Stress-biased rearrangements and preyield behavior in glasses

On the basis of a physical picture of a glass as a mosaic of mesoscopic clusters differing in their yield characteristics, we propose a model for the preyield behavior in glassy materials; the model describes the stress-strain relationship at different strain rates in terms of one reduced variable. A test using experimental data for polycarbonate materials at different rates and temperatures appears favorable for the model. The model may be used to interpolate and extrapolate limited experimental data, and also provides a practical means to assess dynamic heterogeneities within polymeric glasses. When applied at different temperatures, the model gives insight into the dependence of the excitation energies on temperature (glassformer fragility) in the glassy state.     

Computer simulation of self-assembly (crystallization) of oppositely charged nanoparticles with various size distributions

Self-assembly of oppositely charged nanoparticles (NPs) of varying diameters into nanoparticle supracrystals was simulated using a combination of Monte Carlo–Molecular Dynamics techniques in an NVT ensemble. The simulated structures were characterized and classified by probabilistic method of structural invariants based on the Wigner sums of spherical harmonics. It is shown that parameters describing the distributions of the sizes of NP: their average value and the dispersion, are crucially important for the spatial distribution of NPs and the local/global structure of the clusters. The role of energy and entropy effects is briefly discussed. Simulation results are in qualitative agreement with experimental trends.     

Phase front motion in uniaxial ferromagnet

It is well known that in the uniaxial ferromagnet in the presence of an external magnetic field perpendicular to the easy axis (h_x) a continuous phase transition occurs for a critical value of this field. There are metastable and stable states if one includes a small field parallel to the easy axis (h_z). The motion of the relaxation front of the metastable state is investigated. It is found that an “interphase wall of Neel-type” exists, its velocity is proportional to h_z and increases when the critical point is approached.     

Finite perturbation theory-density functional theory calculation of the spin-dipolar contribution to NMR spin-spin coupling constants

In this work an implementation of the FPT-DFT approach for calculating the spin-dipolar contribution to NMR spin-spin coupling constants is presented. This method was tested in a set of small molecules, giving results in excellent agreement when comparing them with values taken from the literature, which were obtained with state-of-the-art calculations. To obtain an insight into the relative importance of the spin-dipolar contribution in unsaturated compounds, calculations of J(F,C), J(F,F) and J(F,H) couplings in 1,2-, 1,3-, and 1,4-difluorobenzenes were performed. An important spin-dipolar contribution to ³ J(F, F) and ⁵ J(F,F) was found, suggesting that this term might be important in some cases. When performing DFT calculations the non-singlet instabilities usually found in unsaturated compounds are overcome.     

Orbital contributions to relativistic corrections of the NMR nuclear magnetic shielding tensor originated in scalar field-dependent operators

In this work relativistic corrections to the magnetic shielding constant (σM), which arise from scalar field-dependent operators (both linear and bilinear), are calculated and decomposed into contributions from molecular orbitals (MOs). Numerical results for the magnetic shielding constant of the heavy nucleus X in closed shell atomic ions X^? and HX compounds (X = F, Cl, Br, I) are presented. The relative importance of inner-shell and valence electrons in the definition of each one of these terms is thus assessed and its relation with their sensitivity to changes in a chemical environment is discussed.     

Experimental, Theoretical and Biological Activity Study on the Acyl-Substituted Benzo-18-crown-6, Dibenzo-18-crown-6 and Dibenzo-24-crown-8

Hexadecanoyl, dihexadecanoyl, dioctadecaneoyl, di-10-undecenoyl, and dicis-9-octadecenoyl derivatives of benzo[18]crown-6, dibenzo[18]crown-6 and dibenzo[24]-8 were synthesized by the condensation of carboxylic acids (palmitic, stearic, oleic and undecenoic acid) with benzo and dibenzo crown ethers in the presence of zinc chloride. The extraction equilibrium constants of such macrocyclic ethers with long side chains were estimated using chloroform/water and dichloromethane/water membranes transfer of Na-PAR (4-(2-pyridylazo)-resorcinol mono sodium monohydrate) with UV–Vis spectroscopy. It was found that they were in the range of 10.88–11.71 in dichloromethane and 8.04–11.77 in chloroform. These results actually show that the Na+ binding effect of macrocyclic ethers depends on the type and the length of side chains. The geometrical properties of the molecules were studied employing semi-empirical calculations by simulated annealing technique. The frontier molecular orbital energies and dipole moments were also examined. The biological activity results showed that the synthesized crown ethers have no activity against the studied microorganisms.     

FLUSHING TIME IN THE CIENFUEGOS BAY,CUBA

We use a high‐resolution numerical model to analyze the hydrodynamical exchange processes of the Cienfuegos Bay with the Caribbean Sea. Results show that freshwater inputs and wind are the main factors controlling the water exchange in the bay. Nine simulation case studies with three wind conditions and three freshwater inputs were carried out. The main conclusion is that the Cienfuegos Bay system has a slow exchange rate, with an average flushing time of 39 days during the rainy period and 50 days during the dry one, for the most probable wind direction. Two new temporary scales for Cienfuegos Bay are discussed: the local e‐flushing time and the flushing lag. The results obtained for these new variables validate the slow exchange capacity of the bay.     

Thermodynamic manifestations of structure fluctuations in liquids

Fluctuations of pressure and internal energy in a low-temperature liquid, generated by structure fluctuations, are related in a general and straightforward way to the differences between the long- and short-time thermodynamic susceptibilities. The relations are similar to, but not identical with, those between thermodynamic fluctuations and susceptibilities in classical statistical mechanics. The derivation is general and straightforward, and based on the assumption that the local structure is a slow-relaxing mode in low-temperature liquids and may be characterized by an order parameter.     

Molecular Interactions and Thermodynamic Aspects of the Complexation Reaction between Gentian Violet and Several Cyclodextrins

The complexation process between gentian violet (CV+) and four different cyclodextrins (-, -, -, and HP--CDs) has been investigated under different reaction conditions (pH, solvent, and temperature) by electronic absorption and 1H NMR (NOE and NOESY) spectroscopies. All the binding constants were determined by the direct spectroscopic method. The H and S complexation values have been evaluated and discussed according to the diverse factors that affect the chemical interactions in these systems. A simple association takes place between the secondary hydroxyl or the hydroxypropyl groups of and HP--cycloamyloses, respectively, with the amine group of the gentian violet, while the binding between CV+ and - or -CDs corresponds to a real inclusion. Also, a CV22+ dimmeric species within the -CD cavity was detected in aqueous solution, while two molecules of -CD react with one molecule of gentian violet in DMSO at 294 K. In all the reaction media the -CD forms 1 : 1 complexes, but in the buffered aqueous solution at pH 7.5 the inclusion is deeper than in the other solvents. It is important to point out that the solvophobic effect is the most important binding factor in the complexation of the CV+ with the - and HP--CDs, while the complexes with -, and -cyclodextrins are mainly stabilized by van der Waals interactions between the guest and the host cavity. In all cases, the inclusion orientation is probably determined by the ion-dipole interactions between gentian violet and the solvent.     

Theoretical modeling of the relationship between Young's modulus and formulation variables for segmented polyurethanes

We describe a new modeling approach to prediction of Young's modulus of segmented polyurethanes. This approach combines micromechanical models with thermodynamic considerations based on the theory of block copolymers. The resulting model predicts both the equilibrium morphology and the “ideal” Young's modulus of a segmented polyurethane polymer as a function of its formulation (hard segment chemical structure, hard segment weight fraction, soft segment equivalent weight) and temperature. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2123–2135, 2007