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991.
Thermal stabilization of polyacrylonitrile (PAN) fibers is an important step in production of carbon fibers. Understanding the onset and temperature range of the stabilization reactions is a key for adjusting processing parameters such as tension, stretching, etc. However, stabilization reactions are very complex and overlap. In order to separate the stabilization reactions, we combined the results of FTIR (Fourier Transform Infrared Spectroscopy), DSC (Differential Scanning Calorimetry), TGA (Thermogravimetry analysis), TMA (Thermomechanical analysis), and densitometry. It was shown that combination of DSC and TGA allowed separation of reactions regardless of the composition of initial PAN fibers. FTIR, TMA and densitometry results verified the validity of proposed method. Accordingly, three special and commercial grade PAN fibers with different chemical composition were studied. FTIR results indicated that during thermal stabilization of PAN fibers chemical reactions including cyclization, oxidation and dehydrogenation occurred in the fibers and a ladder polymer was formed. According to DSC and TGA curves, initiation temperature, temperature range and order of occurrence of these reactions were a function of chemical composition of initial fibers. In fibers containing itaconic acid plus methyl acrylate comonomers, oxidation reactions already started at 175 °C. Cyclization started above 210 °C, and reactions occurring above 250 °C were mainly dehydrogenation. In fibers containing only itaconic acid cyclization initiated above 210 °C, dehydrogenation started after 242 °C and oxidation occurred only after 284 °C. In fibers containing vinyl acetate comonomers, the initial reactions above 240 °C were attributed to cyclization. Oxidation occurred below 290 °C and dehydrogenation started above 290 °C. 相似文献
992.
Dr. Seiji Tsuzuki Wataru Shinoda Dr. Shiro Seki Prof. Yasuhiro Umebayashi Dr. Kazuki Yoshida Prof. Kaoru Dokko Prof. Masayoshi Watanabe 《Chemphyschem》2013,14(9):1993-2001
The stabilization energies (ΔEform) calculated for the formation of the Li+ complexes with mono‐, di‐ tri‐ and tetra‐glyme (G1, G2, G3 and G4) at the MP2/6‐311G** level were ?61.0, ?79.5, ?95.6 and ?107.7 kcal mol?1, respectively. The electrostatic and induction interactions are the major sources of the attraction in the complexes. Although the ΔEform increases by the increase of the number of the O???Li contact, the ΔEform per oxygen atom decreases. The negative charge on the oxygen atom that has contact with the Li+ weakens the attractive electrostatic and induction interactions of other oxygen atoms with the Li+. The binding energies calculated for the [Li(glyme)]+ complexes with TFSA? anion (glyme=G1, G2, G3, and G4) were ?106.5, ?93.7, ?82.8, and ?70.0 kcal mol?1, respectively. The binding energies for the complexes are significantly smaller than that for the Li+ with the TFSA? anion. The binding energy decreases by the increase of the glyme chain length. The weak attraction between the [Li(glyme)]+ complex (glyme=G3 and G4) and TFSA? anion is one of the causes of the fast diffusion of the [Li(glyme)]+ complex in the mixture of the glyme and the Li salt in spite of the large size of the [Li(glyme)]+ complex. The HOMO energy level of glyme in the [Li(glyme)]+ complex is significantly lower than that of isolated glyme, which shows that the interaction of the Li+ with the oxygen atoms of glyme increases the oxidative stability of the glyme. 相似文献
993.
ZHANG Yingying HAN Fangwai ZHANG Mengyao ZHANG Huixin LI Ying WANG Ru ZENG Yongfei LIU Guiyan 《高等学校化学研究》2020,36(5):859-864
Three new Pd-pyridine enhanced precatalyst preparation stabilization and initiation(PEPPSI) complexes with halogen groups on the N-heterocyclic carbene and pyridine were prepared. Their structures have been clearly characterized by nuclear magnetic resonance spectroscopy and X-ray single-crystal diffraction. The effects of the electronic properties of halogen groups on the catalytic activity in the Suzuki-Miyaura cross-coupling of aryl chlorides were investigated. These Pd-PEPPSI complexes could catalyze the cross-coupling reaction efficiently with a low catalyst loading(0.05%, molar ratio) at room temperature and the products were obtained in high yields. 相似文献
994.
Lukas M. Sigmund Fabian Ebner Christoph Jöst Jonas Spengler Nils Gönnheimer Deborah Hartmann Dr. Lutz Greb 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(14):3152-3156
The vital effect of radical states on the pharmacological activity of phenothiazine-based drugs has long been speculated. Whereas cationic radicals of N-substituted phenothiazines show high stability, the respective neutral radicals of N-unsubstituted phenothiazines have never been isolated. Herein, the 1,9-diamino-3,7-di-tert-butyl-N1,N9-bis(2,6-diisopropylphenyl)-10H-phenothiazin-10-yl radical (SQH2.) is described as the first air-stable, neutral phenothiazinyl free radical. The crystalline dark-blue species is characterized by means of EPR and UV/Vis/near-IR spectroscopy, as well as cyclic voltammetry, spectro-electrochemical analysis, single-crystal XRD, and computational studies. The SQH2. radical stands out from other aminyl radicals by an impressive radical stabilization energy and its parent amine has one of the weakest N−H bond dissociation energies ever determined. In addition to serving as open-shell reference in medicinal chemistry, its tridentate binding pocket or hydrogen-bond-donor ability might enable manifold uses as a redox-active ligand or proton-coupled electron-transfer reagent. 相似文献
995.
《Particuology》2022
The magnetic stabilization flow regime could also be created for Geldart-B nonmagnetizable particles provided some magnetizable particles are introduced and the magnetic field is applied. This study aimed to explore the size (dpM) and density (ϿpM) effects of magnetizable particles on its operating range. The upper limit (UmbH) could not be determined from the οPbUg curve but could from analyzing the variation of οPb-fluctuation with increasing Ug. Due to the variation of UmfH (lower limit) with dpM and ϿpM, both UmbHUmfH and (UmbHUmfH)/UmfH were used to quantify the operating range of magnetic stabilization. UmbHUmfH varied hardly with dpM but increased significantly with decreasing ϿpM. (UmbHUmfH)/UmfH increased as dpM or ϿpM decreased. It was more difficult for the nonmagnetizable particles to escape from the network formed by the smaller/lighter magnetizable particles. For the same magnitude of change, dpM had a stronger effect than ϿpM on (UmbHUmfH)/UmfH. Neither UmbHUmfH nor (UmbHUmfH)/UmfH varied monotonously with the minimum fluidization velocity of the magnetizable particles, indicating that no straightforward criterion for matching the magnetizable particles to the given nonmagnetizable particles could be established based on their minimum fluidization velocities to maximize the operating range of magnetic stabilization. 相似文献
996.
《Particuology》2022
The n-body instability is investigated with the soft-sphere discrete element method. The divergence of nearby trajectories is quantified by the dynamical memory time. Using the inverse proportionality between the dynamical memory time and the largest Lyapunov exponent, the soft-sphere discrete element method results are compared to previous hard-sphere molecular dynamics data for the first time. Good agreement is observed at low concentrations and the degree of instability is shown to increase asymptotically with increasing spring stiffness. At particle concentrations above 30%, the soft-sphere Lyapunov exponents increase faster than the corresponding hard-sphere data. This paper concludes with a demonstration of how this case study may be used in conjunction with regression testing and code verification activities. 相似文献
997.
Ying‐Hung So Bruce M. Bell Dave R. Albers 《Journal of polymer science. Part A, Polymer chemistry》2007,45(10):1868-1877
Solid‐state aryl benzobisoxazole and aryl benzobisthiazole compounds photolyzed in the presence of O2 undergo cleavage to produce benzobisoxazoles (or benzobisthiazoles), benzonitriles, and benzamides. A very high percentage of the carbon atoms in one of the two segments from chain cleavage are converted to CO2. This very unusual observation has been carefully confirmed by gas chromatography/mass spectroscopy analysis of the gaseous components in the photolysis vessel before and after photolysis, labeling experiments, and the correlation of the mass reduction in photolyzed polymers and the amount of CO2 that evolves. The rate of CO2 generation is used to compare the relative photostability of aryl benzobisoxazole and aryl benzobisthiazole model compounds, films, and fibers. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1868–1877, 2007 相似文献
998.
On this work it is addressed the problem of how to exploit the dynamic behind a chaotic transient behavior to improve system performance and adaptability to many operational conditions request. The phenomena of chaotic transient is explained as due to the presence of a chaotic saddle in the phase space. Different system operation points can be associated to the set of unstable periodic orbits that exist embedded in the chaotic saddle. A classical control procedure associated with a control of chaos strategy is proposed as a methodology to quickly guide system trajectories among different operation points and to keep the system on a particular operation point. The methodology is applied on an electronic circuit system. 相似文献
999.
We consider the Kirchhoff plate equation and the Bernoulli–Euler plate equation. The energy decay rate in both cases is investigated. Moreover, when we do not have exponential stability in the energy space, we give explicit logarithmic decay estimates valid for regular initial data. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
1000.
During processing polyamides can undergo thermo‐mechanical degradation due to the mechanical stress and to the high temperature applied to the melt. Degradation can occur also during the polyamides lifetime due to many driving forces, like temperature, ultraviolet radiations, etc. The mechanisms of degradation are not well understood and the stabilization with usual stabilizers, like phenol antioxidants, is not so efficient as for other classes of polymers. The aim of this work is the study of the effects of the addition of a new multifunctional additive on the processing and on the thermo‐ and photo‐stabilization of polyamide‐6. This additive is able to prevent the thermo‐mechanical degradation and also to enlarge the molecular weight distribution improving the processability of this polymer in operations where the elongational flow is involved. Moreover, the presence of this additive extends significantly the lifetime of the polyamide when subjected to thermo‐ and photo‐oxidative stress. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献