Metastability in the Potts model on the Cayley tree

Metastability in the ferromagneticp-state Potts model defined on the Cayley tree is discussed. It is shown that the sign of the boundary fieldH _s determines the order of the transition as well as the stability of the low-temperature phase. Lowering the temperature withH _s >0, a system withp<2 (p>2) will display a second (first)-order transition to a metastable (stable) phase. ForH _s >0 a second (first)-order transition to a metastable (stable) phase occurs ifp>2 (p<2). In this case the system also has a residual entropy which is negative forp<2.     

Iodine-poly(2-vinylpyridine-co-styrene-co-divinylbenzene) charge transfer complexes with antibacterial activity

In this work, we have developed three different copolymers based on 2-vinylpyridine, styrene and crosslinked with divinylbenzene (10-30 mol%). The copolymers were morphologically and chemically characterized by apparent density, swelling degree, elemental analysis, Fourier transform infrared spectrophotometry and optical microscopy. The formation of iodine complexes with these copolymers was carried out by two different procedures: with solvent, or not. The influence of the copolymers structure on the capacity of anchoring iodine has been investigated. The antibacterial properties of polymeric charge transfer complexes were determined towards 10³-10⁷ cells/mL dilutions from the auxotrophic AB1157 Escherichia coli strain.     

Effect of ethylene glycol on the thermodynamic and micellar properties of Tween 20

The aggregation behaviour of Tween 20 in ethylene glycol-water mixed solvents has been investigated using surface tension, density, static and dynamic light scattering, and fluorescence measurements. Micellar and surface thermodynamics data were obtained from the temperature dependence of critical micelle concentrations in various aqueous mixtures of ethylene glycol. In order to evaluate the influence of the cosolvent, the differences in the Gibbs energies of micellization of Tween 20 between water and binary solvents were determined. This study allowed us to conclude that the ability of ethylene glycol to act as a structure breaker and its interaction with the surfactant hydrophilic group are the controlling factors of the micellization process. From the evaluation of the thermodynamics of adsorption at the solution-air interface, it was determined that the surface activity of the surfactant decreases slightly with increasing concentration of ethylene glycol at a given temperature. Partial specific volume data, obtained by density measurements, indicate that the fraction of solvent molecules interacting with the micelle, via hydrogen bonds, remained roughly constant. The effect of cosolvent on the size and solvation of the aggregates was analysed by means of static and dynamic light scattering measurements. It was found that the aggregation number decreased, whereas the whole micellar solvation increased with the ethylene glycol content. Micellar micropolarity was examined using two different probes, pyrene and 8-anilinonaphthelene-1-sulfonic acid, and was found to increase with ethylene glycol addition, accompanied by an enhanced solvation. Fluorescence polarization measurements found by using coumarin 6 as a hydrophobic probe revealed an increase in the micellar microviscosity. The observed trends in these microenvironmental properties were ascribed to a participation by ethylene glycol in the micellar solvation layer.     

Thermogravimetric study of some crosslinked copolymers based on poly(acrylonitrile-co-divinylbenzene)

Copolymer networks based on acrylonitrile (AN)/divinylbenzene (DVB) have been investigated by thermogravimetric analysis (TG) to evaluate their thermal stability in nitrogen atmosphere. Thermal stability was determined from TG-DTG curves to investigate the influence of AN and DVB in the synthesis of copolymers on the copolymer thermal properties. The TG and DTG curves of copolymers clearly show two thermodegradation stages. The solid residues produced after thermodegradation stages were analyzed by FTIR and elemental analysis (CHN). The decomposition temperatures were dependent on amount of AN and DVB used as the crosslinking agent. The degradation temperatures of copolymers were influenced by the diluent system during their synthesis. FTIR analyses indicate that the cyclization of the polymer proceeds before any mass loss.     

Molecular orbital calculations on transition metal clusters

Transition metal clusters have been subject of experimental and theoretical interest due to their catalytic activity, as well as their unusual physical properties. Semi-empirical extended Hückel molecular orbital calculations are presented for a series of small metal clusters with nuclearity ranging from 3 to 6. Naked and carbonylated clusters of Fe, Ru, and Os are studied. The charge transfer between ligands shell and metal core is found to be a function of nuclearity, CO coordination and the chemical species forming the bare cluster. The observed magnetic properties of these systems are discussed in terms of their electronic structure and CO-metal charge transfer.     

On the Mechanical Properties of Popgraphene-Based Nanotubes: a Reactive Molecular Dynamics Study

Carbon-based tubular materials have sparked a great interest in future electronics and optoelectronics device applications. In this work, we computationally studied the mechanical properties of nanotubes generated from popgraphene (PopNTs). Popgraphene is a 2D carbon allotrope composed of 5-8-5 rings. We carried out fully atomistic reactive (ReaxFF) molecular dynamics for PopNTs of different chiralities ( and ) and/or diameters and at different temperatures (from 300 up to 1200 K). Results showed that the tubes are thermally stable (at least up to 1200 K). All tubes presented stress/strain curves with a quasi-linear behavior followed by an abrupt drop of stress values. Interestingly, armchair-like PopNTs ( ) can stand a higher strain load before fracturing when contrasted to the zigzag-like ones ( ). Moreover, it was obtained that Young's modulus (Y_Mod) (750–900 GPa) and ultimate strength (σ_US) (120–150 GPa) values are similar to the ones reported for conventional armchair and zigzag carbon nanotubes. Y_Mod values obtained for PopNTs are not significantly temperature-dependent. While the σ_US values for the showed a quasi-linear dependence with the temperature, the exhibited no clear trends.     

Mathematical Modeling of Poly[styrene-co-(ethylene glycol dimethacrylate)] Sulfonation

Kinetics and Catalysis - The production of sulfonated resins is of great interest to be applied in different industrial processes such as esterification, etherification, biodiesel production and...