聚酰亚胺/功能化石墨烯复合材料力学性能及玻璃化转变温度的分子动力学模拟

应用分子模拟方法,建立了聚酰亚胺(polyimide,PI),石墨烯及羧基、氨基、羟基功能化石墨烯模型,探究了聚酰亚胺和石墨烯,聚酰亚胺和功能化石墨烯共混后复合材料的力学性能和玻璃化转变温度(T_g).研究结果表明,羧基修饰的石墨烯与PI复合后材料力学性能增加显著,其杨氏模量和剪切模量分别为4.946 GPa和1.816 GPa.不同官能团修饰的石墨烯引入PI后材料的T_g均有不同程度下降;未修饰的石墨烯与PI复合后,其T_g(559.30 K)较纯PI的T_g(663.57 K)降幅最大;而羧基修饰的石墨烯与PI复合后T_g(601.61 K)降幅最小.计算比较了PI/石墨烯复合材料体系密度、溶解度参数、相互作用能、弹性系数和氢键平均密度,研究发现羧基修饰石墨烯/PI复合材料的密度为1.396 g·cm~(-3),溶解度参数为23.51 J~(1/2)·cm~(-3/2),其相互作用能与氢键平均密度最大,弹性系数显示羧基修饰石墨烯与PI组成的复合材料内部最均匀.计算结果表明,羧基功能化石墨烯可以大幅度提高PI的力学性能,增强石墨烯与PI之间的相互作用可以减少复合材料T_g的降幅程度.此基体间相互作用的研究方法可以作为预测聚合物基纳米复合材料结构与性能的有效工具,以期为材料的设计与应用提供理论指导.     

高压作用下相分离液体玻璃转变的分子动力学研究

采用分子动力学模拟方法,研究了二元混合液体在不同外压作用下的相分离与玻璃转变过程,计算了相分离液体在玻璃转变过程中的结构和动力学特征.研究发现,外压会促进相分离的产生,并提高玻璃转变温度,会使β弛豫出现的温度更高、存在的时间更长,导致系统扩散性降低.同时还发现,相分离液体的玻璃转变过程存在微观不均匀现象. 关键词： 相分离 玻璃转变 分子动力学模拟 外压影响     

Ca70Mg30金属玻璃形成过程热力学、 动力学和结构特性转变机理的模拟研究

采用分子动力学方法对Ca₇₀Mg₃₀合金快速凝固玻璃形成过程进行了计算机模拟, 深入分析了液-固玻璃转变过程热力学、 动力学和结构特性的转变机理, 对不同方法所确立的玻璃转变温度之间的关系进行了探讨. 结果表明: 本模拟计算所获得的Ca₇₀Mg₃₀金属玻璃的结构因子和玻璃转变温度均与实验结果符合, 而且二十面体局域结构对Ca₇₀Mg₃₀金属玻璃的形成起决定性作用. 由于周围原子形成的瞬时"笼子效应", 过冷液体动力学特性逐渐偏离Arrhenius规律而满足模态耦合理论的幂指数规律. 动力学玻璃转变温度接近于微观结构玻璃转变温度, 但高于热力学玻璃转变温度; 而且它们与理想动力学玻璃转变温度之间满足Odagaki关系.     

Atomistic simulation of fcc-bcc phase transition in single crystal Al under uniform compression

By molecular dynamics simulations employing an embedded atom model potential,we investigate the fcc-to-bcc phase transition in single crystal Al,caused by uniform compression.Results show that the fcc structure is unstable when the pressure is over 250 GPa,in reasonable agreement with the calculated value through the density functional theory.The morphology evolution of the structural transition and the corresponding transition mechanism are analysed in detail.The bcc (011) planes are transited from the fcc (11) plane and the (11) plane.We suggest that the transition mechanism consists mainly of compression,shear,slid and rotation of the lattice.In addition,our radial distribution function analysis explicitly indicates the phase transition of Al from fcc phase to bcc structure.     

Atomistic simulation of fccben bcc phase transition in single crystal Al under uniform compression

By molecular dynamics simulations employing an embedded atom model potential, we investigate the fcc-to-bcc phase transition in single crystal Al, caused by uniform compression. Results show that the fcc structure is unstable when the pressure is over 250 GPa, in reasonable agreement with the calculated value through the density functional theory. The morphology evolution of the structural transition and the corresponding transition mechanism are analysed in detail. The bcc (011) planes are transited from the fcc (111) plane and the (111) plane. We suggest that the transition mechanism consists mainly of compression, shear, slid and rotation of the lattice. In addition, our radial distribution function analysis explicitly indicates the phase transition of Al from fcc phase to bcc structure.     

非晶态合金表面的水润湿动力学

采用分子动力学模拟方法研究了改进的Simple pointcharge模型SPC/E水滴在Cu_(50)Zr_(50)非晶薄膜上的润湿行为和铺展过程.通过与CuZr(110)和(100)晶面对比研究发现,水滴在Cu_(50)Zr_(50)非晶薄膜表面上表现出较高的铺展速度.水滴在非晶合金表面的铺展过程中形成了明显的吸附层;而在晶态表面,水滴铺展前沿呈脚状形态.分析结果表明非晶表面的水分子在吸附层内呈现完全无序的单层排列方式,而在晶态表面,特别是(100)晶面,吸附层水分子呈双层有序排列.这种吸附层结构的差异导致了吸附层内水分子方向的差异:非晶表面吸附层内水分子方向倾向平行于表面,而晶态基底上吸附层内的水分子倾向于指向液滴内部.由此造成了非晶表面吸附层中的水分子与液滴内部以相对较弱的氢键相互作用,这使得上层水分子能够较容易扩散至吸附层前沿,促进液滴铺展.     

Molecular simulation study of the glass transition in a soft primitive model for ionic liquids

In this paper, we present a molecular dynamics study of the glass transition for a soft-core primitive model for ionic liquids, in which cations are fully flexible chains of tangent soft spherical monomers, being the positively charged monomer at one of the ends of the chain, and anions as charged soft spheres. We have monitored transport coefficients such as the self-diffusion coefficients and the shear viscosity, as well as correlation functions such as the mean-square displacement, the self-intermediate scattering function, and probes of heterogeneous dynamics such as the van Hove distribution function and the four-points susceptibility. The analysis of these properties indicates that, for a given pressure, the glass transition shows a weak temperature dependence on the cation length, occurring first for short-chain than for long-chain ionic liquids.     

非晶材料玻璃转变过程中记忆效应的热力学

低温下处于非平衡态的非晶材料升温到玻璃转变以上,要先后发生弛豫和回复最终达到平衡过冷液态,其中弛豫过程中释放的能量在回复过程中以等量的方式获取,表现出明显记忆行为.本文基于氧化物、金属与小分子等多种非晶形成体系,全面探讨了在围绕玻璃转变的一个冷却加热循环过程中的焓弛豫特征,建立了弛豫谱,发现弛豫焓在数值上与熔化焓密切相关.基于弛豫焓与非晶材料动力学Fragility之间的关联,展示了非晶体系在动力学极限(m=175)条件下的玻璃转变热力学基本特征,与热力学二级相变进行了对比.研究深化了对非晶弛豫与玻璃转变热力学的理解.     

Atomistic investigation of phase transition in solid argon induced by shock wave transmission

A molecular dynamics (MD) simulation is employed to study the phase transition process in argon induced by shock wave transmission. Deriving the relation between the shock and piston velocities, the theoretical equation of state for argon is presented. Also, argon equation of state is obtained by measuring the quantities directly from simulations to be able to detect the phase transitions. The phase transition is also detected by using argon phase diagram and free energy calculations. A comparison shows good agreement between the theoretical and MD results for the phase transitions. Based on these simulations, it is concluded that under a shock wave transmission with suitable energy, the solid argon experiences a phase transition from solid to liquid and another from liquid to supercritical fluid. By reflecting the shock wave back at the end of its passage, the whole argon may reach the supercritical state.     

Orientation dependence of structural transition in fcc Al driven under uniaxial compression by atomistic simulations

By molecular dynamics simulations employing an embedded atom method potential,we have investigated structural transformations in single crystal Al caused by uniaxial strain loading along the [001],[011] and [111] directions. We find that the structural transition is strongly dependent on the crystal orientations. The entire structure phase transition only occurs when loading along the [001] direction,and the increased amplitude of temperature for [001] loading is evidently lower than that for other orientations. The morphology evolutions of the structural transition for [011] and [111] loadings are analysed in detail. The results indicate that only 20% of atoms transit to the hcp phase for [011] and [111] loadings,and the appearance of the hcp phase is due to the partial dislocation moving forward on {111} fcc family. For [011] loading,the hcp phase grows to form laminar morphology in four planes,which belong to the {111} fcc family; while for [111] loading,the hcp phase grows into a laminar structure in three planes,which belong to the {111} fcc family except for the (111) plane. In addition,the phase transition is evaluated by using the radial distribution functions.     

dsDNA与TiO2 (110)在溶液中相互作用的动力学行为和机制

金红石型TiO2 (110)在DNA传感领域具有广阔的应用前景,然而其与DNA在溶液中相互作用的动力学行为和机制尚不清楚。利用分子动力学模拟,深入研究了双链DNA (dsDNA)在初始时沿轴向平行或垂直于金红石型TiO2 (110)表面时二者在溶液中的相互作用。通过分析dsDNA在吸附后的稳定构型、吸附动力学过程和结构稳定性发现,与一些典型二维纳米材料不同,dsDNA倾向于水平吸附到TiO2 (110)表面。水平吸附不仅使得dsDNA的四种碱基均吸附到TiO2 (110)表面,增加了吸附稳定性,而且不破坏dsDNA的结构稳定性。进而,通过分析dsDNA与TiO2 (110)表面的相互作用能和水分子密度/数量发现,dsDNA的平行吸附可能源于二者之间的短程范德华和长程静电相互作用。此外,纳米级脱湿也增强了dsDNA的吸附。对dsDNA与TiO2 (110)在溶液中的相互作用研究有助于实现TiO2 (110)在DNA传感中的应用。     

Theoretical approaches to the glass transition in simple liquids

Theoretical approaches to the development of an understanding of the behaviour of simple supercooled liquids near the structural glass transition are reviewed and our work on this problem, based on the density functional theory of freezing and replicated liquid state theory, are summarized in this context. A few directions for further work on this problem are suggested.     

非晶合金结构演变影响玻璃形成能力的分子动力学研究

通过分子动力学方法模拟了Ti75Al25、Ni50Zr50和Cu50Zr50非晶合金的玻璃转变过程, 得到并分析了平均原子体积和双体分布函数等结构参数, 并应用Voronoi多面体指数分析法统计了玻璃转变过程中二十面体及类二十面体团簇的数量, 通过分析团簇在玻璃转变过程中种类和数量的涨落趋势, 研究了非晶原子由短程序连接至中程序再至长程无序的动力学演化过程. 结果表明, 非晶合金的玻璃形成能力以及塑性变形能力与动力学演化过程中Voronoi团簇的种类和数量密切相关. 在玻璃转变过程中局部五次对称性高的团簇倾向于连接在一起形成链,从而密排整个空间, 降低系统的动力学行为从而提高玻璃形成能力. 塑性形变倾向于发生在局部五次对称性较低的区域. 在玻璃转变温度附近团簇种类和数量的突变反映出非晶合金的自组织临界行为, 蕴含着丰富的非线性动力学现象.     

The phase behaviour of single polyethylene chains with and without fixing one end

The phase behaviour of a single polyethylene chain is studied by using molecular dynamics simulations. A free chain and a chain with fixing one end are considered here, since the atomic force microscope （AFM） tip can play a significant role in polymer crystallization in experiment. For a free chain, it is confirmed in our calculation that the polymer chain exhibits an extended coil state at high temperatures, collapses into a condensed state at low temperatures, i.e. the coil-to-globule transition that is determined by a high temperature shoulder of the heat capacity curve, and an additional liquid-to-solid transition that is described by a low temperature peak of the same heat curve. These results accord with previous studies of square-well chains and Lennard-Jones homopolymers. However, when one of the end monomers of the same chain is fixed the results become very different, and the chain cannot reach an extended coil-like state as a free chain does at high temperatures, i.e. there exists no coil-to-globule-like transition. These results may provide some insights into the influence of AFM tip when it is used to study the phase behaviour of polymer chains. If the interaction force between AFM tip and polymer monomers is strong, some monomers or one of them can be seen as being fixed by the tip, which is similar to our simulation model, and it is also found that AFM tip could induce polymer crystallization.     

Thermal diffusion near glass transition in Ge-Se glasses measured by photoacoustics

The glass transition in Ge _x Se_1−x ) (0·1 ⩽x ⩽ 0·25) glasses has been investigated using the photoacoustic (PA) technique. It is found that the PA amplitude and phase undergo anomalous changes at the glass transition temperatureT _g. The amplitude has critical minimum and phase has maximum values atT _g. The variation of the thermal diffusivity, determined by measuring the frequency dependence of the PA amplitude and phase, with temperature shows sharp decrease near the glass transition temperature. The temperature dependence of the optical energy gap also has been measured and it shows a decrease with temperature for all compositions, the rate of decrease being higher for temperatures greater thanT _g.     

关于单元体系Cu的玻璃转变过程中动力学不均匀性的研究

基于分子动力学模拟技术,采用非高斯参数α_2(t)来描述单元体系Cu的玻璃转变过程中动力学不均匀性的变化.研究表明在β弛豫阶段,非高斯参数α_2(t)随时间的变化满足幂律函数,而非高斯参数峰值α_(max)和β弛豫时间τ_β与温度的关系均符合Arrhenius函数,且温度越接近玻璃转变温度T_g,体系的动力学不均匀性越明显.     

Electron-driven processes for furan,tetrahydrofuran and 2, 5-dimethylfuran

We present the study of electron-induced processes for biomolecules, furan (C₄H₄O) and tetrahydrofuran (C₄H₈O) as well as an industrial relevant molecule 2, 5-dimethylfuran (C₆H₈O) through various total cross sections for impact energies from threshold to 5 keV. We have employed spherical complex optical potential to calculate total cross sections Q_T and total elastic cross sections Q_el. Since these molecules are complex molecules, we have used the group additivity rule which incorporates molecular properties as well as geometry of the target to evaluate these cross sections. Dependence of Q_T and Q_el on number of electrons and correlation of dipole polarisability (α) and ionisation potential (I) with peak of total ionisation cross sections Q_ion is studied and important inferences are noted. The present results for these applied molecules are compared with previous data wherever available. This is the maiden attempt to report Q_el and Q_T for 2, 5-dimethylfuran.     

聚丙烯酸酯玻璃化转移温度QSPR的研究

对聚丙烯酸酯的定量构性关系(QSPR)研究具有重要意义。采用分子电性作用失量(MEIV)表征聚丙烯酸酯的分子结构,运用多元线性回归(MLR)建立定量结构玻璃化转移温度相关(QSPR)模型,同时采用逐步回归结合统计检测筛选模型变量,建立了22个聚丙烯酸酯玻璃化转移温度（Tg）与其结构间的多元线性回归方程。另外采用内部及外部双重验证的办法深入分析和检验模型的稳定性。建模的复相关系数(Rcum)、留一法(LOO)交互校验复相关系数(Qcum)和外部样本校验复相关系数(Qext)分别为0.982、0.971和0.922。表明用MEIV对聚丙烯酸酯分子结构信息表达较好,所建QSPR模型的稳定性和预测能力良好。     

Simulations of the flipping images and microparameters of molecular orientations in liquids according to molecule string model

The relaxation dynamics of liquids is one of the fundamental problems in liquid physics, and it is also one of the key issues to understand the glass transition mechanism. It will undoubtedly give enlightenments on understanding and calculating the relaxation dynamics if the molecular orientation flipping images and relevant microparameters of liquids are studied. In this paper, we first give five microparameters to describe the individual molecular string (MS) relaxation based on the dynamical Hamiltonian of the MS model, and then simulate the images of individual MS ensemble, at the same time calculate the parameters of the equilibrium state. The results show that the main molecular orientation flipping image in liquids (including supercooled liquid) is similar to the random walk. In addition, two pairs of the parameters are equal, and one can be ignored compared with the other. This conclusion will effectively reduce the difficulties in calculating the individual MS relaxation based on the single-molecule orientation flipping rate of general Glauber type, and the computer simulation time of interaction MS relaxation. Moreover, the conclusion has no doubt of the reference significance for solving and simulating the multi-state MS model.     

环氧树脂高温分子链松弛与玻璃化转变特性

环氧树脂是电力设备中广泛应用的一种绝缘材料, 其介电性能受到分子链运动特性的影响. 本文制备了直径为50 mm、厚度为1 mm的环氧树脂试样, 采用差示扫描量热仪和宽频介电谱仪测试了环氧树脂的玻璃化转变温度和介电特性. 实验结果表明, 环氧树脂的玻璃化转变温度为105 ℃, 在玻璃化转变温度以上, 高频段出现了由分子链段运动造成的松弛过程, 低频段出现了由载流子在材料中迁移造成的直流电导过程. 发现环氧树脂不同尺寸分子链段的松弛时间不同, 其松弛时间分布较宽, 计算得到了分子链段在不同温度下的松弛时间分布特性. 分子链松弛峰频率和直流电导随温度的变化关系服从Vogel-Tammann-Fulcher公式. 拟合实验结果得到分子链松弛峰频率和直流电导的Vogel温度和强度系数. 由Vogel温度计算得到了与差示扫描量热测试结果一致的玻璃化转变温度, 约为102 ℃. 结果表明玻璃化转变温度以上环氧树脂的自由体积增大, 分子链段有足够的空间来响应外电场从而产生分子链松弛极化, 载流子有足够的能量在材料中迁移形成电导.