拉伸取向聚合物2H NMR谱的键涨落模型Monte Carlo模拟

采用三维键涨落模型（BFM）的Monte Carlo计算机模拟方法,模拟了两种分别具有高取向和低取向链微观结构的拉伸取向聚合物²H NMR谱. 讨论了排除体积效应与链的均方末端距分布对²H NMR谱线的影响. 结果表明,排除体积效应对高取向和低取向聚合物²H谱劈裂的影响是不同的,较小均方末端距的链决定²H谱劈裂,而较大均方末端距的链使²H谱产生长的拖尾. 采用BFM计算机模拟与²H NMR实验的结合是研究拉伸取向聚合物网络微观结构的有力手段.     

‘Monte Carlo’ computer simulation of chain molecules III. Simulation of n-alkane molecules

Considering a realistic model for isolated n-alkane molecules, their configurational behaviour has been explored employing a ‘Monte Carlo’ method. The present study shows that the behaviour of the short molecules is unlike that of the longer molecules in regard to the variation with temperature of the mean-square end-to-end distance and radius of gyration. For the short molecules the mean-square end-to-end distance and the radius of gyration are decreasing functions of temperature, but they increase with temperature in the case of the longer molecules. The macromolecules assume random-walk behaviour at approximately 600 K. The computed heat capacity versus temperature curves disclose configurational transitions in the chains.     

Configurational-conformational statistics of stereoirregular poly(methyl methacrylate)s

The mean-square radius of gyration of poly(methyl methacrylate) (PMMA) with Bemoullian statistics was theoretically investigated utilizing the method of periodic condition. The dependent curve of the characteristic ratio of mean-square end-to-end distance on periodic microstructure length increases monotonously with the length and goes gradually to its asymptotic value for atactic PMMA chains (P _m = 0.5). Conformational energies E _α ～1.3 kcal mol^?1 and E _β ～-0.6 kcal mol^?1 of the two-state scheme are acceptable, from which the derived meansquare end-to-end distance and mean-square radius of gyration conform to the experimental data. The ratio ?S²?/χ of atactic PMMA (P _m = 0.2) as a function of the degree of polymerization χ first increases with increasing chain length, then passes through a maximum at χ = 30, and finally decreases to its asymptotic value, which is in reasonable agreement with the experimental measurements reported elsewhere. The ratio ?S²?/χ as a function of stereochemical composition indicates that theoretical and experimental data are in accord for isotactic and atactic PMMA, but small-angle neutron scattering (SANS) measurements are higher than the calculated values for syndiotactic PMMA.     

Simulation of light scattering by a particle on a film-coated substrate using coupled-dipole method

The intensity of light scattered by a submicron particle on a film-coated semiconductor substrate is calculated as a function of the thickness of the film using the coupled-dipole method. The result of calculation reproduces the experimentally observed features, i.e., the oscillatory dependence of the scattering intensity on the thickness and the enhancement of the scattering intensity for very thin films. The enhancement is reproduced only when the dipole-dipole interaction between the particle and the substrate is included in the calculation. Using the method we propose, the scattering intensity can be calculated for an arbitrary size and shape of particle on an arbitrary thickness of film.     

Mobility in thin polymer films ranging from local segmental motion, Rouse modes to whole chain motion: A coupling model consideration

Large increases of mobility of local segmental relaxation observed in polymer films as the film thickness is decreased, as evidenced by decreases of the glass temperature, are not found for relaxation mechanisms that have longer length scales including the Rouse relaxation modes and the diffusion of entire polymer chains. We show that the coupling model predictions, when extended to consider polymer thin films, are consistent with a large increase of the mobility of the local segmental motions and the lack of such a change for the Rouse modes and the diffusion of entire polymer chains. There are two effects that can reduce the coupling parameter of the local segmental relaxation in thin films. One is the chain orientation that is induced parallel to the surface when the film thickness h becomes smaller than the end-to-end distance of the chains and the other is a finite-size effect when h is no longer large compared to the cooperative length scale. Extremely thin ( ≈ 1.5 nm) films obtained by intercalating a polymer into layered silicates have thickness significantly less than the cooperative length scale near the bulk polymer glass transition temperature. As a result, the coupling parameter of the local segmental relaxation in such thin films is reduced almost to zero. With this plausible assumption, we show the coupling model can explain quantitatively the large decrease of the local segmental relaxation time found experimentally. Received 1 August 2001 and Received in final form 1 December 2001     

A colloidal model system with tunable disorder: Solid-fluid transition and discontinuities in the limit of zero disorder

We study a colloidal model system where disorder can be continuously tuned from no disorder --corresponding to a system that can crystallize-- to large disorder where geometrical frustration occurs. The model system consists of colloidal particles with screened electrostatic repulsion. They can only move on single lines which are parallel and equidistant to each other. We introduce disorder by modulating the particle line density. The system exhibits a solid-to-fluid transition which we study by the structure factor and the temporal evolution of the mean-square distance of nearest neighbors on neighboring lines. A determining feature is the occurrence of discontinuities when disorder is tuned to zero. We observe that the peak height of the pair correlation function in the solid phase does not extrapolate to the value of the perfect crystal. Similarly, the mean interaction energy and the screening length at which the solid-fluid transition occurs seem to be discontinuous when the limit of zero disorder is approached.     

Modelling self-assembling of colloid particles in multilayered structures

Simulations of particle multilayer build-up in the layer by layer (LbL) self-assembling processes have been performed according to the generalized random sequential adsorption (RSA) scheme. The first (precursor) layer having an arbitrary coverage of adsorption centers was generated using the standard RSA scheme pertinent to homogeneous surface. Formation of the consecutive layers (up to 20) was simulated by assuming short-range interaction potentials for two kinds of particles of equal size. Interaction of two particles of different kind resulted in irreversible and localized adsorption upon their contact, whereas particles of the same kind were assumed to interact via the hard potential (no adsorption possible). Using this algorithm theoretical simulations were performed aimed at determining the particle volume fraction as a function of the distance from the interface, as well as the multilayer film roughness and thickness as a function of the number of layers. The simulations revealed that particle concentration distribution in the film was more uniform for low precursor layer density than for higher density, where well-defined layers of closely packed particles appeared. On the other hand, the roughness of the film was the lowest at the highest precursor layer density. It was also predicted theoretically that for low precursor layer density the film thickness increased with the number of layers in a non-linear way. However, for high precursor layer density, the film thickness increased linearly with the number of layers and the average layer thickness was equal to 1.58 of the particle radius, which is close to the closely packed hexagonal layer thickness equal to 1.73. It was concluded by analysing the existing data for colloid particles and polyelectrolytes that the theoretical results can be effectively exploited for interpretation of the LbL processes involving colloid particles and molecular species like polymers or proteins.     

Analysis of oscillations of concentration in bounded binary mixtures taking into account surface effects

We consider a binary mixture bordering a substrate and vacuum and forming a film. The distribution of one of the mixture components is described by the Cahn-Hillard equation with asymmetric boundary conditions taking into account (wetting) processes occurring at the boundary and in surface layers of the film and, probably, the effect of a constant external magnetic field. The parameters of the problem characterize the bulk correlation length, pair interaction intensity in the nearest neighbor approximation at the film boundary, and the renormalized magnetic field acting on the spins in the surface layers of the system. The range of parameters generating oscillations of the concentration over the film thickness is determined. Subranges of parameters generating the preset number of oscillations over the film cross section are indicated. The analytic results are qualitatively confirmed in numerical experiments.     

扫描近场拉曼显微镜中“热点”处场增强的数值模拟(英文)

在本文中我们提出了一种近场扫描光学拉曼显微镜的数值模型,其中,用一个银粒子来模拟扫描尖,一个薄的银膜层来模拟样品的衬底。我们模拟了这种模型在p极化的隐失波的照射下,其热点处的场增强。通过变化银粒子的半径,银膜的厚度以及银粒子与银膜之间的距离,我们得出了产生最强场增强的三个参数     

Correction-to-Scaling Exponents for Two-Dimensional Self-Avoiding Walks

We study the correction-to-scaling exponents for the two-dimensional self-avoiding walk, using a combination of series-extrapolation and Monte Carlo methods. We enumerate all self-avoiding walks up to 59 steps on the square lattice, and up to 40 steps on the triangular lattice, measuring the mean-square end-to-end distance, the mean-square radius of gyration and the mean-square distance of a monomer from the endpoints. The complete endpoint distribution is also calculated for self-avoiding walks up to 32 steps (square) and up to 22 steps (triangular). We also generate self-avoiding walks on the square lattice by Monte Carlo, using the pivot algorithm, obtaining the mean-square radii to ≈ 0.01% accuracy up to N=4000. We give compelling evidence that the first non-analytic correction term for two-dimensional self-avoiding walks is Δ₁=3/2. We compute several moments of the endpoint distribution function, finding good agreement with the field-theoretic predictions. Finally, we study a particular invariant ratio that can be shown, by conformal-field-theory arguments, to vanish asymptotically, and we find the cancellation of the leading analytic correction.     

Non-imaging ray-tracing for sputtering simulation with apodization

Although apodization patterns have been adopted for the analysis of sputtering sources, the analytical solutions for the film thickness equations are yet limited to only simple conditions. Empirical formulations for thin film sputtering lacking the flexibility in dealing with multi-substrate conditions, a suitable cost-effective procedure is required to estimate the film thickness distribution. This study reports a cross-discipline simulation program, which is based on discrete particle Monte-Carlo methods and has been successfully applied to a non-imaging design to solve problems associated with sputtering uniformity. Robustness of the present method is first proved by comparing it with a typical analytical solution. Further, this report also investigates the overall all effects cause by the sizes of the deposited substrate, such that the determination of the distance between the target surface and the apodization index can be complete. This verifies the capability of the proposed method for solving the sputtering film thickness problems. The benefit is that an optical thin film engineer can, using the same optical software, design a specific optical component and consider the possible coating qualities with thickness tolerance, during the design stage.     

Tracer studies on f-actin fluctuations

We present a study on the fluctuations of semiflexible actin filaments using fluorescence videomicroscopy, focusing on the end-to-end fluctuations of single filaments. In order to specifically measure the position of the polymer's ends, we developed a novel noninvasive method that consists of annealing short end tags to the filaments. This allows us to probe polymer fluctuations to a very high accuracy. We compared the distribution of the end-to-end distance with recent theoretical results, and found excellent agreement. We also studied the dynamics of the mean-square end-to-end distance deltaR2(t) and orientation of the ends, deltaTheta(2)(t), finding power laws t(3/4) and t(1/4), respectively. Scaling behavior for deltaR2(t) is observed over several decades in relaxation time in agreement with theoretical results.     

Effect of Polymer-Substrate Interactions on the Surface Morphology of Polymer Blend Thin Films: Comparison between Simulation and Experiment

Microphase and macrophase separation phenomena can simultaneously appear in ABA/C copolymer blend systems due to the immiscibility among monomers A, B, and C. In this work, the surface morphologies and compositions of ABA/C blend thin films confined between two walls, which were used to mimic SEBS/PMMA films, have been simulated by a lattice Monte Carlo (MC) method. The effect of the polymer-wall interaction on the surface morphologies and compositions of thin films was investigated as a function of blend composition and film thickness. It is shown that the simulated surface morphologies of thin films resulting from the macrophase separation between copolymer ABA and homopolymer C and the microphase separation between block A and block B in ABA copolymer are similar to the experimental surface morphology of SEBS/PMMA polymer blend films observed by atomic force microscope (AFM). The effect of substrate on the surface morphologies by MC simulation is qualitatively consistent with the experimental results. The composition profiles of thin films are given to characterize the micro- and macrophase separation in thin films. It is indicated that the surface energy of the substrate (substrate/air) plays a crucial role on the surface composition. For a fixed surface, the adsorptions of polymer on the substrate and film thickness are also important.     

Modeling and prediction of transmission laser bonding process between titanium coated glass and PET based on response surface methodology

Laser bonding parameters play a very significant role in determining the quality of laser transmission bonding between PET films and titanium films coated glass sheets. In order to achieve good bond strength and minimal bond width, three key process parameters affecting the bond quality of transmission laser bonding, namely, laser power, bond speed and film thickness were optimized by response surface methodology in this paper. Response surface methodology (RSM) was used to develop mathematical models between the key process parameters and the desired responses and the central composite design (CCD) was utilized to conduct experimental planning. The samples were tested using an electromechanical universal micro-tester in order to determine bond strength. The morphology of the bonded area was observed with an optical microscope. The interaction effect of main process parameters on bond quality was researched. Design Expert analysis indicated that the best laser power, bond speed and film thickness on bond quality were 11.2 W, 4 mm/s and 163 nm, respectively. Finally, the experimental results are consistent with the predicted, which illustrates that the developed mathematical models can predict the responses adequately.     

应力对La0.83Sr0.17MnO3薄膜输运性能的影响

采用溶胶-凝胶方法在Si(111)上制备了LSMO(x=0.17)薄膜.研究了块体材料和不同厚度薄膜R -T曲线、红外光谱和X射线衍射.结果表明，LSMO薄膜属于正交晶体结构，薄膜取向与膜厚度 有关，当膜厚度为450nm或680nm时，主要取向〈200〉，而膜厚度为900nm时取向为〈020〉 ：根据离子对相互作用能和谐振子模型，得到了红外吸收与Mn—O—Mn键长和键角关系式，6 00cm^-1附近红外吸收与晶格常数b的变化有关；块体与薄膜的金属—绝缘体转变 温度(T_MI)存在较大差别，薄膜转变温度显著低于块体，并与厚度有一定关系. 认为是LSMO薄膜中的应力诱导了晶格常数变化，引起键角改变及JT效应是转变温度变化的主 要原因. 关键词： 单晶硅 晶格常数 金属—绝缘体转变温度 应力诱导     

光溅射镀膜均匀性的优化模拟

为了改善脉冲激光溅射沉积大面积薄膜的均匀性,发展了基片离轴旋转的扫描技术.根据基片离轴旋转的基本原理和等离子体羽空间余弦分布规律,建立了径向膜厚分布公式.数值模拟了各种因素对基片离轴旋转扫描沉积薄膜均匀性的影响.分析表明,优化粒子束中心与基片中心偏置距离、溅射点与基片的距离是改善基片离轴旋转扫描镀膜均匀性的主要途径.同时也考虑了电机转速、镀膜时间和激光重频的影响.通过参量优化,当均匀度要求在95％时,计算得到薄膜的最大半径超过40 mm.     

星形两嵌段共聚高分子薄膜微相分离的耗散粒子动力学模拟

利用耗散粒子动力学模拟研究了两类典型的星形两嵌段共聚高分子薄膜的微相分离行为. 结果表明,(A_x)₄(B_y)₄体系的相图较(A_xB_y)₄体系具有明显的对称性,且前者比后者更易发生相分离,与体相中的情况相近,并可归因于两类星形高分子在结构上的差异;组成相同的星形共聚高分子体系在体相与薄膜中所形成的介观结构之间存在对应关系;减小薄膜厚度与加大A-B组分间排斥强度均有利于增强所形成结构的有序性. 同样是缘于分子结构差异,两类星形高分 子薄膜中的均方回旋半径随体系组分分率的变化规律几乎相反.     

A Monte Carlo and continuum study of mechanical properties of nanoparticle based films

A combination Monte Carlo and equivalent-continuum simulation approach was used to investigate the structure-mechanical property relationships of titania nanoparticle deposits. Films of titania composed of nanoparticle aggregates were simulated using a Monte Carlo approach with diffusion-limited aggregation. Each aggregate in the simulation is fractal-like and random in structure. In the film structure, it is assumed that bond strength is a function of distance with two limiting values for the bond strengths: one representing the strong chemical bond between the particles at closest proximity in the aggregate and the other representing the weak van der Waals bond between particles from different aggregates. The Young’s modulus of the film is estimated using an equivalent-continuum modeling approach, and the influences of particle diameter (5–100 nm) and aggregate size (3–400 particles per aggregate) on predicted Young’s modulus are investigated. The Young’s modulus is observed to increase with a decrease in primary particle size and is independent of the size of the aggregates deposited. Decreasing porosity resulted in an increase in Young’s modulus as expected from results reported previously in the literature.     

高斯波束对界面附近离轴球形粒子的轴向声辐射力

从声波的散射理论出发,利用级数展开法得到高斯波束的波束因子,推导其对阻抗边界下离轴球形粒子声辐射力.针对刚性球与液体球两种球形粒子进行数值模拟,与自由空间的情况进行比较.讨论边界反射系数、粒子与边界距离、束腰半径以及离轴角度与距离等对声辐射力的影响.仿真结果表明：边界反射系数的增大会引起声辐射力的增加,但不改变峰值的位置;在合适的频率处,可以产生负向声辐射力;声辐射力随粒子与边界距离呈周期性变化;束腰半径的影响主要体现在中高频;随着粒子偏离传播轴的距离和角度增大,声辐射力明显衰减.该研究为利用高斯波束实现对粒子的操纵提供理论基础.     

Pressure effect on the melting point of alkali metals

The mean-square displacement of alkali metals is studied theoretically using our local Heine-Abarenkov-type model potential in the perturbational scheme. The temperature-dependent mean-square displacement of alkali metals decreases as function of the compressed volume. Lindemann's criterion for melting x_m, which is defined as the ratio of two times the root-mean-square displacement to the nearest-neighbour distance, is found to be nearly constant for five alkali metals. The volume effect on the melting temperature of alkali metals is studied by keeping x_m constant. The obtained melting curve increases as function of the compressed volume and are qualitatively in good agreement with the observed tendency for alkali metals.