Laminated viscoelastic plate with inclusions for passive underwater acoustic control

The effectiveness of a fluid-loaded sandwiched viscoelastic layer in suppressing acoustic scattering and radiation has been estimated using the Thomson-Haskell matrix method. The GU (Gaunaurd and Uberall) resonance theory has been adopted to handle three types of viscoelastic substrates with air-filled inclusions. The effects of the varying thickness of the substrates, the volume fractions of the inclusions and frequencies of incident waves have been studied to determine the optimal strategies of signal reduction. In the investigation, the monopole resonance phenomena of the inclusions have been taken into account. Project supported by the National Natural Science Foundation of China (No. 10172039).     

直裂纹短圆棒拉伸试样的J积分估算公式

直裂纹短圆棒拉伸试样特别适用于芯棒一类原材料的断裂韧度测试。本文在局部化约束塑性条件下，采用Ｚａｈｏｏｒ的推导方法［１－３］，获得了这种试样的非线性断裂参数Ｊ积分的估算公式。     

Effect of endgroup modification on dynamic viscoelastic relaxation and motion of hyperbranched poly(ether ketone)s

Fluoro‐terminated hyperbranched poly(ether ketone) (FHPEK) was synthesized and its end groups were modified with alkyl compounds of different chain lengths, i.e., hexyloxy (C6), dodecyloxy (C12), and octadecyloxy, (C18), to produce alkyl‐modified HPEKs (HPEK‐C6, HPEK‐C12, and HPEK‐C18, respectively). Master curves were constructed by using the time‐temperature superposition principle. The horizontal shift factors, a_T, used for the construction of the master curves were fit using the William‐Landel‐Ferry (WLF) equation. From the fitting parameters, the apparent activation energy, E_a, was estimated. With increasing alkyl chain length, the E_a values were found to decrease in the order FHPEK > HPEK‐C6 > HPEK‐C12, and then increase for HPEK‐C18. The average relaxation time, τ_HN, was determined by fitting of the dynamic moduli G′(ω) and G″(ω) to the empirical Havriliak‐Negami equation. Similarly, the τ_HN values decreased in the order of FHPEK > HPEK‐C6 > HPEK‐C12, and then increased for HPEK‐C18. This indicates that the endgroup modification with short alkyl chains (C6, C12) increased the molecular mobility due to the internal plasticization effect of these alkyl chains. Modification with the longer alkyl chain (C18) retarded the molecular motion through an antiplasticization effect caused by summation of nonpolar hydrophobic interactions between long hydrocarbon chains. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2079–2089, 2008     

Synthesis and properties of poly(isobutyl methacrylate‐co‐butanediol dimethacrylate‐co‐methacryl polyhedral oligomeric silsesquioxane) nanocomposites

Poly[isobutyl methacrylate‐co‐butanediol dimethacrylate‐co‐3‐methacrylylpropylheptaisobutyl‐T8‐polyhedral oligomeric silsesquioxane] [P(iBMA‐co‐BDMA‐co‐MA‐POSS)] nanocomposites with different crosslink densities and different polyhedral oligomeric silsesquioxane (MA‐POSS) percentages (5, 10, 15, 20, and 30 wt %) were synthesized by radical‐initiated terpolymerization. Linear [P(iBMA‐co‐MA‐POSS)] copolymers were also prepared. The viscoelastic properties and morphologies were studied by dynamic mechanical thermal analysis, confocal microscopy, and transmission electron microscopy (TEM). The viscoelastic properties depended on the crosslink density. The dependence of viscoelastic properties on MA‐POSS content at a low BDMA loading (1 wt %) was similar to that of linear P(iBMA‐co‐MA‐POSS) copolymers. P(iBMA‐co‐1 wt % BDMA‐co‐10 wt % MA‐POSS) exhibited the highest dynamic storage modulus (E′) values in the rubbery region of this series. The 30 wt % MA‐POSS nanocomposites with 1 wt % BDMA exhibited the lowest E′. However, the E′ values in the rubbery region for P(iBMA‐co‐3 wt % BDMA‐co‐MA‐POSS) nanocomposites with 15 and 30 wt % MA‐POSS were higher than those of the parent P(iBMA‐co‐3 wt % BDMA) resin. MA‐POSS raised the E′ values of all P(iBMA‐co‐ 5 wt % BDMA‐co‐MA‐POSS) nanocomposites in the rubbery region above those of P(iBMA‐co‐5 wt % BDMA), but MA‐POSS loadings < 15 wt % had little influence on glass‐transition temperatures (T_g's) and slightly reduced T_g values with 20 or 30 wt % POSS. Heating history had little influence on viscoelastic properties. No POSS aggregates were observed for the P(iBMA‐co‐1 wt % BDMA‐co‐MA‐POSS) nanocomposites by TEM. POSS‐rich particles with diameters of several micrometers were present in the nanocomposites with 3 or 5 wt % BDMA. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 355–372, 2005     

高功率LDA紧密环绕抽运Nd:YLF放大器实验研究

 采用高功率LDA紧密环绕抽运10mm×10mm Nd:YLF和两级双程离轴行波放大结构，实现了对5mm×5mm口径激光的放大。分析并实验研究了在不同抽运电流及不同抽运延时条件下，激光放大器及两级双程离轴放大系统小信号增益的规律。实验得到两级双程放大系统的小信号净增益可达600。     

Fluctuation regime in the viscoelastic properties of block copolymer solutions

We report measurements of the dynamic shear moduli for solutions of three symmetric styrene isoprene diblock copolymers in the neutral solvent di-octyl phthalate. For each sample the concentration range was selected to extend from the disordered regime to the (lamellar) ordered state, with the emphasis placed on the intermediate regime, where large amplitude composition fluctuations are clearly evident. Temperatures from 0° to nearly 100°C were employed. The fluctuations were most clearly revealed when the data were plotted as phase angle versus reduced frequency, where the frequency axis was scaled by the longest relaxation time in the absence of fluctuations. The dynamic shear moduli and the apparent longest relaxation times, all normalized to their respective values in the absence of fluctuations, increased markedly as the order-disorder transition was approached. The rate of increase was consistent with recent theoretical predictions of Fredrickson, Larson, and Helfand, but the magnitude of the effect is larger than anticipated by theory. The onset of visible fluctuation effects depends on molecular weight in a manner that suggests that entanglements play an important role in determining the coupling of the fluctuations to the viscoelastic properties. This is consistent with previous measurements of translational diffusion, which demonstrated that entanglements severely diminish the mobility of lamellar-forming block copolymers along the interfaces between microdomains.Dedicated to Prof. John D. Ferry on the occasion of his 85th birthday.     

Some tests of constitutive equations for entangled polymers based on planar-extension flow histories in a periodically constricted channel

Stress and velocity were determined locally by birefringence measurements and laser Doppler velocimetry for a mildly entangled polystyrene solution flowing at steady state in a rectangular channel with sinusoidally varying wall spacing. Having measured both the velocity and stress fields, we were able to test constitutive equations locally, i.e., without solving the equations of motion for the entire flow. Four were examined for the periodic planar extensions on the channel centerplane: the Newtonian model, the Lodge network model, the Doi-Edwards tube model, and the Wagner-Schaeffer modification of Doi-Edwards. High enough Weissenberg and Deborah numbers were reached to produce sizable departures from the Newtonian predictions. The Doi-Edwards model underpredicted the stress, as did Wagner-Schaeffer, although to a lesser extent. Predictions of the Lodge model were best of all, a surprising result in view of its inadequacy for simple shear deformations. The predictions of the Lodge model, without parameter adjustment, agreed remarkably well with the planar extension data over the accessible range for our apparatus: Deborah numbers up to 2.0, extensional Weissenberg numbers up to 6.5, and a maximum extension ratio of about 2.3.     

碾压混凝土坝层面影响带渐变规律分析模型

针对碾压混凝土坝层面影响带具有渐变特性，基于复合材料分析思想，利用串联和并联模型，建立了碾压混凝土坝层面影响带渐变规律分析模型；并根据模型的不同状况，提出了，确定层面影响带厚度瞬时弹性模量、延迟弹性模量以及粘性系数等的方法，揭示了决定层面影响带性质的主要计算参数的渐变规律，表征了层面的渐变特性；在上述研究的基础上，建立了相应的三维粘弹性分析模型，研制了相应的各部分分析程序；实例表明，提出的确定层面影响带主要计算参数的方法以及建立的分析模型比较好地反映了碾压混凝土坝的工作性态。     

Synthesis and aqueous solution properties of hydrophobically modified anionic acrylamide copolymers

In this investigation, hydrophobically modified polyacrylamide with low amounts of anionic long‐chain alkyl was synthesized by the free radical polymerization in deionized water. This water‐soluble copolymerization method is more convenient compared with the traditional micellar copolymerization methods. The copolymers were characterized using Fourier transform infrared, ¹H NMR, and the molecular weight and polydispersity were determined using gel permeation chromatography. The solution behavior of the copolymers was studied as a function of composition, pH, and added electrolytes. As NaCl was added to solutions of AM/C₁₁AM copolymers or pH was lowered, the shielding or elimination of electrostatic repulsions between carboxylate groups of the C₁₁AM unit lead to coil shrinkage. The steady shear viscosity and dynamic shear viscoelastic properties in semidilute, salt‐free aqueous solutions were conducted to examine the concentration effects on copolymers. In addition, the shear superimposed oscillation technique was used to probe the structural changes of the network under various stresses or shear conditions. We prepared hydrophobically modified polyacrylamide with N‐alkyl groups in the aqueous medium. The advantage of this method is that the production is pure without surfactants. These results suggest that the unique aqueous solution behavior of the copolymers is different from conventional hydrophobically associating acrylamide. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2465–2474, 2008     

Bulk viscoelastic contribution to the wet‐sliding friction of rubber compounds

The wet‐sliding friction characteristics of rubber compounds made of high cis‐polybutadiene were examined with a British pendulum skid tester at room temperature. Three series of compounds were prepared—unfilled or filled with carbon black at two different levels. The bulk viscoelastic properties as characterized by the bulk glass‐transition temperature for the compounds were systematically adjusted by changing the crosslinking density via sulfur vulcanization. In fact, the dynamic mechanical glass‐transition temperature for the compounds ranges between approximately ?100 and 20 °C. Consequently, the wet‐sliding friction of these rubber compounds is dramatically affected. With increasing compound glass‐transition temperature, the wet‐sliding friction increases to a maximum and then decreases. However, the rate of increase or decrease varies with the amount of filler in the compounds. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 757–771, 2003