基于运动约束解过约束并联机构变形协调方程

提出利用运动约束关系来间接求解过约束并联机构变形协调方程．首先介绍了该方法的原理,接着分别针对平面和空间过约束并联机构,详述该方法的解决步骤,结果验证了该方法的正确性,从中还可看出该方法在求解复杂过约束并联机构时非常简洁,最后介绍了采用该方法解决多度过约束问题．     

Analysis of dimensionally stable copolyimide with a low‐level residual stress

Copolyimide thin film, which has low‐level stress and stress relaxation induced by water sorption, was characterized for potential applications as an encapsulant, a stress‐relief buffer, and in interlayer dielectrics. The polyimides examined were poly(p‐phenylene pyromellitimide) (PMDA‐PDA) and poly(p‐phenylene biphenyltetracarboximide) (BPDA‐PDA) as well as their random copolyimides with various compositions. These copolyimide films exhibited good combinations of physical and mechanical properties with low thermal expansion coefficients, residual stress, and moisture‐induced stress–relaxation behavior by appropriately selecting the ratios of the dianhydride component. For these polyimides, the residual stress increased in the range of −8.1–7.5 MPa, whereas stress relaxation induced by water uptake decreased in the range of 10.3–4.7 MPa with increasing BPDA contents, respectively. The major factor in determining the magnitude of the stress behavior induced by both the thermal mismatch and water uptake in films should be the morphological factors such as chain rigidity, chain orientation, crystallinity, and microvoids. Their morphological structures were examined by wide angle X‐ray diffraction and a prism coupler, and the thermal properties were measured using a dynamic mechanical thermal analyzer as well as thermomechanical analysis. Overall, the candidate for the low level stress buffer application from the PMDA/BPDA‐PDA copolyimide was the 30/70 (= PMDA/BPDA in molar ratio) copolyimide. This copolyimide showed no residual stress after curing at 400 °C and relatively insensitive stress relaxation to ambient humidity. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 796–810, 2001     

Open spaces and relaxation processes in the subsurface region of polypropylene probed by monoenergetic positron beams

Open spaces and relaxation processes in the subsurface region of isotactic polypropylene were investigated by monoenergetic positron beams. From measurements of the lifetime spectra of positrons, the size of the open spaces in the subsurface region (≤ 0.2 μm) was found to be larger than that in the bulk; their differences were estimated as 20% at 295 K and 10% at 395 K. From conventional positron annihilation experiments, the glass‐transition temperatures, T_{g (upper)}and T_g(lower) were determined as 306 K and 278 K, respectively. These transition temperatures were associated with the onset temperatures of the molecular motions under the constraint imposed by crystalline regions and those free from the constraint, respectively. In the subsurface region, although the onset temperatures of the molecular motions were close to those in the bulk, the molecular motions above T_{g (lower)} were found to be suppressed. The annihilation characteristics of positrons with different implantation energies were also discussed. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 101–107, 2000     

Molecular motion and relaxation studies of aliphatic polyureas by thermal windowing thermally stimulated depolarization current technique

Molecular motion and relaxation studies using a thermal windowing thermally stimulated depolarization current (TW‐TSDC) were performed for aliphatic polyureas 7 and 9. Global thermally stimulated depolarization current gave three characteristic major peaks corresponding to the α, β, and γ relaxation modes at 78.5, −44, and −136°C for polyurea 7 and at 80, −50, and −134°C for polyurea 9, respectively. The α relaxation is related to the large‐scale molecular motion due to micro‐Brownian motion of long‐range segments. This relaxation is significantly related to the glass‐transition temperature. The β relaxation is caused by the local thermal motion of long‐chain segments. The γ relaxation is caused by the limited local motion of hydrocarbon sections. Temperature dependence of relaxation times was expressed well using Vogel–Tammann–Fulcher (VTF) expression. 3‐D simulation of dielectric constants of dielectric strength and loss factor were performed in the frequency range from 10⁻⁶ to 10⁴ Hz and temperature range from −150 to 250°C, using the relaxation parameters obtained from the TW‐TSDC method. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 88–94, 2000     

The effect of residual acetate groups on the structure and properties of vinyl alcohol–ethylene copolymers

Vinyl alcohol–ethylene (VAE) copolymers, commercially manufactured by hydrolysis of the corresponding vinyl acetate–ethylene copolymers, can contain small amounts of unhydrolyzed vinyl acetate. This article shows the influence of these residual groups on the structure of the resulting copolymers, studied by nuclear magnetic resonance and wide‐angle X‐ray scattering. Thermal and mechanical properties of these materials were investigated by differential scanning calorimetry, thermogravimetry, drawing behavior, birefringence measurements, and dynamic mechanical analysis. The structure of the copolymers is considerably affected by the volume of the residual acetate groups, bigger than that of the hydroxyl ones, which hinders the crystallization process. In relation to the thermal and mechanical properties, the temperature and enthalpy of melting as well as the Young's modulus and yield stress, decrease as vinyl acetate molar fraction increases. Moreover, the α and β relaxations are shifted to lower temperatures as residual content in the copolymer is raised. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 573–583, 2000     

Unimodal molecular weight distribution of commercial polymers from viscoelastic data

This article presents a method that provides the molecular weight distribution (MWD) of polymeric material from rheological data. The technique has been developed to deal with linear polymers with a log‐normal molecular weight distribution. The rheological data must include the shear storage modulus, G′(ω), and the shear loss modulus, G″ (ω), ranging from the terminal zone to the rubberlike zone. It was not necessary to achieve the relaxation spectrums via the extremely unstable problem of inverting integral equations. The method has been tested with different polymers (polydimethylsiloxane, polyisoprene, random copolymer of ethylene and propylene, and polystyrene) and the calculated MWDs were in good agreement with experimental data. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1539–1546, 2000     

产能约束下的供应商竞争

买方市场中,占据主导地位的零售商通常会引入生产同质产品的两家或多家供应商,以满足自身的需求,并通过供应商之间的竞争获取更高的利益。价格竞争是供应商为了争夺市场份额的重要手段。然而,在产能充足的前提下,竞争的最终结果是双方均不获利。本文从产能角度出发,运用报童模型分析了两个生产同质产品的供应商在不同产能下,竞争对供应商和零售商的影响。通过分析对比产能对称和不对称两种情况,我们发现竞争对于零售商来说总是有利的,而对于相互竞争的供应商来说,产能大小应该限定在一定范围内,并从中获得一些管理学方面启示。     

Effect of High Humidity Exposure to Wheat Starch Based High Conducting Flexible Polymer-in-Salt-Electrolyte

Polymer-in-salt-electrolytes (PISEs) are an important class of electrolytes as they carry the promise of faster and single ion transport. Unfortunately, due to unavailability of a suitable polymer host PISE has still not reached to commercial level. In the present work, using a novel synthesis protocol developed by the group, glutaraldehyde crosslinked wheat starch has been successfully modified with sodium iodide (NaI) to synthesize a flexible PISE membrane with desired electrochemical properties. Present paper reports the effect of crosslinker and exposure to high humidity ambience on electrochemical and morphological properties. It has been established that on exposure to higher humidity atmosphere starch-based PISEs stabilize at lower resistance value, but with higher ion relaxation time, which indicates that effect of high humidity treatment is more on salt dissociation instead of assisting the ion transport. The studied materials have conductivity ≈0.01 S cm⁻¹ range with ESW >2.5 V, ensuring its usability in electrochemical devices. The developed synthesis protocol does not require any complicated synthesis route and/or sophisticated instrument hence the overall process is economical also, adding up to its potentiality for energy device fabrication.     

冲击加载下20钢中弹性前驱波衰减与应力松弛行为实验研究

 通过改变样品厚度，对平面冲击加载下20钢的弹性前驱波的波幅衰减和应力松弛进行了实验研究。采用激光速度干涉测速仪（VISAR）实测了样品后自由面速度历史，采样频率达到1 ns，保证了实验结果的准确性。实验结果显示：Hugoniot弹性极限随着传播距离呈指数衰减，在所研究的样品厚度范围内，Hugoniot弹性极限减小了44%；应力松弛行为和弹性前驱波的上升沿时间也依赖于传播距离；冲击加载的强度对材料动态屈服行为的影响很小。     

Medium effect on dielectric relaxation behaviors of 4A zeolite bulk dispersion system

Dielectric spectra of 4A zeolite particles bulked in deionized water (4A/W) or cyclohexane (4A/C) were measured respectively under different temperatures, and two dielectric relaxations were found in both of 4A/W and 4A/C. Because of existence of water molecules in pores, both low and high frequency relaxation times (τ_LFR and τ_HFR) of 4A/Wdecreasedmore sharply than 4A/C with increasing temperature. According to the temperature dependence of τ_LFR and τ_HFR, the activation energies of Na⁺ interacting with framework under hydrated and dehydrated environments were calculated respectively, and the original properties of Na⁺ in dehydrated zeolite were obtained by analyzing the dielectric properties of 4A/C. Furthermore, a transient state of Na⁺ motion with temperature was found in 4A/C system.