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排序方式: 共有820条查询结果,搜索用时 31 毫秒
31.
James M. Wescott Tae Ho Yoon David Rodrigues Laura A. Kiefer Garth L. Wilkes James E. McGrath 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(8):1071-1085
Abstract Poly(aryl imide)-poly(dimethyl siloxane) randomly segmented copolymers were synthesized by essentially a one-step solution imidization process in a solvent system consisting of predominately o-dichlorobenzene with a small amount of n-methylpyrolidone. This solvent combination was selected because of its ability to afford homogeneous solutions throughout the polymerization process. This enabled copolymers of any desired poly(dimethyl siloxane) composition to be prepared. A hydrolytically stable triphenylphosphine oxide containing diamine, bis(3-amino-phenoxy-4′-phenyl)phenylphosphine oxide, was utilized as a chain extender and together with oxydiphthalic anhydride formed the hard segment in these copolymers. The soft segment was formed from α,ω-aminopropyl poly(dimethyl siloxane) oligomers of controlled molecular weight. The presence of phosphorus and silicon contributes several unique properties to the system, including enhanced solubility, thermal stability, and flame resistance. High molecular weight copolymers containing up to 60% (w/w) of the poly(dimethyl siloxane) segments were successfully prepared using this method. Gel permeation chromatography analysis, based on a universal calibration curve in CHCl3, was performed to determine the molecular weights and distribution. These copolymers with 40-60% (w/w) poly(dimethyl siloxane) exhibited upper Tg values ranging from 130 to 180°C and showed substantial char yields at 750°C in air, which increased with siloxane content. Dynamic mechanical analysis confirmed the anticipated microphase behavior by the presence of two separate glass-transition regions. Both small angle x-ray scattering and transmission electron microscopy measurements determined on well-characterized transparent cast films were used to better demonstrate the multiphase nature of these copolymers. 相似文献
32.
Transparent hybrid materials have been prepared by incorporating either an alumina or titania alkoxide sol with triethoxysilane end-capped polytetramethylene oxide (PTMO) oligomer. In order to avoid undesirable precipitation after the addition of water, a β-dicarbonyl ligand, ethyl acetoacetate, was used. Triethoxysilane endcapped polytetramethylene oxide with a molecular weight of 2000 (g/mol) was chosen to react with either the alumina or titania sol to demonstrate the features of the reaction scheme. The extent of reaction leading to network formation has been qualitatively followed by infrared spectroscopy. A series of solid films were prepared by varying the molar ratio of either aluminum tri(sec-butoxide) or titanium tetraisopropoxide to ethyl acetoacetate, and the weight fraction of alumina or titania to that of PTMO. The refractive index as well as the general structure-property behavior of these hybrid materials were studied. 相似文献
33.
Dr. Adrien Bourdolle Dr. Mustapha Allali Dr. Anthony D'Aléo Dr. Patrice L. Baldeck Dr. Kenji Kamada Prof. J. A. Gareth Williams Dr. Hubert Le Bozec Dr. Chantal Andraud Dr. Olivier Maury 《Chemphyschem》2013,14(14):3361-3367
The synthesis of tris(2‐thenoyltrifluoroacetonate)lanthanide(III) complexes featuring a diethylaminostyryl‐2,2′‐bipyridine coligand was achieved for lanthanum; the near‐infrared (NIR) emitters neodymium, erbium, and ytterbium; and the transition‐metal yttrium. The photophysical properties were thoroughly studied, and it was demonstrated that the conjugated bipyridine ligand acts as a good antenna for the sensitization of the NIR emitters. The two‐photon absorption (TPA) properties of all five complexes were investigated by using both two‐photon excited fluorescence and the Z‐scan method. We demonstrate that the nature of the rare earth ion has almost no influence on the TPA properties centered on the conjugated bipyridyl ligand. Finally, we show that YbIII is sensitized by a two‐photon antenna effect, and that NdIII is mostly sensitized by a one‐photon process involving direct excitation of forbidden f–f transitions. 相似文献
34.
Dr. Dongpeng Yan Dr. Dejan‐Krešimir Bučar Dr. Amit Delori Bhavnita Patel Dr. Gareth O. Lloyd Prof. William Jones Prof. Xue Duan 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(25):8213-8219
Multi‐component organic nanocrystals that are comprised of two or more supramolecular building blocks can be used to extend the design and assembly scope of solid molecular materials. Herein, we report the use of ultrasonication to prepare halogen‐bonded stilbene‐based nano‐cocrystals that exhibit different photoemission properties, including one‐ and two‐phonon emission and fluorescence lifetimes, relative to those of macrodimensional crystals. The structural transformation from nano‐cocrystals into nanocrystals upon heating results in a luminescence red‐shift from greenish blue to yellow. The temperature‐dependent ratiometric luminescence may allow such nano‐cocrystals to be used as fluorescent sensors and thermosensitive materials. 相似文献
35.
36.
Y. C. Jean Y. Rhee Y. Lou D. Shelby G. L. Wilkes 《Journal of Polymer Science.Polymer Physics》1996,34(17):2979-2985
Two-dimensional angular correlation of annihilation radiation (2D-ACAR) experiments have been performed on an uniaxially oriented polymer, bisphenol-A polycarbonate. The two-dimensional hole structures of unoriented and oriented polymers, determined from the momentum distributions, are found to be spherical and ellipsoidal, respectively. The average hole radii are determined to be 2.9 Å in the unoriented sample and between 2.3 and 3.8 Å in the oriented sample. Applications of the 2D-ACAR method to image the three-dimensional hole structures of polymeric materials are discussed. © 1996 John Wiley & Sons, Inc. 相似文献
37.
Gareth J. Rowlands Author Vitae 《Tetrahedron》2010,66(9):1593-501
38.
Duncan G. Wynn Gareth HumphriesVéronique Morisson-Iveson James NairneIan M. Newington Joanna PassmoreLars-Göran Wistrand 《Tetrahedron letters》2011,52(24):3068-3071
Novel dimeric iodinated contrast agents with low osmolality have been prepared and evaluated with the aim of improving the already good safety profile of such agents. The aim of low osmolality was achieved, and the viscosity of these dimeric agents was also found to be beneficially lower than current dimeric agents in clinical use. 相似文献
39.
Bruno Casella Gareth Roberts Osnat Stramer 《Methodology and Computing in Applied Probability》2011,13(4):835-854
A broad class of implicit or partially implicit time discretizations for the Langevin diffusion are considered and used as
proposals for the Metropolis–Hastings algorithm. Ergodic properties of our proposed schemes are studied. We show that introducing
implicitness in the discretization leads to a process that often inherits the convergence rate of the continuous time process.
These contrast with the behavior of the naive or Euler–Maruyama discretization, which can behave badly even in simple cases.
We also show that our proposed chains, when used as proposals for the Metropolis–Hastings algorithm, preserve geometric ergodicity
of their implicit Langevin schemes and thus behave better than the local linearization of the Langevin diffusion. We illustrate
the behavior of our proposed schemes with examples. Our results are described in detail in one dimension only, although extensions
to higher dimensions are also described and illustrated. 相似文献
40.