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排序方式: 共有9339条查询结果,搜索用时 15 毫秒
141.
Min Chan Kim David M. Phillips Myung S. Jhon Xiaoding 《Chemical Engineering Communications》2004,191(1):1-16
The surface diffusion characteristics of nonpolar perfluoropolyether (PFPE) Z on carbon surfaces are investigated in two regimes, submonolayer and multilayer, for nano-thin films. For the submonolayer regime, the two-dimensional, cubic van der Waals equation of state is applied to determine the dependence of the surface diffusion coefficient on the film thickness, as experimental surface diffusion coefficients increase with increasing film thickness. For the multilayer regime, a conventional fluid mechanics analysis with position dependent viscosity and a van der Waals disjoining pressure gradient is applied to investigate the surface diffusion characteristics. The present theoretical analysis qualitatively agrees with the experimental results. 相似文献
142.
The onset of hydrodynamical instability induced by impulsive spin-down to rest in a cylinder containing a Newtonian fluid
is analyzed by using propagation theory. It is well-known that the primary transient swirl flow is laminar, but with initial
high velocities secondary motion sets in at a certain time. The dimensionless critical time Τc to mark the onset of instability is presented here as a function of the Reynolds number Re. Available experimental data indicate
that for large Re deviation of the velocity profiles from their momentum diffusion occurs starting from a certain time Τ≈4Τc. This means that secondary motion is detected at this characteristic time. It seems evident that during Τc⪯Τ⪯4Τc, secondary motion is relatively very weak and the primary diffusive momentum transfer is dominant. 相似文献
143.
Polyester polyurethane cationomers are prepared using polyester polyol of molecular weight 2000 as soft segments, N-methyl diethanolamine as chain extender, glycolic acid as quaternization agent, methyl ethyl ketone as solvent, and 4,4'-diphenylenemethylene diisocyanate (MDI) as diisocyanate. Properties of the films cast from solutions and emulsions are studied by infrared spectroscopy, dynamic mechanical analysis, thermogravimetric analysis, differential scanning calorimetry, and tensile-elongation testing. Ionization can cause phase separation and produces increased cohesion in the hard domains. The dispersion process can be divided into three stages involving a separation of hard segment aggregates due to adsorption of water on their surface, water entering into hard segment microionic lattics, and finally a rearrangement of agglomerates to form microspheres. The dispersion can disrupt the order in the hard domains and an increased phase separation. 相似文献
144.
The manufacture of polymer composites through resin transfer molding (RTM) or structural reaction injection molding (SRIM) involves the impregnation of a fibrous reinforcement in a mold cavity with a reactive polymer resin. The design of RTM and SRIM operations requires an understanding of the various parameters, such as materials properties, mold geometry, and mold filling conditions, that affect the resin impregnation process. Modeling provides a potential tool for analyzing the relationships among the important parameters. The present work provides the physical model and finite element formulations for simulating the mold filling stage. Resin flow through the fibers is modeled using two-dimensional Darcian flow. Simultaneous resin reaction and heat transfer among resin, mold walls, and fibers are considered in the model. The proposed technique emphasizes the use of the least squares finite element method to solve the convection dominated mass and energy equations for the resin. Excellent numerical stability of the proposed technique provides a powerful numerical method for the modeling of polymer processing systems characterized by convection dominated transport equations. Results from example numerical studies for SRIM of polyurethane/glass fiber composites were presented to illustrate the application of the proposed model and numerical scheme. 相似文献
145.
Won Seok Lyoo Chan Sik Park Joon Ho Kim Han Do Ghim Ji Youl Lee Jinwon Lee Chul Joo Lee 《应用聚合物科学杂志》2003,88(3):832-839
Vinyl pivalate (VPi) was suspension‐polymerized to synthesize high molecular weight (HMW) poly(vinyl pivalate) (PVPi) with a high conversion above 95% for a precursor of syndiotacticity‐rich HMW poly(vinyl alcohol) (PVA). Also, the effects of the polymerization conditions on the conversion, molecular weight, and degree of branching (DB) of PVPi and PVA prepared by the saponification of PVPi were investigated. Bulk polymerization was slightly superior to suspension polymerization in increasing the molecular weight of PVA. On the other hand, the latter was absolutely superior to the former in increasing the conversion of the polymer, indicating that the suspension polymerization rate of VPi was faster than that of the bulk one. These effects could be explained by a kinetic order of a 2,2′‐azobis(2,4‐dimethylvaleronitrile) concentration calculated by the initial rate method. Suspension polymerization of VPi at 55°C by controlling various polymerization factors proved to be successful in preparing PVA of HMW [number‐average degree of polymerization (Pn): 8200–10,500], high syndiotactic diad content (58%), and very high yield (ultimate conversion of VPi into PVPi: 94–98%). In the case of the bulk polymerization of VPi at the same conditions, the maximum Pn and conversion of 10,700–11,800 and 32–43% were obtained, respectively. The DB was lower and the Pn was higher with PVA prepared from PVPi polymerized at lower initiator concentrations. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 832–839, 2003 相似文献
146.
Lin C.H.; Chan F.C.H.; Hwang J.K.; Lyu P.C. 《Protein engineering, design & selection : PEDS》1999,12(7):589-595
Conantokin-T (con-T) and conantokin-G (con-G) are two highlyhomologous peptide toxins found in Conus venom. The former isa 21-residue peptide with four -carboxyglutamic acid (Gla) residues(at positions 3, 4, 10 and 14), while the latter is a 17-residuepeptide with five -carboxyglutamic acid residues (at positions3, 4, 7, 10 and 14). Despite the apparent similarity in numberand relative positions of the -carboxyglutamic acid residues,113Cd-NMR studies indicated a distinct metal binding behaviorfor con-G and con-T. There appears to be four binding sitesin con-G in contrast to one metal binding site in con-T. Toelucidate the mode of calcium binding by the -carboxyglutamicacid residues in these conantokins, we designed various analogouspeptides with their -carboxyglutamic acid replaced by otheramino acid residues. 113Cd-NMR experiments on conantokin analoguesreveal that the major difference in the number of metal bindingsites between con-G and con-T is due to the residue at position7. We also performed molecular simulations to calculate therelative binding free energies of several potential bindingsites. Based on our theoretical and experimental results, wepropose a `four-site' binding model for conantokin-G and a `single-site'binding model for conantokin-T. 相似文献
147.
针对正交车铣复合加工的生产效率和生产成本多目标的切削用量优化问题,确定了切削速度、轴向进给量、工件转速为优化设计变量。建立了正交车铣复合加工的优化目标函数。从机床、工件、刀具的特性等方面考虑,建立了约束条件方程。采用统一目标法求解了正交车铣复合加工的切削用量优化. 相似文献
148.
M-distinct extension算法通过将数据添加、删除、更新三种情况分开处理,在删除情况下引入伪记录的概念,使得攻击者无法得出插入和删除数据之间的联系;另一方面提出事实:数据记录的不同属性在不同的发布用途中其信息权重并不相同,根据事实,引入属性权重的概念,减少算法在分配和划分两个阶段的循环次数,提高算法的效率。 相似文献
149.
150.
利用近红外光谱技术,以不同产地的羊绒、羊毛纤维为研究对象,结合主成分分析法和多元线性回归方法分别建立了羊绒、羊毛定性分析模型和羊绒-羊毛混纺纤维的定量分析模型.对模型的验证结果表明:定性分析模型能够准确地鉴别出羊绒与羊毛纤维;定量分析模型的相关系数和预测标准偏差分别达到了0.998 1和1.206 1,能够对羊绒-羊毛混纺纤维的含量进行准确的预测;近红外光谱技术用于羊绒、羊毛定性及定量分析具有可行性. 相似文献