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排序方式: 共有55条查询结果,搜索用时 824 毫秒
1.
Thermal
degradation of vinylidene chloride/[4-(t-butoxycarbonyloxy)phenyl]methyl
acrylate copolymers
B. A. Howell D. A. Spears P. B. Smith 《Journal of Thermal Analysis and Calorimetry》2006,85(1):115-117
Vinylidene chloride polymers containing comonomer
units capable of consuming evolved hydrogen chloride to expose good radical-scavenging
sites might be expected to display greater thermal stability than similar
polymers containing simple alkyl acrylates as comonomer. Incorporation of
a comonomer containing the phenyl t-butyl
carbonate moiety into a vinylidene chloride polymer has the potential to afford
a polymer with pendant groups which might interact with hydrogen chloride
to expose phenolic groups. Copolymers of vinylidene chloride with [4-(t-butoxycarbonyloxy)phenyl]methyl acrylate have been
prepared, characterized, and subjected to thermal degradation. The degradation
has been characterized by thermal and spectroscopic techniques. The degradation
of vinylidene chloride/[4-(t-butoxycarbonyloxy)phenyl]methyl
acrylate copolymers is much more facile than the same process for similar
copolymers containing either [4-(isobutoxycarbonyloxy)phenyl]methyl acrylate
or methyl acrylate, a simple alkyl acrylate, as comonomer. During copolymer
degradation, [4-(t-butoxycarbonyloxy) phenylmethyl
acrylate units are apparently converted to acrylic acid units by extensive
fragmentation of the sidechain. Thus, the phenyl t-butyl
carbonate moiety does function as a labile acid-sensitive pendant group but
its decomposition in this instance leads to the generation of a phenoxybenzyl
carboxylate capable of further fragmentation. 相似文献
2.
Josephine L. Y. Kong Kenneth G. Spears Paul A. Loach 《Photochemistry and photobiology》1982,35(4):545-553
Abstract— The fluorescence properties of a covalently-linked porphyrin-quinone complex and its zinc derivative were studied in a variety of organic solvents. The kinetics of fluorescence decay for both the quinone and hydroquinone oxidation states were measured in acetonitrile, dichloromethane, dimethyl-formamide, and pentane. The fluorescence yield and kinetics of decay at room temperature were little affected in the porphyrin or zinc porphyrin complexes when the attached quinone was reduced. However, for these complexes the fluorescence yield and lifetimes were both substantially decreased in acetonitrile and dichloromethane when the quinone was in its oxidized state. These latter decay kinetics were not explainable by a process having a single exponential decay. On the other hand, little fluorescence quenching or lifetime shortening was observed in dimethylformamide or pentane, indicating unique solvent dependencies for the quenching process. Evidence was obtained for photoproduced charge separation from EPR measurements on the covalently-linked zinc porphyrin-quinone complex. The EPR data showed equivalent concentrations of a Zn porphyrin cation radical and a benzoquinone anion radical in acetonitrile or dichloromethane at both room temperature and 77 K. The charge separated state rapidly decayed at room temperature (in sub-millisecond times) but was quite stable at 77 K. It is concluded that light-induced charge separation in acetonitrile and dichloromethane at room temperature may occur from the excited singlet state with a high quantum efficiency. A photoproduced charge separated state also occurred when the covalently-linked complexes were incorporated into egg yolk phosphatidylcholine liposomes. The quantum yield for radical formation in this latter system was 0.1 and the lifetimes of the radical species formed were many minutes. 相似文献
3.
Companion animal diets may contain up to 50% starch, derived from cereal grains. The amount of resistant starch (RS) in an ingredient depends on the origin and form of the ingredient and on the processing conditions to which the ingredient has been exposed. Extrusion has proven to be a means of optimizing utilization of starch by companion animals. Although the RS fraction of starch typically decreases by extrusion, retrogradation can result in increased concentrations of this fraction. Limited research exists regarding the effects of RS in companion animal nutrition and gastrointestinal health. Existing in vitro and in vivo research indicates that certain RS sources are readily fermented in the large bowel, producing short-chain fatty acids, whereas others are less fermentable, resulting in excellent laxation properties. Feeding dogs a diet high in RS may result in an increase in fecal bulk due to an increased excretion of microbial matter in those cases where RS is highly fermentable, or to indigestibility of the RS source in other cases. RS has a role to play as a potential proxy for dietary fiber, especially for those companion animals fed diets high in protein and fat and devoid of traditional dietary fiber. 相似文献
4.
Navin A Anthony DW Aumann T Baumann T Bazin D Blumenfeld Y Brown BA Glasmacher T Hansen PG Ibbotson RW Lofy PA Maddalena V Miller K Nakamura T Pritychenko BV Sherrill BM Spears E Steiner M Tostevin JA Yurkon J Wagner A 《Physical review letters》2000,84(2):266-269
We derive the formalism to obtain spatial distributions of collisional correlation times for macroscopic particles undergoing granular flow from pulsed gradient spin echo nuclear magnetic resonance diffusion data. This is demonstrated with an example of axial motion in the shear flow regime of a 3D granular flow in a horizontal rotating cylinder at one rotation rate. 相似文献
5.
High thermoelectric figures of merit in PbTe quantum wells 总被引:2,自引:0,他引:2
High-quality Pb1−xEuxTe/PbTe multiple quantum wells (MQWs) have been grown by molecular beam epitaxy. The measured 300K thermoelectric properties
have been compared with that of the best bulk PbTe. This experimental investigation is the first detailed study of MQW structures
designed to improve ZT of thermoelectric materials and has resulted in a breakthrough in the decades-long ZT ≅ 1 barrier for
a room-temperature thermoelectric material. A value of Z2DT >1.2 has been achieved for these PbTe quantum wells. 相似文献
6.
David P. Spears Helmut J. Fischbeck Thomas A. Carlson 《Journal of Electron Spectroscopy and Related Phenomena》1975,6(5):411-420
Satellite structures have been observed in the photoelectron spectra of the core shells of nitric oxide, nitrous oxide and water. An attempt has been made to characterize the satellite structure in terms of monopole transitions resulting from electron shake-up. For this purpose comparisons of the observed excitation energies were made with optical data for both (1) the neutral molecules and (2) the analogous equivalent-core molecular ions. Using these energy comparisons and population densities for various molecular orbitals some assignments are made of the orbitals involved in electron shake-up as a function of the core vacancy. 相似文献
7.
T. C. Harman P. J. Taylor D. L. Spears M. P. Walsh 《Journal of Electronic Materials》2000,29(1):L1-L2
Following the experimentally observed Seebeck coefficient enhancement in PbTe quantum wells in Pb1−xEuxTe/PbTe multiple-quantum-well structures which indicated the potential usefulness of low dimensionality, we have investigated
the thermoelectric properties of PbSexTe1−x/PbTe quantum-dot superlattices for possible improved thermoelectric materials. We have again found enhancements in Seebeck
coefficient and thermoelectric figure of merit (ZT) relative to bulk values, which occur through the various physics and materials
science phenomena associated with the quantum-dot structures. To date, we have obtained estimated ZT values approximately
double the best bulk PbTe values, with estimated ZT as high as about 0.9 at 300 K. 相似文献
8.
With an ever-growing emphasis on sustainable synthesis, aerobic C–H activation (the use of oxygen in air to activate C–H bonds) represents a highly attractive conduit for the development of novel synthetic methodologies. Herein, we report the air mediated functionalisation of various saturated heterocycles and ethers via aerobically generated radical intermediates to form new C–C bonds using acetylenic and vinyl triflones as radical acceptors. This enables access to a variety of acetylenic and vinyl substituted saturated heterocycles that are rich in synthetic value. Mechanistic studies and control reactions support an aerobic radical-based C–H activation mechanism.Herein we disclose a novel method for the aerobic C–H activation of ethereal-based heterocycles to generate various α-functionalised building blocks. 相似文献
9.
10.
Two forces, service needs and technological advances, are driving the extension of the Integrated Services Digital Network (ISDN) into the broadband area. First, this paper identifies several business and residential services which provide a market pull for broadband ISDN (BISDN). Secondly, it reviews recent advances in optical transmission and broadband switching which provide a technological push. Finally, it describes an experimental broadband prototype constructed to help us better understand the transmission, switching, signaling protocol, software, and service capabilities required for BISDN. 相似文献