Coarse-grained molecular dynamics simulations of ionic polymer networks

The stress-strain behavior of cross-linked polymeric networks was investigated using molecular dynamics simulations with a coarse-grained representation of the repeating units. The network structure was formed by dynamically cross-linking the reactants placed between two rigid layers comprised of particles of the same type. We studied two types of networks which differ only by one containing ionic pairs that amount to 7% of the total number of bonds present. The stress-strain curves were obtained after imposing deformation in tensile and shear modes to the networks and measuring their stress response. Under both forms of deformations there was improvement in the level of stress that the material could bear. Moreover, the time dependent behavior of the improvement in mechanical properties signified a self-healing mechanism.     

Coarse-grained molecular dynamics simulations on size effect of glassy polyethylene particles

The size effect on mechanical properties of glassy polyethylene (PE) nanoscale particles has been investigated by extensive coarse-grained molecular dynamics simulations. The diameter of the PE particles varies in the range of 5-40 nm, we confirm that the particle's behaviour under compressive stress strongly depends on its size-the smaller the particle diameter is, the stiffer the particle behaves. The present mechanical responses of compressed particles are in good agreement with our previous experimental phenomena of micron-sized polymer particles measured by a nanoindentation-based flat punch method. Possible reasons for the size effect are discussed.     

Lattice dynamics and molecular dynamics simulation of complex materials

In this article we briefly review the lattice dynamics and molecular dynamics simulation techniques, as used for complex ionic and molecular solids, and demonstrate a number of applications through examples of our work. These computational studies, along with experiments, have provided microscopic insight into the structure and dynamics, phase transitions and thermodynamical properties of a variety of materials including fullerene, high temperature superconducting oxides and geological minerals as a function of pressure and temperature. The computational techniques also allow the study of the structures and dynamics associated with disorder, defects, surfaces, interfaces etc.     

Load transfer of graphene/carbon nanotube/polyethylene hybrid nanocomposite by molecular dynamics simulation

Load transfer of the graphene/carbon nanotube (CNT)/polyethylene hybrid nanocomposite is studied here from molecular dynamics (MD) simulations. Simulations of this composite material under uniaxial tension were conducted by varying CNT’s position and diameter in the polymer matrix. The obtained results show that: (1) The peak strength of stress and strain evolution in the polymer matrix is lower than the peak strength of the graphene/graphene and graphene/polymer interfaces. Hence, the damage zone is always located in the polymer matrix. (2) Agglomerated two-layer graphenes do not possess an increased value in the peak strength compared with single-layer graphene-reinforced polymer nanocomposite (PNC), while two separate layers of graphene show slightly higher peak strength. (3) The largest peak strength is observed before CNT moves to the center of the polymer matrix. The damage location moves from the upper to the lower part of CNT when the CNT is located at the centre of polymer matrix. (4) The influence of the CNT diameter on the peak strength is not obvious, while the damage location and shape in the polymer matrix changes with respect to varying CNT diameters. In addition, the damage zone always falls outside the interphase zone.     

Monte carlo simulation for molecular gas dynamics

The dynamics of low-density flows is governed by the Boltzmann equation of the kinetic theory of gases. This is a nonlinear integro-differential equation and, in general, numerical methods must be used to obtain its solution. The present paper, after a brief review of Direct Simulation Monte Carlo (DSMC) methods due to Bird, and Belotserkovskii and Yanitskii, studies the details of theDSMC method of Deshpande for mono as well as multicomponent gases. The present method is a statistical particle-in-cell method and is based upon the Kac-Prigogine master equation which reduces to the Boltzmann equation under the hypothesis of molecular chaos. The proposed Markoff model simulating the collisions uses a Poisson distribution for the number of collisions allowed in cells into which the physical space is divided. The model is then extended to a binary mixture of gases and it is shown that it is necessary to perform the collisions in a certain sequence to obtain unbiased simulation. A part of the material in this paper was presented at the Minisymposium 2 of the ICIAM 87 held in Paris during June 29-July 3, 1987.     

Dual-resolution molecular dynamics simulation of antimicrobials in biomembranes

Triclocarban and triclosan, two potent antibacterial molecules present in many consumer products, have been subject to growing debate on a number of issues, particularly in relation to their possible role in causing microbial resistance. In this computational study, we present molecular-level insights into the interaction between these antimicrobial agents and hydrated phospholipid bilayers (taken as a simple model for the cell membrane). Simulations are conducted by a novel ‘dual-resolution’ molecular dynamics approach which combines accuracy with efficiency: the antimicrobials, modelled atomistically, are mixed with simplified (coarse-grain) models of lipids and water. A first set of calculations is run to study the antimicrobials'' transfer free energies and orientations as a function of depth inside the membrane. Both molecules are predicted to preferentially accumulate in the lipid headgroup–glycerol region; this finding, which reproduces corresponding experimental data, is also discussed in terms of a general relation between solute partitioning and the intramembrane distribution of pressure. A second set of runs involves membranes incorporated with different molar concentrations of antimicrobial molecules (up to one antimicrobial per two lipids). We study the effects induced on fundamental membrane properties, such as the electron density, lateral pressure and electrical potential profiles. In particular, the analysis of the spontaneous curvature indicates that increasing antimicrobial concentrations promote a ‘destabilizing’ tendency towards non-bilayer phases, as observed experimentally. The antimicrobials'' influence on the self-assembly process is also investigated. The significance of our results in the context of current theories of antimicrobial action is discussed.     

Mechanical properties of carbon nanotube by molecular dynamics simulation

The mechanical properties of single-walled carbon nanotube (SWCNT) are computed and simulated by using molecular dynamics (MD) in this paper. From the MD simulation for an armchair SWCNT whose diameter is 1.2 nm and length is 4.7 nm, we get that its Young modulus is 3.62 TPa, and tensile strength is 9.6 GPa. It is shown that the Young modulus and tensile strength of armchair SWCNTs are 12 order higher than those of ordinary metal materials. Therefore we can draw a conclusion that carbon nanotubes (CNT) belong to a particular material with excellent mechanical properties.     

Characterizing mechanical properties of graphite using molecular dynamics simulation

The mechanical properties of graphite in the forms of single graphene layer and graphite flakes (containing several graphene layers) were investigated using molecular dynamics (MD) simulation. The in-plane properties, Young’s modulus, Poisson’s ratio, and shear modulus, were measured, respectively, by applying axial tensile stress and in-plane shear stress on the simulation box through the modified NPT ensemble. In order to validate the results, the conventional NVT ensemble with the applied uniform strain filed in the simulation box was adopted in the MD simulation. Results indicated that the modified NPT ensemble is capable of characterizing the material properties of atomistic structures with accuracy. In addition, it was found the graphene layers exhibit higher moduli than the graphite flakes; thus, it was suggested that the graphite flakes have to be expanded and exfoliated into numbers of single graphene layers in order to provide better reinforcement effect in nanocomposites.     

填料对聚合物基复合材料摩擦学行为的影响

本文综述了用于聚合物填充增强改性的填料种类、对摩擦学性能的影响以及填料的作用机理 ,同时讨论了填料的表面改性以及填料的选择标准。提出了今后研究填料对聚合物改性研究应注意的问题 ,从而对研究、开发、应用性能优异的聚合物基复合材料具有指导意义     

Classical molecular dynamics simulation of uranium monocarbide (UC)

Inter-atomic potential parameters of UC, for a partially ionic potential model, were found out by fitting method using experimental lattice parameter data in an improvised way. The inter-atomic potential used here is essentially a combined potential model that has earlier been used to simulate UO₂ system. The calculated lattice parameters, bulk modulus and coefficient of thermal expansion of UC, using the current potential parameters, are in good agreement with the experimental values. The model also shows that there is no Bredig transition in UC system as contrary to UO₂, where the same potential model was used.     

Preparation and photochromic behavior of crosslinked polymer thin films containing polyoxometalates

A series of reversible photochromic nanocomposite films were prepared by entrapping phosphotungstic acid (PWA) and molybdenumphsophoric acid (PMoA) into P(VP-BVA), which was a crosslinked polymer based on N-vinylpyrrolidone (VP) and bisvinyl-A (BVA). The microstructure, photochromic behavior and mechanism of the films were studied with transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible spectra (UV-vis) and electron resonance spectra (ESR). The TEM image showed that the polyoxometalates particles had regular microstructure with narrow size distribution (average diameter of 30 nm) in hybrid films. FT-IR results showed that the Keggin geometry of polyoxometalates (POM) was still preserved inside the composites and strong coulombic interaction between POM and crosslinked polymer matrix was built. Irradiated with ultraviolet light, the transparent films changed from colorless to blue and showed reversible photochromism. Oxygen plays an important role during the bleaching process. PMoA/P(VP-BVA) film had higher photochromic efficiency and slower bleaching reaction than PWA/P(VP-BVA) film. The characteristic signals of W (V) or Mo (V) in ESR spectra indicated that electron transfer occurred between the organic substrates and heteropolyanions under UV irradiation, which induced heteropolyanions to heteropolybules with simultaneous oxidation of the organic substrates.     

Atomic scale structure and dynamics characteristics simulation of amorphous SrTiO3 by molecular dynamics

The atomic scale structure and dynamics characteristics of amorphous SrTiO₃ was simulated by molecular dynamics with potential function including Coulomb interaction, short range repulsion potential, Van der Waals interaction and Morse potential. From the energy and volume’s dramatic increase during heating, the melting point was estimated to be about 2440 K, in good agreement with the experimental value. The amorphous SrTiO₃ was obtained by quenching the liquid to room temperature. The correlation function and coordination numbers of the crystalline, liquid, and amorphous states were analyzed. The diffusion coefficients at various temperatures calculated from the auto correlation function of velocity verified that the melting occurred at 2440 K.     

Ab initio molecular dynamics simulation of pressure-induced phase transformation in BeO

Ab initio molecular dynamics (MD) method has been used to study high pressure-induced phase transformation in BeO based on the local density approximation (LDA) and the generalized gradient approximation (GGA). Both methods show that the wurtzite (WZ) and zinc blende (ZB) BeO transforms to the rocksalt (RS) structure smoothly at high pressure. The transition pressures obtained from the LDA method are about 40 GPa larger than the GGA result for both WZ → RS and ZB → RS phase transformations, and the phase transformation mechanisms revealed by the LDA and GGA methods are different. For WZ → RS phase transformations both mechanisms obtained from the LDA and GGA methods are not comparable to the previous ab initio MD simulations of WZ BeO at 700 GPa based on the GGA method. It is suggested that the phase transformation mechanisms of BeO revealed by the ab initio MD simulations are affected remarkably by the exchange–correlation functional employed and the way of applying pressure.     

SAXS/WAXS/DSC study of temperature evolution in nanopolymer electrolyte

Soft matter polymer electrolytes as nanostructured materials are very attractive components for batteries and for opto-electronic devices as a new generation of dye-sensitized solar cells. (PEO)₈ZnCl₂ polymer electrolytes were prepared from PEO and ZnCl₂. The nanocomposites (PEO)₈ZnCl₂/TiO₂ themselves contained TiO₂ nanograins. In this work, the influence of TiO₂ nanograins or the morphology and ionic conductivity of the nanocomposite was systematically studied by small-angle X-ray scattering (SAXS) simultaneously recorded with wide-angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC) at the synchrotron ELETTRA. Shown by previous impedance spectroscopy measurements (IS), the room temperature conductivity of nanocomposite polymer electrolyte increased more than two times above 65 °C, relative to pure composites of PEO and salts. The SAXS/DSC measurements yielded insight into the temperature-dependent changes of the grains of the electrolyte as well as into the effects of heating and cooling rates. The crystal structure and temperatures of melting and crystallization of the nanosize grains was revealed by the simultaneous WAXS measurements.     

Application of level set method in simulation of surface roughness in nanotechnologies

One of the limiting factors in applications of plasma etching in nanotechnologies in general will be the control of plasma induced roughness or perhaps control of surface roughness by plasma etching. In this paper we consider roughening of nanocomposite materials during plasma etching for two etching modes (isotropic and anisotropic) by using a level set method. It was found that the presence of two phases with different etch rates (the ratio of the two etch rates is s and the abundance of one phase is p) affects the evolution of the surface roughness and that the etch rate is higher during the isotropic process as compared to the anisotropic process for all values of s and p. At the same time, in case of isotropic process, the higher s leads to a higher overall etch rate. The obtained results apart from their theoretical relevance, have practical implications for surface treatment of nanocomposite materials.     

A simulation study on the effect of spring-shaped fillers on the viscoelasticity of rubber nanocomposite

Background/PurposeRubber nanocomposites have been widely used in many engineering fields due to their unique properties such as high elasticity and viscoelasticity. Much attention has been paid to the viscoelasticity of rubbers because it directly relates to the performance of the rubber products.MethodsBased on the micromechanical theory, the finite element method is used to analyze the effect of elastic modulus and volume content of spring-shape nanofillers on the dynamic viscosity of composites.ResultsThe simulation results show that there is an optimal elastic modulus of spring-shape nanofillers to make the loss factor a minimum. There is a threshold value of spring-shape nanofiller content for the dissipation energy density of composite.ConclusionThe elastic modulus of spring-shape nanofillers has a large effect on the loss factor of composites. The selection of elastic modulus of spring-shape nanofillers is critical for applications of composites. The efficiency of spring-shape nanofillers in reducing the dynamic viscosity of composites is so high that volume content of spring-shape nanofillers as low as 0.1% can greatly reduce the loss factor of composites with bonding interface.     

A review of recent research on mechanics of multifunctional composite materials and structures

In response to the marked increase in research activity and publications in multifunctional materials and structures in the last few years, this article is an attempt to identify the topics that are most relevant to multifunctional composite materials and structures and review representative journal publications that are related to those topics. Articles covering developments in both multiple structural functions and integrated structural and non-structural functions since 2000 are emphasized. Structural functions include mechanical properties like strength, stiffness, fracture toughness, and damping, while non-structural functions include electrical and/or thermal conductivity, sensing and actuation, energy harvesting/storage, self-healing capability, electromagnetic interference (EMI) shielding, recyclability and biodegradability. Many of these recent developments are associated with polymeric composite materials and corresponding advances in nanomaterials and nanostructures, as are many of the articles reviewed. The article concludes with a discussion of recent applications of multifunctional materials and structures, such as morphing aircraft wings, structurally integrated electronic components, biomedical nanoparticles for dispensing drugs and diagnostics, and optically transparent impact absorbing structures. Several suggestions regarding future research needs are also presented.     

A molecular dynamics simulation on surface tension of liquid Ni and Cu

Molecular dynamics simulations of liquid transition metals Ni and Cu have been performed with the tight-binding potential model. The surface tensions of the liquid metals at different temperatures are evaluated using both methods of calculating the work of cohesion and of using the mechanical expression for the surface stress. The calculated surface tension data are compared with available experimental values. The simulated results for Ni are in good agreement with experiment, but those for Cu show about 10–20% underestimation. Comparing with the mechanical method, the data of surface tension calculated using the method of cohesive work show remarkable dependence on temperature, and the estimated temperature coefficients of liquid Ni and Cu are consistent with the experimental data.     

Ar-Kr低温界面传热过程分子动力学模拟

运用非平衡分子动力学原理和LJ势函数仿真研究氩(Ar)与氪(Kr)之间的界面层传热问题,模拟其界面传热的能量变化过程.仿真结果表明,即使界面粗糙度为0,低温固体界面热阻仍然存在.平均温度为40 K,粗糙度为0时,氩(Ar)与氪(Kr)之间界面热阻为0.15～0.18 Wm2/K,相对误差小于17%.