Kinetics of the Photopolymerization of Vinyl Monomers by Bis(Isopropylxanthogen) Disulfide. Design of Block Copolymers

Bis(isopropylxanthogen) disulfide (BX) has been used as a photoinitiator with various vinyl monomers at 30°C. The kinetics of polymerization of styrene (St) and methyl methacrylate (MMA) at 30°C were studied for various concentrations of monomer and initiator. The observed deviations in polymerization rate from simple kinetic theory could be explained in terms of primary radical termination. The fraction of primary radical terminating chains was obtained as a function of various concentrations. The ratio of the rate constants for chain initiation and chain termination by a primary radical was determined to be 3.34 ± 10⁷ for St and 2.60 ± 10⁷ for MMA. The number-average degree of polymerization (DP _n) of polymers obtained by photopolym-erization with BX was found to increase linearly with conversion. However, the DP _n extrapolated to zero conversion was in good agreement with that calculated on the basis of the kinetic scheme. It was found that BX had interesting properties for the design of block copolymers, i.e., BX acts as a terminator and a chain transfer agent as well as an initiator in these polymerizations. The polymers obtained with BX contained two reactive isopropyl xanthate groups bonded at their chain ends, which could also act as macrophotoinitiators.     

Synthesis of 1,4-diaryl[60]fullerenes by bis-hydroarylation of C60 and their use in solution-processable, thin-film organic photovoltaic cells

An AlCl₃-mediated Friedel-Crafts reaction of arenes with C₆₀ affords two-fold hydroarylated compounds, C₆₀Ar₂H₂, which upon deprotonation with ^tBuOK and oxidation with CuBr·SMe₂ yield 1,4-diaryl[60]fullerenes, C₆₀Ar₂ (Ar = Ph, 4-Me-C₆H₄, 3,4-Me₂-C₆H₃, and 4-Ph-C₆H₄) in good yield. A solution-processed, thin-film organic photovoltaic device using C₆₀(4-PhC₆H₄)₂ as electron acceptor material showed a 2.3% power conversion efficiency.     

Detailed studies of a high-capacity electrode material for rechargeable batteries, Li2MnO3-LiCo(1/3)Ni(1/3)Mn(1/3)O2

Lithium-excess manganese layered oxides, which are commonly described by the chemical formula zLi(2)MnO(3)-(1-z)LiMeO(2) (Me = Co, Ni, Mn, etc.), are of great importance as positive electrode materials for rechargeable lithium batteries. In this Article, Li(x)Co(0.13)Ni(0.13)Mn(0.54)O(2-δ) samples are prepared from Li(1.2)Ni(0.13)Co(0.13)Mn(0.54)O(2) (or 0.5Li(2)MnO(3)-0.5LiCo(1/3)Ni(1/3)Mn(1/3)O(2)) by an electrochemical oxidation/reduction process in an electrochemical cell to study a reaction mechanism in detail before and after charging across a voltage plateau at 4.5 V vs Li/Li(+). Changes of the bulk and surface structures are examined by synchrotron X-ray diffraction (SXRD), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectroscopy (SIMS). SXRD data show that simultaneous oxygen and lithium removal at the voltage plateau upon initial charge causes the structural rearrangement, including a cation migration process from metal to lithium layers, which is also supported by XAS. This is consistent with the mechanism proposed in the literature related to the Li-excess manganese layered oxides. Oxygen removal associated with the initial charge on the high voltage plateau causes oxygen molecule generation in the electrochemical cells. The oxygen molecules in the cell are electrochemically reduced in the subsequent discharge below 3.0 V, leading to the extra capacity. Surface analysis confirms the formation of the oxygen containing species, such as lithium carbonate, which accumulates on the electrode surface. The oxygen containing species are electrochemically decomposed upon second charge above 4.0 V. The results suggest that, in addition to the conventional transition metal redox reactions, at least some of the reversible capacity for the Li-excess manganese layered oxides originates from the electrochemical redox reaction of the oxygen molecules at the electrode surface.     

Two Bessel Bridges Conditioned Never to Collide,Double Dirichlet Series,and Jacobi Theta Function

It is known that the moments of the maximum value of a one-dimensional conditional Brownian motion, the three-dimensional Bessel bridge with duration 1 started from the origin, are expressed using the Riemann zeta function. We consider a system of two Bessel bridges, in which noncolliding condition is imposed. We show that the moments of the maximum value is then expressed using the double Dirichlet series, or using the integrals of products of the Jacobi theta functions and its derivatives. Since the present system will be provided as a diffusion scaling limit of a version of vicious walker model, the ensemble of 2-watermelons with a wall, the dominant terms in long-time asymptotics of moments of height of 2-watermelons are completely determined. For the height of 2-watermelons with a wall, the average value was recently studied by Fulmek by a method of enumerative combinatorics.     

Visualizing of skin chromophore concentrations by use of RGB images

A method is proposed for visualizing simply the concentrations of melanin, oxygenated blood, and deoxygenated blood in skin tissue using digital RGB images. The total blood concentration and oxygen saturation can also be reconstructed. Monte Carlo simulation of light transport specifies a relation between the chromophore concentrations and Commission Internationale de l'Eclairage XYZ, which are compatible with the common RGB working space. Experiments with a tissuelike agar gel phantom demonstrated the possibility of the method. In vivo imaging of a human hand during forearm occlusion demonstrated the ability of the method to evaluate hemodynamics of skin tissue.     

Discovering network behind infectious disease outbreak

Stochasticity and spatial heterogeneity are of great interest recently in studying the spread of an infectious disease. The presented method solves an inverse problem to discover the effectively decisive topology of a heterogeneous network and reveal the transmission parameters which govern the stochastic spreads over the network from a dataset on an infectious disease outbreak in the early growth phase. Populations in a combination of epidemiological compartment models and a meta-population network model are described by stochastic differential equations. Probability density functions are derived from the equations and used for the maximal likelihood estimation of the topology and parameters. The method is tested with computationally synthesized datasets and the WHO dataset on the SARS outbreak.     

Spatial variation of superconducting properties of Gd123 Bulk superconductors with magnetic particles addition

Magnetic flux trapping and the homogeneity of the flux pinning are essential problems in the practical application of high-temperature superconductors. We have conducted study on the role of addition of soft magnetic Fe-B alloy particles contribute to the enhancement of the critical current density (J_c) under wide-range of magnetic field. Magnetic flux trapping was enhanced in Gd123 bulk superconductor with suitable amount of magnetic particles addition. In addition, it can be effective as pinning center enhance the J_c of the bulk in both the a–b growth sector and the c-growth sector under magnetic field. However, the T_c of the Gd123 bulk was decreased obviously by addition of magnetic particles. The study on the spatial variation of superconducting properties indicates that the performance of the upper part of the bulk is better than the bottom. By comparing the superconducting properties of the Gd123 bulk with magnetic particles addition and without magnetic particles addition, we concluded that there is a trace of the formation of homogeneous pinning properties in the magnetic particles addition Gd123 bulk.     

Structure of χ3-Borophene Studied by Total-Reflection High-Energy Positron Diffraction (TRHEPD)

We have investigated the structure of χ₃-borophene on Ag(111), a monolayer material of boron atoms, via total-reflection high-energy positron diffraction (TRHEPD). By comparing the experimental rocking-curves with ones for several structures calculated by using dynamical diffraction theory, we confirmed that the χ₃-borophene layer has a flat structure. The distance from the topmost layer of the metal crystal is 2.4 Å, which is consistent with results reported by X-ray standing wave-excited X-ray photoelectron spectroscopy. We also demonstrated that the in-plane structure of χ₃-borophene is compatible with the theoretical predictions. These structural properties indicate that χ₃-borophene belongs to a group of epitaxial monolayer sheets, such as graphene, which have weak interactions with the substrates.