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991.
992.
Marie‐France Llauro Julien Loiseau Fernande Boisson Frdric Delolme Catherine Ladavire J. Claverie 《Journal of polymer science. Part A, Polymer chemistry》2004,42(21):5439-5462
Low‐molecular‐weight poly(acrylic acid) (PAA) was synthesized by reversible addition fragmentation chain transfer polymerization with a trithiocarbonate as chain‐transfer agent (CTA). With a combination of NMR spectroscopy and matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry, the PAA end‐groups of the polymer were analyzed before and after neutralization by sodium hydroxide. The polymer prior to neutralization is made up of the expected trithiocarbonate chain‐ends and of the H‐terminated chains issued from a reaction of transfer to solvent. After neutralization, the trithiocarbonates are transformed into thiols, disulfides, thiolactones, and additional H‐terminated chains. By quantifying the different end‐groups, it was possible to demonstrate that fragmentation is the rate limiting step in the transfer reaction. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5439–5462, 2004 相似文献
993.
John Boyle Ica Manas‐Zloczower DonaldL. Feke 《Particle & Particle Systems Characterization》2004,21(3):205-212
The dispersion behavior of agglomerates of several grades of fumed silica in poly(dimethyl siloxane) liquids has been studied as a function of particle morphology and applied flow conditions. The effects of primary particle size and aggregate density and structure on cohesivity were probed through tensile and shear strength tests on particle compacts. These cohesivity tests indicated that the shear strength of particle compacts was two orders of magnitude higher than the tensile strength at the same overall packing density. Experiments carried out in both steady and time‐varying simple‐shear flows indicate that dispersion occurs through tensile failure. In the steady‐shear experiments,enhanced dispersion was obtained at higher levels of applied stress and, at comparable levels of applied stress, dispersion was found to proceed faster at higher shear rates. Experiments conducted in time‐varying flows further corroborated the results obtained in tensile cohesivity tests. Experiments in which the mean and maximum stresses in the time‐varying flows were matched to the stresses produced in steady shear flows highlight the influence of flow dynamics on dispersion behavior. 相似文献
994.
995.
Jau-Jiun Chen Soohwan Jang F. Ren Yuanjie Li Hyun-Sik Kim D. P. Norton S. J. Pearton A. Osinsky S. N. G. Chu J. F. Weaver 《Journal of Electronic Materials》2006,35(4):516-519
Wet etch rates at 25°C for Zn0.9Mg0.1O grown on sapphire substrates by pulsed laser deposition (PLD) were in the range 300–1100 nm · min−1 with HCl/H2O (5×10−3−2×10−2 M) and 120–300 nm · min−1 with H3PO4/H2O (5×10−3−2×10−2 M). Both of these dilute mixtures exhibited diffusion-limited etching, with thermal activation energies of 2–3 kCal · mol−1. By sharp contrast, the etch rates for ZnO also grown on sapphire by PLD were much slower in similar solutions, with rates
of 1.2–50 nm · min−1 in HCl/H2O (0.01–1.2 M) and 12–54 nm · min−1 in H3PO4/H2O (0.02–0.15 M). The etching was reaction limited over the temperature range 25–75°C, with activation energies close to 6
kCal · mol−1. The resulting selectivity of Zn0.9Mg0.1O over ZnO can be a high as ∼400 with HCl and ∼30 with H3PO4. 相似文献
996.
Auvergne R. Saint‐Loup R. Joly‐Duhamel C. Robin J. J. Boutevin B. 《Journal of polymer science. Part A, Polymer chemistry》2007,45(7):1324-1335
The synthesis and characterization of photopolymerizable unsaturated polyester resins based on PET waste are described. The resins came from a depolymerization process based on the glycolysis of PET by diethylene glycol (DEG). Different molecular weights of glycolysates were synthesized. Then, the latter was functionalized by a methyl hemiester of maleic acid to obtain unsatured α,ω‐bismaleate PET oligomers. In the presence of an electron donor monomer, such as triethylene glycol divinyl ether, these electron acceptor oligomers were copolymerized by way of charge‐transfer complexes under UV irradiation. The reaction was monitored in situ by real‐time IR spectroscopy to study the kinetics of photopolymerization. This one was studied in relation with the physical and chemical characteristics of oligoesters and the composition of mixtures containing divinyl ethers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1324–1335, 2007 相似文献
997.
Christopher K. Yee‐Chan Robert C. Scogna Richard A. Register 《Journal of Polymer Science.Polymer Physics》2007,45(10):1198-1204
In the idealized two‐phase model of a semicrystalline polymer, the amorphous intercrystalline layers are considered to have the same properties as the fully‐amorphous polymer. In reality, these thin intercrystalline layers can be substantially influenced by the presence of the crystals, as individual polymer molecules traverse both crystalline and amorphous phases. In polymers with rigid backbone units, such as poly(etheretherketone), PEEK, previous work has shown this coupling to be particularly severe; the glass transition temperature (Tg) can be elevated by tens of degrees celsius, with the magnitude of the elevation correlating directly with the thinness of the amorphous layer. However, this connection has not been explored for flexible‐chain polymers, such as those formed from vinyl‐type monomers. Here, we examine Tg in both isotactic polystyrene (iPS) and syndiotactic polystyrene (sPS), crystallized under conditions that produce a range of amorphous layer thicknesses. Tg is indeed shown to be elevated relative to fully‐amorphous iPS and sPS, by an amount that correlates with the thinness of the amorphous layer; the magnitude of the effect is severalfold less than that in PEEK, consistent with the minimum lengths of polymer chain required to make a fold in the different cases. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1198–1204, 2007 相似文献
998.
Development of a low cost microwave mixer using a broad-band substrate integrated waveguide (SIW) coupler 总被引:2,自引:0,他引:2
Ji-Xin Chen Wei Hong Zhang-Cheng Hao Hao Li Ke Wu 《Microwave and Wireless Components Letters, IEEE》2006,16(2):84-86
A low cost single-balanced mixer is designed using a newly designed 90/spl deg/ substrate integrated waveguide (SIW) 3-dB coupler, which takes the advantages of low cost, low profile, and high performance. An X-band single-balanced SIW mixer is designed and fabricated with a standard printed circuit board process. Measured conversion loss of 6.8dB and the wide-band response from 8.5 to 12GHz are presented. 相似文献
999.
封装工业正在推动着印制线路板技术水平朝着半导体要求的方向发展。高密度印制线路板正广泛用于网络路由器、自动测试设备和服务器领域。高密度要求(>150 IO/cm~2)正成为客户的普通需求。为满足现有的高密度封装要求,由于线宽、线距和通孔孔径正接近传统制作极限,普遍的方法就是不断增加印制线路板的层数。这些特征正驱动着印制线路板在基材、图像转移、蚀刻、电镀、阻焊、测试流程和对位系统的变化。此文主要讲述了印制线路板特性的变化趋势和利用对位模式和雷利分布去预测满足高级对位要求的能力以及印制线路板各流程中有关对准度的控制要点。 相似文献
1000.
短信市场巨额收入的刺激,再加上市场监管力度不够,使得一些服务提供商(SP)片在追求收益而忽视消费者需求,导致短信服务走入了一些误区,这已经引起一系列负面效应,要解决此问题,SP应树立服务理念,根据客户的需求来提供服务内容和服务方式;运营商需改进商业模式,应考虑对SP提供的业务内容和配套服务进行规范,用户要维护自身权益,政府则应加强监管能力。 相似文献