Relaxation nuclear magnetic resonance imaging (R-NMRI) of desiccation in M9787 silicone pads.

The production and aging of silicone materials remains an important issue in the weapons stockpile due to their utilization in a wide variety of components and systems within the stockpile. Changes in the physical characteristics of silicone materials due to long term desiccation has been identified as one of the major aging effects observed in silicone pad components. Here we report relaxation nuclear magnetic resonance imaging (R-NMRI) spectroscopy characterization of the silica-filled and unfilled polydimethylsiloxane (PDMS) and polydiphenylsiloxane (PDPS) copolymer (M9787) silicone pads within desiccating environments. These studies were directed at providing additional details about the heterogeneity of the desiccation process. Uniform NMR spin-spin relaxation time (T2) images were observed across the pad thickness indicating that the drying process is approximately uniform, and that the desiccation of the M9787 silicone pad is not a H2O diffusion limited process. In a P2O5 desiccation environment, significant reduction of T2 was observed for the silica-filled and unfilled M9787 silicone pad for desiccation up to 225 days. A very small reduction in T2 was observed for the unfilled copolymer between 225 and 487 days. The increase in relative stiffness with desiccation was found to be higher for the unfilled copolymer. These R-NMRI results are correlated to local changes in the modulus of the materia

Dynamics of Uranyl Peroxide Nanocapsules

Discrete aqueous metal oxide polyionic clusters that include aluminum polycations, transition-metal polyoxometalates, and the actinyl peroxide clusters have captivated the interest of scientists in the realm of both their fundamental and applied chemistries. Yet the counterions for these polycations or polyanions are often ignored, even though they are imperative for solubility, crystallization, purification, and even templating cluster formation. The actinyl peroxide clusters have counterions not only external, but internal to the hollow peroxide capsules. In this study, we reveal the dynamic behavior of these internal alkali counterions via solid-state and liquid NMR experiments. These studies on two select cluster geometries, those containing 24 and 28 uranyl polyhedra, respectively, show that the capsules-like clusters are not rigid entities. Rather, the internal alkalis both have mobility inside the capsules, as well as exchange with species in the media in which they are dissolved. The alkali mobilities are affected by both what is inside the clusters as well as the composition of the dissolving medium

Proceedings of the 23rd annual Central Plains irrigation conference

Presented at Proceedings of the 23rd annual Central Plains irrigation conference held in Burlington, Colorado on February 22-23, 2011.Includes bibliographical references

Synthesis of Allylnickel Aryloxides and Arenethiolates: Study of Their Dynamic Isomerization and 1,3-Diene Polymerization Activity

A new family of allylnickel(I1) complexes, [Ni(η^3-+allyl)(µ-X)]_2 (X = ArO, ArS), have been synthesized by anion metathesis of the sodium or lithium salts of aryloxides or arenethiolates with [Ni(η^3-allyl)(µ-Br)]_2. The complexes are proposed to be dimeric and to consist of a mixture of cis and trans isomers. A dynamic process rapidly equilibrates the cis and trans isomers of the pentafluorophenoxide, 2,6-difluorophenoxide, and 3,5-bis(trifluoromethyl)phenoxide complexes on the ^1H NMR time scale. The 2,6-dimethylphenoxide, 2,6-diisopropylphenoxide, 2,4,6- tris(trifluoromethyl)phenoxide, and pentafluorothiophenoxide complexes are static at room temperature. A variable-temperature NMR study of the 3,5-bis(trifluoromethyl)phenoxide complex provided activation enthalpy and entropy values of 12.9 kcal/mol and -6.6 cal/ (K mol), respectively. Allyl rotation or cleavage of one of the µ-X bridges is proposed as the mechanism for the isomerization. The pentafluorophenoxide, 3,5-bis(trifluoromethyl)phenoxide, and 2,4,6-tris(trifluoromethy1)phenoxide complexes initiate the rapid polymerization of 1,3-cyclohexadiene and 1,3-butadiene to form high-molecular weight, 1,4-linked polymers

Solid-State Dynamics of Uranyl Polyoxometalates

Understanding fundamental uranyl polyoxometalate (POM) chemistry in solution and the solid state is the first step to defining its future role in the development of new actinide materials and separation processes that are vital to every step of the nuclear fuel cycle. Many solid-state geometries of uranyl POMs have been described, but we are only beginning to understand their chemical behavior, which thus far includes the role of templates in their self-assembly, and the dynamics of encapsulated species in solution. This study provides unprecedented detail into the exchange dynamics of the encapsulated species in the solid state through Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) spectroscopy. Although it was previously recognized that capsule-like molybdate and uranyl POMs exchange encapsulated species when dissolved in water, analogous exchange in the solid state has not been documented, or even considered. Here, we observe the extremely high rate of transport of Li⁺ and aqua species across the uranyl shell in the solid state, a process that is affected by both temperature and pore blocking by larger species. These results highlight the untapped potential of emergent f-block element materials and vesicle-like POMs.Keywords: polyoxometalate, solid-state NMR, uranyl, proton MAS NMR, ion-exchang

Share of afghanistan populace in hepatitis B and hepatitis C infection's pool: is it worthwhile?

There is a notable dearth of data about Hepatitis B Virus (HBV) and Hepatitis C Virus(HCV) prevalence in Afghanistan. Awareness program and research capacity in the field of hepatitis are very limited in Afghanistan. Number of vulnerabilities and patterns of risk behaviors signal the need to take action now

Non-Newtonian Couette-Poiseuille flow of a dilute gas

The steady state of a dilute gas enclosed between two infinite parallel plates in relative motion and under the action of a uniform body force parallel to the plates is considered. The Bhatnagar-Gross-Krook model kinetic equation is analytically solved for this Couette-Poiseuille flow to first order in the force and for arbitrary values of the Knudsen number associated with the shear rate. This allows us to investigate the influence of the external force on the non-Newtonian properties of the Couette flow. Moreover, the Couette-Poiseuille flow is analyzed when the shear-rate Knudsen number and the scaled force are of the same order and terms up to second order are retained. In this way, the transition from the bimodal temperature profile characteristic of the pure force-driven Poiseuille flow to the parabolic profile characteristic of the pure Couette flow through several intermediate stages in the Couette-Poiseuille flow are described. A critical comparison with the Navier-Stokes solution of the problem is carried out.Comment: 24 pages, 5 figures; v2: discussion on boundary conditions added; 10 additional references. Published in a special issue of the journal "Kinetic and Related Models" dedicated to the memory of Carlo Cercignan

Fuel traps: mapping stability via water association.

Hydrogen storage is a key enabling technology required for attaining a hydrogen-based economy. Fundamental research can reveal the underlying principles controlling hydrogen uptake and release by storage materials, and also aid in characterizing and designing novel storage materials. New ideas for hydrogen storage materials come from exploiting the properties of hydrophobic hydration, which refers to water s ability to stabilize, by its mode of association, specific structures under specific conditions. Although hydrogen was always considered too small to support the formation of solid clathrate hydrate structures, exciting new experiments show that water traps hydrogen molecules at conditions of low temperatures and moderate pressures. Hydrogen release is accomplished by simple warming. While these experiments lend credibility to the idea that water could form an environmentally attractive alternative storage compound for hydrogen fuel, which would advance our nation s goals of attaining a hydrogen-based economy, much work is yet required to understand and realize the full potential of clathrate hydrates for hydrogen storage. Here we undertake theoretical studies of hydrogen in water to establish a firm foundation for predictive work on clathrate hydrate H{sub 2} storage capabilities. Using molecular simulation and statistical mechanical theories based in part on quantum mechanical descriptions of molecular interactions, we characterize the interactions between hydrogen and liquid water in terms of structural and thermodynamic properties. In the process we validate classical force field models of hydrogen in water and discover new features of hydrophobic hydration that impact problems in both energy technology and biology. Finally, we predict hydrogen occupancy in the small and large cages of hydrogen clathrate hydrates, a property unresolved by previous experimental and theoretical work