 4O.2 compound in a dielectric medium at room temperature‐A theoretical approach

The molecular ordering of N‐(4‐n‐butoxybenzylidene)‐4′‐ethyaniline [4O.2] in a dielectric medium (benzene) has been carried out on the basis of statistical mechanics and intermolecular forces. The atomic net charge and atomic dipole moment at each atomic centre has been evaluated using the CNDO/2 method. The modified Rayleigh‐Schrodinger perturbation theory with multicentered‐multipole expansion method has been employed to evaluate the long‐range intermolecular interactions, while ‘6‐exp’ potential function has been assumed for the short‐range interactions. The total interaction energy values obtained through these computations were used to calculate the probability of each configuration in a dielectric medium at room temperature (300K) using the MB‐formula. The flexibility of various interacting configuration has been studied in terms of variation of probability due to departure from the most probable configuration. A comparative picture of the molecular parameters like total energy, binding energy and total dipole moment has been given. An attempt has been made to analyse the characteristic features of nematogenic 4O.2 compound in terms of its relative order. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Heat Capacity and Thermodynamic Properties of p'-Substituted p-n-Hexyloxybenzylideneaniline. I. p-n-Hexyloxybenzylideneamno-p'-Benzonitrile (HBAB)

Abstract

The heat capacity of the nematogenic liquid crystal, HBAB, has been measured between 15 K and 385 K by using an adiabatic calorimeter. The crystal-crystal phase transition has been discovered at 27 K below the crystal-nematic phase transition temperature. The transition temperatures, the enthalpies and the entropies of the three phase transitions have been determined: T ₁ = 306.98 K, ΔH _t = 5.11 kJ mol^?1, ΔS _t = 16.7 JK^?l; T _m = 334.05 K, ΔH _m = 23.77 kJ mol^?1, ΔS_m = 71.2 J K^?l mol^?1; and T _c = 375.10 K, ΔH _c = 1.75 kJ mol^?1, ΔS _c = 3.2 J K^?1 mol^?1, respectively. The thermodynamic functions of HBAB from 0 K to 385 K have been determined from the heat capacity data and the enthalpies of the transitions. Two crystal modifications, one yellow and granular form and the other white and needle-like form, have been obtained during the course of the preparation of the sample. It turned out that the yellow form was the stable crystal and the white the metastable modification. The crystal-crystal phase transition has been discussed as an onset of partial melting from the entropy consideration. In this connection the total entropies of the transitions, 91.1 J K^?1 mol^?1 has been proposed to be an important measure of melting.     

The Guest-Host Interaction in Liquid Crystals: 1. A New Fast Dichroic Shutter

A new fast optical shutter, based on the guest-host interaction, that uses a 2 mm optical path length, has been demonstrated. Using a suitable pleo-chroic dye as guest in a cyanobiphenyl nematic host it has been shown that the shutter can be turned on and off in a few milliseconds. The electro-optic cell uses transverse electric fields, and field strength, frequency and temperature studies have been made. It has been shown that both increases and decreases in transient light levels may be obtained by changing the polarisation state of the incident light.     

Crystal and molecular structure of C.I. Solvent Yellow 2, 1-phenylazo-4 (N,N-dimethylamine)-phenyl

The crystal structure of C.I. Solvent Yellow 2, C₁₄H₁₅N₃, has been determined by single crystal X-ray diffraction techniques. It crystallizes in the orthorhombic system witha=6.059(2) Å,b=7.398(1) Å,c=27.467(4) Å, space groupP2₁2₁2₁,Z=4. The structure has been solved by direct methods and least-squares has been completed on three dimensional data (688 reflections, MoK radiation). The hydrogen positions have been found but only their positional parameters refined. Final residual 0.055 (all positive intensity data). The molecule exists as an azo-compound and is almost planar. The molecules are packed in columns parallel to theb axis, within each column they are packed in layers almost parallel to the (130) or (130) plane depending on the column. The molecules are linked by van der Waals' forces.     

Growth conditions and electric properties of KDP crystals. I. Conductivity measurements

Conductivity of potassium dihydrophosphate (KDP) crystals grown from solutions with different pH has been studied with due regard for the sectorial structure and the impurity composition of the crystals grown. The dependence of the conductivity parameters on these two factors is established. The data obtained lead to a conclusion on the formation of the real structure important for various electro-optical applications of KDP crystals and provide the optimization of growth conditions.     

ThePoranthera corymbosa alkaloids,I. Crystal and absolute molecular structure of porantherine hydrobromide

The alkaloid porantherine, C₁₅H₂₃N, has been isolated fromPoranthera corymbosa Brogn. (family Euphorbiaceae). It forms a stable hydrobromide which crystallizes in the orthorhombic space groupP2₁2₁2₁, with cell dimensionsa=11·689,b = 15·190 andc = 7·721 Å. Its structure has been solved by the heavy-atom method and refined to anR of 0·063. The molecular skeleton has the form of a tertiary amine, with three fused six-membered rings centred on the nitrogen atom and a two-carbon atom bridge which holds the whole molecule rigid. The absolute configuration has been determined from the differences between Bijvoet pairs of reflexions.     

Bandwidth narrowing in n-type many valley semiconductors. II. Self-consistent many-body and unrestricted Hartree-Fock cluster approaches

A recent one-band model for impurity states in n-type many-valley semiconductor has been extended by self-consistent many-body and unrestricted Hartree-Fock approaches. The formulations are presented in order to discuss the shallow donor states in semiconductors. The impurity band splits into two Hubbard bands, presenting different aspects of the impurity states when the many-valley effects of the host conduction band are considered. These effects turn out to be essential for a better interpretation of the theoretical and experimental results. The results agree with some theoretical calculations and experimental findings.     

Molecular dynamics simulation of La2O3-Na2O-SiO2 glasses. II. The clustering of La cations

Clustering of high-field strength rare-earth ions in silicate glasses has been experimentally observed for a wide range of concentrations. Clustering has also been observed by molecular dynamics (MD) computer simulations over a range 1-10 mol% in soda silicate glasses. Although there have been numerous experimental studies, atomic-level details of the mechanisms that lead to clustering remain unclear. Coupling experiment with MD simulations is essential to uncovering the factors that lead to clustering. In this work, MD computer simulations are used to verify that clustering found in previous MD simulations is not an artifact of the simulation method. This work also provides clues as to the mechanism of atomic-level clustering.     

Structure analysis by reduced data. III. Method of interexperimental minimization

Being logically justified, a new algorithm of the interexperimental minimization (IEM) has been formally described. The IEM method is the generalization of the optimization method to the case of several independent measurements and is characterized by the following basic features: (1) the structural model refined is divided into the basic model and the model-perturbation; (2) the goal function of the method has four terms, of which the first two correspond to independent measurements, the third one, to averaged measurements, and the fourth one, to the normalized interexperimental difference; (3) in the IEM refinement, the weighting scheme adequately reflecting the accuracy of the experimental data is automatically formed; (4) the interexperimental minimization method includes the algorithm for reducing interparametric correlations; (5) the basic criterion of the IEM method is the statistical test of reproducibility of the results; (6) the IEM method uses the quantum-mechanical and molecular-dynamics calculations to normalize the experimental data; and (7) the physical reliability of the results obtained upon the refinement by the IEM method also depends on the coupling relations imposed onto the structural and functional parameters.     

Study of the dissolution and growth of salt single crystals in inhomogeneous acoustic fields: II. A focused acoustic field

The growth and dissolution of the isolated (100) face of a KDP crystal at exposure of the phase boundary to focused acoustic fields have been investigated. Visualization tools have been developed and the scale and dynamics of the arising acoustic flows have been established. A quadratic dependence of the dissolution rate on the sound pressure has been obtained; such a dependence is in agreement with the theoretical concepts about the dependence of the mass exchange rate in the acoustic boundary layer on the field parameters. A significant growth response of the face under study, normally exposed to ultrasound along the acoustic axis in the prefocal and postfocal planes of a spherical concentrator (f = 1.4 MHz), has been revealed. It is shown that the mechanisms of mass exchange enhancement in these acoustic modes are radically different. The results obtained show a possibility of controlling growth and dissolution of crystals by varying the parameters of an inhomogeneous acoustic field.     

Electromagnetic emission under uniaxial compression of ice: III. Dynamics and statistics of dislocation avalanches and cracks

Statistical analysis of the plastic deformation steps and fracture of polycrystalline ice has been performed. It is established that an increase in deformation leads to gradual evolution of the statistics of amplitudes of mesoscopic deformation jumps from random (with a Poisson distribution of dislocation avalanche amplitudes) to “critical” (with a power-law distribution), which indicates occurrence of long-range correlations of the dislocation mesodynamics of deformed polycrystalline ice. The state of self-organized criticality at subcritical ice fracture has been revealed from the power-law statistics of the amplitudes of electric pulses and pauses between them, flicker-noise structure, and almost monofractal character of signals.     

The boat conformation of α-parachloral. Notes on the revised structures of α- andβ-parachloral

The boat conformation of-parachloral [2,4,6-tris(trichloromethyl)-1,3,5-trioxane] has been confirmed by X-ray crystallography. Evidence for a chair conformation based upon calculations of dipole moments has been shown to be unreliable, but the possibility of this existing in solution has been studied by¹H and¹³C NMR spectroscopy. The X-ray structures of both- and-parachloral have been redetermined.     

Numerical study for diffusion processes in dissolution and growth of heterostructures formed by LPE. Part I. Isothermal conditions

The paper deals with computer simulation for growth and dissolution of ternary CdHgTe compounds in liquid-phase epitaxy (LPE) systems. The mathematical model accounts for the matter transport in both solid and liquid phases and the equilibrium phase diagram. The study displays the effect of solid-state diffusion on compositional grading at the substrate-layer boundary, on the formation of concentration distribution in both phases and on interface motion. The simulation has been done for isothermal conditions.     

Atomic structure and enormous anisotropy of thermal expansion in NiSi single crystals. I. Refinement of structure models

The atomic structure of NiSi single crystals has been studied by the methods of X-ray diffraction analysis at 295 and 418 K. The refinement of the structure models with due regard for anharmonicity of atomic vibrations revealed the pronounced anisotropy of thermal vibrations and the considerable contribution of the fourth-order anharmonicity. The maps of the probability density function of atomic displacements and single-particle potentials of both atoms were constructed.     

Determination of the Near-Atomic Structure of Non-Purified Oligomeric E. coli Proteins

Crystallography Reports - The roughly purified extract of E. coli proteins has been studied by cryoelectron microscopy, the class-sums containing 2D projections of two proteins...     

Polarization of Molecular ions in NaTCNQ and TTF. TCNQ Crtstals

Self-consistent electrostatic intersite potentials and atomic charges have been calculated. Lattice cohesion energy for NaTCNQ has been reproduced when polarization of TCNQ- anion in the lattice was taken into account.     

Structural analysis according to reduced data: VIII. Refinement of the extended model of aspherical atoms

A program for the refinement of the model of aspherical atoms within the Stewart-Hansen-Coppens formalism has been developed. Deformation scattering up to the 8th expansion order in multipoles has been taken into account for the first time. The program was tested for 11 crystals. The effect of the result of interpolation of radial scattering curves on the model parameters is considered. The importance of introduction of multipoles of high (5th–8th) orders into the model for a number of crystals is shown. The use of the extended multipole model for a silicon crystal revealed some new specific features of the electronic structure: consideration of multipoles up to the 7th order makes it possible to explain the intensity of the forbidden 222 reflection.     

Thermodynamic and experimental studies of the CVD of A-15 superconductors I. Nb3Ga

This paper deals with the experimental and thermodynamic study of the CVD synthesis of Nb₃Ga layers on various metallic and insulting substrates using the coreduction of mixed halides by hydrogen. Thermodynamic equilibrium in the seven-components system Nb-Ga-H-Cl-Si-O-Ar has been calculated using the method of minimization of the system Gibbs free energy as a function of the variables directly available in the CVD system. The chosen variables were the chloride ratio, the reduction and dilution parameters and the temperature of the deposition zone. The equilibrium compositions were calculated for the two composition limits of the A-15 phase: NbGa_0.15 and Nb₃Ga. They are presented in the form of CVD phase diagrams. A CVD reactor has been set up and more than one hundred measurements have been made in order to check the validity of the equilibrium calculations. The comparisons between equilibrium and experimental results show a good agreement and lead to a better understanding of the chemistry and thermodynamics of the system.