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A novel fluorinated bis(ether amine) monomer, 2,3-bis(4-amino-2-trifluoromethylphenoxy)naphthalene, was prepared through the nucleophilic aromatic substitution reaction of 2-chloro-5-nitrobenzotrifluoride and 2,3-dihydroxynaphthalene in the presence of potassium carbonate, followed by catalytic reduction with hydrazine and Pd/C. A series of new fluorine-containing polyimides having inherent viscosities of 0.54 to 1.10 dl/g were synthesized from the diamine with various commercially available aromatic dianhydrides using a standard two-stage process with thermal imidization of poly(amic acid) films. These polyimides were highly soluble in a variety of organic solvents, and most of them afforded transparent, light-colored, and tough films with good tensile strengths. These polyimides exhibited glass transition temperatures (Tgs) of 247-300 °C and showed no significant decomposition below 500 °C under either nitrogen or air atmosphere. Except for the polyimide derived from pyromellitic dianhydride, the polyimide films were almost colorless, with an ultraviolet-visible absorption cutoff wavelength below 400 nm and low b∗ values (a yellowness index) of 10.7-41.9. These polyimides had dielectric constants of 3.09 to 3.65 (1 MHz) and moisture absorptions in the range of 0.2-0.3 wt%. 相似文献
194.
Polyester‐based polyurethane/nano‐silica composites were obtained via in situ polymerization and investigated by Fourier‐transform infrared spectroscopy (FTIR), or FTIR coupled with attenuated total reflectance (FTIR‐ATR), Transmission electron microscopy (TEM), atomic force microscopy (AFM), an Instron testing machine, dynamic mechanical analysis (DMA) and ultraviolet‐visible spectrophotometry (UV‐vis). FTIR analysis showed that in situ polymerization provoked some chemical reactions between polyester molecules and nano‐silica particles. FTIR‐ATR, TEM and AFM analyses showed that both surface and interface contained nano‐silica particles. Instron testing and DMA data showed that introducing nano‐silica particles into polyurethane enhanced the hardness, glass temperature and adhesion strength of polyurethane to the substrate, but also increased the resin viscosity. UV‐vis spectrophotometry showed that nano‐silica obtained by the fumed method did not shield UV radiation in polyurethane films. Copyright © 2003 Society of Chemical Industry 相似文献
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XiaYinshui A.E.A.Almaini WuXunwei 《电子科学学刊(英文版)》2003,20(3):194-201
Using state assignment to minimize power dissipation and area for finite state machines is computationally hard.Most of published results show that the reduction of switching activity often trades with area penalty.In this paper,a new approach is proposed.Experimental results show a significant reduction of switching activity without area penalty compared with previous publications. 相似文献
197.
High-power 1320-nm wafer-bonded VCSELs with tunnel junctions 总被引:8,自引:0,他引:8
V. Jayaraman M. Mehta A.W. Jackson S. Wu Y. Okuno J. Piprek J.E. Bowers 《Photonics Technology Letters, IEEE》2003,15(11):1495-1497
A new long-wavelength vertical-cavity surface-emitting laser structure is described that utilizes AlGaAs-GaAs mirrors bonded to AlInGaAs-InP quantum wells with an intracavity buried tunnel junction. This structure offers complete wavelength flexibility in the 1250-1650 nm fiber communication bands and reduces the high free-carrier losses and bonded junction voltage drops in previous devices. The intracavity contacts electrically bypass the bonded junctions to reduce threshold voltage. N-type current spreading layers and undoped AlGaAs mirrors minimize optical losses. This has enabled 134/spl deg/C maximum continuous-wave lasing temperature, 2-mW room-temperature continuous-wave single-mode power, and 1-mW single-mode power at 80/spl deg/C, in various devices in the 1310-1340 nm wavelength range. 相似文献
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Souheng Wu 《Polymer Engineering and Science》1992,32(12):823-830
The characteristic ratio C∞ and the entanglement molecular weight Me are two key molecular parameters that control melt viscoelasticity, solid mechanical (brittle/ductile) behavior, and adhesion of polymers. We show that the characteristic ratio C∞ and the entanglement molecular weight Me can be predicted from chemical structure by group additivity with uncertainties usually less than ~ 7% for C∞ and ~ 15% for Me, comparable with the accuracies of experimental values. 相似文献
200.
介绍一种用于钢筋混凝土框架二阶弹塑性分析的简化单元模式。它考虑了轴力的二阶效应、横截面的塑化和塑性区长度等非线性因素的影响。利用杆件截面的弯矩—曲率关系,可以直接由弹性杆件的转角—位移方程建立单元的非线性刚度矩阵。经与试验结果比较,说明计算结果是正确的。 相似文献