Epoxy resins and epoxy blends studied by near infra-red spectroscopy

The cure and the final network of epoxy resins have been investigated by numerous techniques, nevertheless a clear understanding of this network structure has not yet been achieved. FTIR analysis of polymeric materials provides highly precise measurements that are widely interpretable in terms of chemical structure. Yet the high absorption of fundamental bands requires careful sample preparation to reduce the thickness of the sample or special reflection techniques are needed. Furthermore, the occurrence of overlapping bands for epoxy resin (N-H and O-H vibrations in the 3000 cm⁻¹ region) renders the quantitative analysis in the region mid IR particularly difficult. However, the overtone and combination bands are 10–100 times weaker than the fundamental ones and are observed in near infrared (NIR) region. Longer pathlengths than Mid IR ones can be used allowing transmission analysis of thick samples (1-20 mm) without special preparation. NIR absorption bands have different intensities depending on the anharmonicity of vibrations. The strongest absorption bands are due to protons connected to carbon, nitrogen, oxygen. Hydrogen bonding due to inter- and intramolecular interactions can cause band broadening, peak position shifts and intensity variations. NIR spectroscopy is therefore a useful technique to investigate polymeric materials and was used to study the cure reactions of various epoxy resins cured with amine hardener. Using different NIR techniques (reflectance, transmission and microscopy) we will briefly present some results concerning hydrogen bonding between epoxy and amine hardener before curing, epoxy resins, glass/epoxy composites and epoxy/PES (polyethersulfone) blends.     

Temperature dependence of collective states in the random-phase approximation

We investigate the temperature dependence of collective states in the framework of the random-phase approximation at finite temperature. We show that sum rules can be extended to collective energies at finite temperature. Numerical methods are developed to solve the RPA equations at finite temperature. Results are presented and discussed in the case of ⁴⁰Ca for isovector dipole and isoscalar octupole vibrations, using oscillator wave functions and a zero-range force. We show that the broadening of giant dipole resonances observed experimentally, appears as a natural consequence of the structure of the RPA equations. Comparison is made with the schematic model for which the temperature dependence of collective states can be worked out analytically.     

Microscopic structure of Er-related optically active centers in crystalline silicon

A successful observation and analysis of the Zeeman effect on the lambda approximately 1.54 microm photoluminescence band in Er-doped crystalline MBE-grown silicon are presented. The symmetry of the dominant optically active centers is conclusively established as orthorhombic I(C(2v)) with g axially approximately 18.39 and g radially approximately 0. In this way the long standing puzzle as regards the paramagnetism of optically active Er-related centers in silicon is settled. Preferential generation of a single type of an optically active Er-related center confirmed in this study is essential for photonic applications of Si:Er.     

SCF CI calculations of the K-shell ionization potential of carbon in methane and in the fluoromethanes

The vertical ionization potentials of the K-shell of carbon have been calculated in CH₄, CH₃F, CH₂F₂, CHF₃ and CF₄. The ab initio SCF CI approach has been used for the first three compounds with an attempt to rationalize the choice of the atomic orbital basis sets. The ionization potentials of CHF₃ and CF₄ have been extrapolated from the proceeding results. The effect of the CI cannot be neglected in either of these compounds, but it can in CH₄.     

Optimum coasting flight in a horizontal plane

Busemann's concept of optimum altitude for rectilinear coasting flight has been extended to flight in a horizontal plane. The reachable domain of a vehicle in coasting flight in a horizontal plane, starting from an initial velocity, is obtained. It is shown that the area covered first increases with the altitude and then decreases to zero when the ceiling is reached. In particular, there exists an altitude for maximum longitudinal range and another altitude for maximum lateral range. Optimum variations in the lift coefficient and the bank angle to reach the boundary of the footprint are discussed.     

Intermediate-thrust arcs and their optimality in a central,time-invariant force field

This paper presents the general equations of the intermediate-thrust arcs in a general, time-invariant, central force field. Two families of planar arcs, namely, the family of Lawden's spirals in the equatorial plane of an oblate planet and the family of intermediate-thrust arcs in a gravitational field of the form /r ⁿ , have been considered in detail. The Kelley-Contensou condition has been used to test their optimality condition. It is shown that, in the first case, there exist portions of the arcs at a finite distance satisfying the condition, while, in the second case, the entire family satisfies the condition forn 3. Hence, in a perturbed Newtonian gravitational force field, the intermediate-thrust arcs, under certain favorable conditions, can be part of an optimal trajectory.     

Analytical solutions of a class of optimum orbit modifications

This paper presents the complete analytical solution of several fundamental problems in orbital correction. The initial orbit is represented by a given point in the phase space, while the final orbit is constrained to stay in a given curve which can be bounded, unbounded, or composed of a finite number of segments of different curves. The inclusion of atmospheric maneuver as part of the optimum process is discussed; its analytical treatment can be carried out by modifying the final state to include the set of orbits having their perigee at the boundary of the atmosphere.The selection of the apogee and perigee distances as state variables gives a symmetric form to the problem and results in a linear differential equation of the first order for the ratio of the adjoint variables. The introduction of a curve of comparison, called the separatrix, facilitates the discussion of the existence of a corner on an optimal trajectory.This work was supported by NASA Contract No. NASr 54(06).     

Identification of Potential Anti-Neuroinflammatory Inhibitors from Antarctic Fungal Strain Aspergillus sp. SF-7402 via Regulating the NF-κB Signaling Pathway in Microglia

Microglia play a significant role in immune defense and tissue repair in the central nervous system (CNS). Microglial activation and the resulting neuroinflammation play a key role in the pathogenesis of neurodegenerative disorders. Recently, inflammation reduction strategies in neurodegenerative diseases have attracted increasing attention. Herein, we discovered and evaluated the anti-neuroinflammatory potential of compounds from the Antarctic fungi strain Aspergillus sp. SF-7402 in lipopolysaccharide (LPS)-stimulated BV2 cells. Four metabolites were isolated from the fungi through chemical investigations, namely, 5-methoxysterigmatocystin (1), sterigmatocystin (2), aversin (3), and 6,8-O-dimethylversicolorin A (4). Their chemical structures were elucidated by extensive spectroscopic analysis and HR-ESI-MS, as well as by comparison with those reported in literature. Anti-neuroinflammatory effects of the isolated metabolites were evaluated by measuring the production of nitric oxide (NO), tumor necrosis factor (TNF)-α, and interleukin (IL)-6 in LPS-activated microglia at non-cytotoxic concentrations. Sterigmatocystins (1 and 2) displayed significant effects on NO production and mild effects on TNF-α and IL-6 expression inhibition. The molecular mechanisms underlying this activity were investigated using Western blot analysis. Sterigmatocystin treatment inhibited NO production via downregulation of inducible nitric oxide synthase (iNOS) expression in LPS-stimulated BV2 cells. Additionally, sterigmatocystins reduced nuclear translocation of NF-κB. These results suggest that sterigmatocystins present in the fungal strain Aspergillus sp. are promising candidates for the treatment of neuroinflammatory diseases.     

Methyl Internal Rotation in Fruit Esters: Chain-Length Effect Observed in the Microwave Spectrum of Methyl Hexanoate

The gas-phase structures of the fruit ester methyl hexanoate, CH₃-O-(C=O)-C₅H₁₁, have been determined using a combination of molecular jet Fourier-transform microwave spectroscopy and quantum chemistry. The microwave spectrum was measured in the frequency range of 3 to 23 GHz. Two conformers were assigned, one with C_s symmetry and the other with C₁ symmetry where the γ-carbon atom of the hexyl chain is in a gauche orientation in relation to the carbonyl bond. Splittings of all rotational lines into doublets were observed due to internal rotation of the methoxy methyl group CH₃-O, from which torsional barriers of 417 cm⁻¹ and 415 cm⁻¹, respectively, could be deduced. Rotational constants obtained from geometry optimizations at various levels of theory were compared to the experimental values, confirming the soft degree of freedom of the (C=O)-C bond observed for the C₁ conformer of shorter methyl alkynoates like methyl butyrate and methyl valerate. Comparison of the barriers to methyl internal rotation of methyl hexanoate to those of other CH₃-O-(C=O)-R molecules leads to the conclusion that though the barrier height is relatively constant at about 420 cm⁻¹, it decreases in molecules with longer R.