Banana-shaped side chain liquid crystalline siloxanes

Eight banana-shaped side chain liquid crystalline oligomers and polymers have been synthesized by hydrosilylation of vinyl-terminated bent-core mesogens with trimethylsilyl-terminated siloxanes. The synthesized oligomers and polymers, and their olefinic precursors, were investigated by polarizing optical microscopy (POM), differential scanning calorimetry, X-ray diffraction (XRD), electro-optical experiments and Maldi-Tof. The short-tailed olefins form a Col_r mesophase, whereas those with longer chains exhibit the SmCP_A mesophase. All the oligomers and polymers studied show liquid crystalline properties and do not crystallize upon cooling. Most oligomers with around four repeating siloxane units, show a lamellar (layer) structure and antiferroelectric switching properties, the SmCP_A phase. XRD shows that the layer spacings are hardly influenced by the length of the terminal tails. The oligomer prepared from the smallest olefinic precursor, having the shortest alkyl tail, shows an XRD pattern reminiscent of a columnar phase, although POM displays domains of opposite chirality, and no switching behaviour could be detected. The polymers with around 35 repeating siloxane units are liquid crystalline, but due to their high viscosity a thorough characterization of the liquid crystalline phases was impossible.     

Light absorption of (oxy-)hemoglobin assessed by spectroscopic optical coherence tomography

The combination of optical coherence tomography and spectroscopy may allow for highly localized, quantitative measurements of tissue spectral properties. We present, for the first time to our knowledge, quantitative measurements of the absorption coefficients of phantoms and of hemoglobin and oxygenated hemoglobin with spectroscopic optical coherence tomography (SOCT). Our results suggest that SOCT will be able to provide localized, quantitative oxygenation measurements.     

A semiclassical model for the vibrational excitation of spherical-top molecules in collisions with ions

A simple theory is presented to explain the previously observed single-mode vibrational excitation of the spherical-top molecules CH₄, CF₄ and SF₆ in collisions with H⁺ and Li⁺. The theory is based on a three-dimensional forced-oscillator model which has been modified to take account of many independent harmonic oscillators. For small-angle collisions the linear driving forces are the dipole-, polarizability- and quadrupole-derivatives taken from IR, Raman spectroscopy and simple estimates, respectively. To explain the results at larger angles near and beyond the rainbow it has been necessary to introduce short-range repulsive forces between the ions and the outer atoms of the molecule. For small angles both the predicted first moment of the energy transfer and the time-of-flight spectra agree quantitatively with the experimental results. At large angles, for which only the first moment of the energy is available, good qualitative agreement is obtained after a slight adjustment of the potential parameters. The energy transfer as a function of time is calculated and shows a different oscillatory behavior for the proton and Li⁺-ion systems. Also the effect of intra-mode coupling is investigated and shown to have only a small effect on the overall energy transfer. The paper closes with a discussion of the implications of these experiments and the possible role of rotational excitation. The field strengths in these ion scattering experiments are shown to be greater than in the strongest focused Q-switched laser pulses.     

The application of organic compounds as thermochemical agents in spectrochemical analysis: The effects of tetraethylammonium iodide in the determination of impurities in materials containing molybdenum

The reducing action of some aliphatic and aromatic hydrocarbon derivatives on the oxides of Me, W, Ti, Zr, Nb and Ta was compared to that of graphite powder. The investigation showed that among the organic compounds studied tetraethylammonium iodide gives a better separation of the volatile elements As, Te, Sn, Pb, Bi, Cd, Ag and In, which commonly occur in the above oxides as impurities, from the molybdenum and better line intensities than when using graphite powder. The possible reduction and iodide forming reactions which take place in the electrode cavity in the presence of tetraethylammonium iodide have been established by using calculated values for the change in the Gibbs free energy and the results of X-ray analyses of sample residues obtained after arcing. This reagent was also found to stabilize the are colunm temperature, a necessary condition for its successful application as a spectroscopic buffer.     

Über holomorphe Abbildungen projektiv-algebraischer Mannigfaltigkeiten auf komplexe Räume

Ohne ZusammenfassungUnterstützt durch Air Force Office of Scientific Research, AF 49 (638)-253, S/A1.