Self-assembly at room temperature of thermally stable discrete and extended oligomers of polycyclic aromatics on Ag(100): induced dipoles and cooperative effects

Thermally stable nanoarchitectures are realized on the Ag(100) surface by self-assembly of asymmetrically substituted arenes. The process is instigated by adsorption-induced molecule → surface charge transfer that gives rise to in-plane dipole moments. Observation and calculation indicate that cooperative interactions further enhance the stability of these polarizable systems.     

Preparation,heat treatment and photoluminescence properties of V-doped ZnO–SiO2–B2O3 glasses

Four glasses in ZnO–SiO₂–B₂O₃ ternary system were prepared by the melt quenching method with the objective of optimizing sub-nanosecond emission over the UV region of zinc borosilicate glasses used in superfast scintillators. The effect of vanadium addition and heat treatment on phase formation, microstructure and photoluminescence properties of the glasses was characterized by means of DTA, XRD, SEM and fluorescence spectrophotometer. Vanadium contributed to the near-band-edge emission in two ways, by introducing donor levels in the energy band of ZnO particles and by facilitating the precipitation of ZnO and willemite crystals. Furthermore, nucleation of willemite and zinc oxide phases, which are both the origins of the intense emission bands in the UV region, was facilitated with increasing either the time or temperature of heat treatments. Photoluminescence spectra showed the elimination of the visible emission band which is favorable in scintillating glasses.     

Amino modified mesostructured silica nanoparticles for efficient adsorption of methylene blue

In this work, mesostructured silica nanoparticles (MSN(AP)) with high adsorptivity were prepared by a modification with 3-aminopropyl triethoxysilane (APTES) as a pore expander. The performance of the MSN(AP) was tested by the adsorption of MB in a batch system under varying pH (2-11), adsorbent dosage (0.1-0.5gL(-1)), and initial MB concentration (5-60mgL(-1)). The best conditions were achieved at pH 7 when using 0.1gL(-1) MSN(AP) and 60mgL(-1)MB to give a maximum monolayer adsorption capacity of 500.1mgg(-1) at 303K. The equilibrium data were evaluated using the Langmuir, Freundlich, Temkin, and Harkins-Jura isotherms and fit well to the Freundlich isotherm model. The adsorption kinetics was best described by the pseudo-second order model. The results indicate the potential for a new use of mesostructured materials as an effective adsorbent for MB.     

Modeling of Shape Memory Induction and Recovery in Heat‐Shrinkable Polymers

Summary: A mechanical model was developed to describe qualitatively and quantitatively the stress‐strain‐time behavior of a prepared shape memory crosslinked polyethylene during hot stretching, stress relaxation under 200% strain at high temperature and strain recovery of the heat shrinkable polymer. The stress‐strain, the stress relaxation and the irrecoverable strain behavior of the model were established by driving the constitutive equation, which could qualitatively represent the behavior of the real material. By choosing significant values for the parameters of the proposed model, an excellent fit was obtained between the experimental behavior of the polymer and that predicted by the model. It was also revealed that the main source responsible for the imperfect recovery of the induced strain observed was the stress relaxation occurring during the stretch holding‐cooling time step.

Stress relaxation of crosslinked polyethylene under 200% strain at 160 °C.     

Thermally Stable Magnetic Polyurethane Nanocomposites Prepared from Functionalized Polybutadiene: Novel Approach to the Polybutadiene Chemical Modification

Russian Journal of Applied Chemistry - In this work, preparation and characterization of new classes of polybutadiene-based polyols and polyurethanes investigated with consecutive reactions namely,...     

Pool boiling heat transfer characteristics of graphene-based aqueous nanofluids

Journal of Thermal Analysis and Calorimetry - In the present study, experiments on pool boiling heat transfer of graphene nanofluids on a flat heater surface (40 mm diameter) were...     

Synthesis,Characterization and Physicomechanical Properties of Novel Water-based Biodegradable Polyurethane Dispersion

Novel water-based biodegradable polyurethane dispersions with an aim to develop environmentally friendly materials, including medicine, various industries, have been prepared in this study. Biodegradable ionic polyurethanes (IPU) were synthesized based on polyols from renewable resources, such as castor oil (CO), in the presence of a polyester polyol and polyethylene glycol (PEG) with hydrophilic property and 1,6-hexamethylene diisocyanate. 1,4-Butanediol and dibutyltin dilaurate, were used as a chain extender and catalyst, respectively. The comprehensive investigations of the structure and properties of five types of synthesized polyurethanes demonstrated biodegradability relationship of these polyurethanes with their structure and composition. In this research effects of different types and content of polyols on biodegradability and physico mechanical properties of prepared PUDs were investigated. The structure, properties and physico mechanical and application behavior of mentioned materials were characterized by ¹H NMR, FTIR spectroscopy, thermogravimetric analysis (TG/DTG) and dynamic mechanical thermal analysis (DMTA). The adhesion properties were measured by pull off test as well. Particle size was measured by dynamic light scattering (DLS) methods. The biodegradability of prepared polyurethane dispersions was confirmed by water uptake, hydrolytic and enzymatic degradation in phosphate buffer saline (PBS) with lipase enzyme in PBS. Results showed that by the incorporation of natural components into the polymer chain, adjusting of hydrophilic and hydrolytic liability properties of soft segments and especial relevant designs, useful polyurethane can be synthesized with desirable property of biodegradability and dispersion stability. Except for one sample, other samples were decomposed totally in enzymatic media.     

First-principles study of superabundant vacancy formation in metal hydrides

Recent experiments have established the generality of superabundant vacancies (SAV) formation in metal hydrides. Aiming to elucidate this intriguing phenomenon and to clarify previous interpretations, we employ density-functional theory to investigate atomic mechanisms of SAV formation in fcc hydrides of Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au. We have found that upon H insertion, vacancy formation energies reduce substantially. This is consistent with experimental suggestions. We demonstrate that the entropy effect, which has been proposed to explain SAV formation, is not the main cause. Instead, it is the drastic change of electronic structure induced by the H in the SAV hydrides, which is to a large extent responsible. Interesting trends in systems investigated are also found: ideal hydrides of 5d metals and noble metals are unstable compared to the corresponding pure metals, but the SAV hydrides are more stable than the corresponding ideal hydrides, whereas opposite results exist in the cases of Ni, Rh, and Pd. These trends of stabilities of the SAV hydrides are discussed in detail and a general understanding for SAV formation is provided. Finally, we propose an alternative reaction pathway to generate a SAV hydride from a metal alloy.     

A combinatorial approach to the electron correlation problem

Starting from a path-integral formulation of quantum statistical mechanics expressed in a space of Slater determinants, we develop a method for the Monte Carlo evaluation of the energy of a correlated electronic system. The path-integral expression for the partition function is written as a contracted sum over graphs. A graph is a set of distinct connected determinants on which paths can be represented. The weight of a graph is given by the sum over exponentially large numbers of paths which visit the vertices of the graph. We show that these weights are analytically computable using combinatorial techniques, and they turn out to be sufficiently well behaved to allow stable Monte Carlo simulations in which graphs are stochastically sampled according to a Metropolis algorithm. In the present formulation, graphs of up to four vertices have been included. In a Hartree-Fock basis, this allows for paths which include up to sixfold excitations relative to the Hartree-Fock determinant. As an illustration, we have studied the dissociation curve of the N(2) molecule in a VDZ basis, which allows comparison with full configuration-interaction calculations.     

Controlling organic reactions on silicon surfaces with a scanning tunneling microscope: theoretical and experimental studies of resonance-mediated desorption

The dynamics of tip-induced, resonance-mediated bond-breaking in complex organic adsorbates is studied theoretically and experimentally. Desorption of benzene from a Si(100) surface is found to be efficient and sensitive to voltage, the measured yield rising from below 10(-10) to ca. 10(-6) per electron within a ca. 0.8 V range at low (< 100 pA) current. A theoretical model, based upon first principles electronic structure calculations and quantum mechanical wavepacket simulations, traces these observations to multi-mode dynamics triggered by a transition into a cationic resonance. The model is generalized to provide understanding of, and suggest a means of control over, the behaviour of different classes of organic adsorbates under tunneling current.