MD simulations of the collision between a copper ion and a polyethylene surface: an application to the plasma-insulating material interaction

To allow a better understanding of the physical phenomena occurred between a plasma and an insulating material, we have developed a specific MD code to study this type of interaction. We report results of MD simulations of the interaction of an incoming copper ion with a polyethylene crystal surface. Three initial incoming velocities and four impact angle values are used to check the influence of both the incident energy and impact direction to the resulting surface damage. When the incoming ion velocity is sufficiently high, MD results show that the impact can cause bond breaking leading to uncoordinated carbon atoms and free hydrogen atoms. The values of local temperatures associated with the structural changes show a possible ablation of the polyethylene surface.     

Roles of cyclic units in copolyethylene crystallization: a molecular dynamics simulation

Molecular dynamics (MD) simulations of several polyethylene copolymer chains containing 1,2-, 1,3- or 1,4-disubstituted cyclopentane or hexamethylene structures in the main chain (with 500 CH₂) are performed to investigate the influence of cyclic units on the crystallization properties of polyethylene (PE). From the isothermal relaxation process it is found that they generally collapse to a globule via a local collapse process. The copolymer chains containing 1,2-disubstituted cycloparaffin structures form more kinks and take shorter time to totally collapse into a single globule than the others. Moreover, from the morphology of the crystal structures after annealing it is found that the copolymer chains containing 1,2-disubstituted cycloparaffin structures can yield more ordered structures with cyclic units rejected to the fold surface. For the copolymer chains containing 1,3- or 1,4-disubstituted cycloparaffin, the lamellar structures are not perfect and some cyclic units are always incorporated in the crystalline phase.     

Diffusion of single alkane molecule in carbon nanotube studied by molecular dynamics simulation

Full atomistic molecular dynamics simulations have been used to study the diffusion of alkane molecule in single wall carbon nanotube (SWCNT), with different alkane chain lengths and nanotube diameters. In this paper, we calculated the self-diffusion coefficient, mean-square gyration and bond-orientation order parameter of alkane molecule and the average intermolecular interaction energy per segment between SWCNT and alkane. Furthermore, structure of alkane in SWCNT was characterized through the radial distribution function, with results showing that the self-diffusion coefficient is related to the nanotube diameter. The component of mean-square gyration in z-direction scales with alkane chain length in SWCNT(9,9) like N1.07±0.04, which is in good agreement with the prediction from scaling theory for polymers. The obtained results show that nanotube diameter and alkane chain length are important factors affecting the behavior of one-dimensional confined alkanes.     

Graphitization of small diamond cluster — Molecular dynamics simulation

Molecular dynamics simulation was used to study graphitization process of a small diamond cluster at 1200, 1500, and 1800 K. The cluster was in the shape of a sphere of about 3 nm in diameter, and interaction between carbon atoms was described by the reactive bond order potential. Results obtained for 1500 K showed transformation of diamond nanoparticle into a carbon onion with diamond-like core and graphite layers in its outer shell. At 1800 K the process was faster and graphitization more effective. The whole final cluster was basically comprised of the onion structure, but it was irregular and separation between layers ranged from 0.2 to 0.3 nm.     

Molecular dynamics simulation of diffusion of gases in pure and silica-filled poly(1-trimethylsilyl-1-propyne) [PTMSP]

Nanocomposites have been extensively applied, and molecular dynamics simulation techniques have been applied to study the diffusion of gases (H₂, O₂, N₂, CO₂, CH₄, n-C₄H₁₀) through pure and filled with silica particle poly(1-trimethylsilyl-1-propyne) [PTMSP]. The aim for this research is to explore and investigate the effect of silica particle on the diffusion of gases in polymer. The diffusion coefficients of gases were determined via NVT molecular dynamics simulation using the COMPASS force field up to 500 or 1000 ps simulation time. We have focused on the effect of the concentration and the size of the silica particles on diffusion coefficients of gases and the changes of free volume and translational dynamics and intermolecular energies. It has been found that the addition of silica particle to PTMSP increased the diffusion coefficients of gases by enhancing the free volume of polymer.     

Fluctuation-assisted nucleation in the phase separation/crystallization of iPP/OBC blends

This work mainly focused on the nucleation behavior in iPP/OBC (isotactic polypropylene/polyolefin block copolymers) blends with two distinct OBCs. The influence of composition and molecular structure of the OBC component on the crystallization kinetics of the blends was investigated systematically with the aim to better understand the interplay between the two coupled phase transitions in the blends: macrophase separation and crystallization. The isothermal crystallization kinetics showed component and composition dependence in iPP/OBC blends. All the blends in the studied range have enhanced nucleation ability of iPP than the pure iPP under identical conditions. Furthermore, the distinct macrophase separation morphology resulting from the different compatibility between the various OBCs and iPP caused remarkable diversity between the blends: the nuclei density is qualitatively higher (or the nucleation rate is qualitatively faster) in the more compatible blends, and this enhancement of nucleation can be depressed by imposing a macrophase separation process before crystallization. The crystal nuclei from the phase separated matrix were preferentially formed at the interface of the phase domains, and then grew toward and into the iPP-rich phase. It is postulated that the increased nuclei density and/or nucleation rate followed the fluctuation-assisted nucleation mechanism: the enhanced concentration fluctuation at the interfacial area created by the spinodal decomposition played an important role in the nucleation behavior of iPP/OBC blends. The decreased interface areas with increased domain sizes after deeper phase separation, coupled with a more depressed concentration fluctuation, are responsible for lower nuclei density after long time annealing for phase separation.     

Molecular dynamics simulations of deformation mechanisms of amorphous polyethylene

Molecular dynamics simulations were used to study deformation mechanisms during uniaxial tensile deformation of an amorphous polyethylene polymer. The stress-strain behavior comprised elastic, yield, strain softening and strain hardening regions that were qualitatively in agreement with previous simulations and experimental results. The chain lengths, number of chains, strain rate and temperature dependence of the stress-strain behavior was investigated. The energy contributions from the united atom potential were calculated as a function of strain to help elucidate the inherent deformation mechanisms within the elastic, yield, and strain hardening regions. The results of examining the partitioning of energy show that the elastic and yield regions were mainly dominated by interchain non-bonded interactions whereas strain hardening regions were mainly dominated by intra-chain dihedral motion of polyethylene. Additional results show how internal mechanisms associated with bond length, bond angle, dihedral distributions, change of free volume and chain entanglements evolve with increasing deformation.     

Molecular dynamics simulation of diffusion of O2 and CO2 in amorphous poly(ethylene terephthalate) and related aromatic polyesters

The diffusion of small molecules through polymers is important in many areas of polymer science, such as gas barrier and separation membrane materials, polymeric foams, and in the processing and properties of polymers. Molecular simulation techniques have been applied to study the diffusion of oxygen and dioxide of carbon as small molecule penetrants in models of bulk amorphous poly(ethylene terephthalate) and related aromatic polyesters. A bulk amorphous configuration with periodic boundary conditions is generated into a unit cell whose dimensions are determined for each of the simulated aromatic polyesters in the cell to have the experimental density. The aim for this research is to explore and investigate the diffusion of gases through bulk amorphous poly(ethylene terephthalate) and related aromatic polyesters. The diffusion coefficients for O₂ and CO₂ were determined via NVE molecular dynamics simulations using the Dreiding 2.21 molecular mechanics force field over a range of temperatures (300, 500 and 600 K) using up to 30 ns simulation time. We have focussed on the influence of the temperature, polymer dynamics, number of aromatic rings, ortho-, meta-, para-isomers, density and free volume distribution on the diffusion properties. Correlation of diffusion coefficients with free volume, temperature, number of aromatic rings, ortho-, meta- and para-isomers was found.     

A molecular dynamics simulation study on the crystallization of 22,8-polyurethane

By means of the molecular dynamics (MD) simulation, the crystallization mechanism of 22,8-polyurethane which contains hydrogen-bond units is investigated and the results show that the crystallization process at a fixed temperature can be characterized by three stages: (1) The extended chain collapses to a globular random coil; (2) The random coil reorganizes into an ordered lamellar structure; (3) Accompanied with the segments clustering due to the hydrogen-bond formation, the lamellar develops with local defects. Two kinds of hydrogen-bond, which are formed between NH group and CO group (N-H?OC), and between NH group and urethane alkoxy oxygen (N-H?O), respectively, are found to play an important role in the crystallization process of 22,8-polyurethane. Furthermore, the effect of temperature on the crystallization is also studied by selecting three temperatures 200, 300 and 400 K. The lower the crystal temperature is, the slower the crystallization rate is and the stronger the hydrogen-bonding interactions are presented. This is in harmony with the experimental results.     

Time evolution of dynamic heterogeneity in a polymeric glass: a molecular dynamics simulation study

Dynamic heterogeneity, where it is noticed in molecular dynamics (MD) simulations that, for example, conformational transition rates vary greatly from bond to bond, is characteristic of polymeric glasses. The phenomenon can be attributed to the fact that certain local bond sequences are more capable of conformational rearrangement than others. These local sequences become fixed sites when the overall chain trajectory is frozen-in in the glass. Although this is no doubt the case, because of the relatively short times of MD trajectories and the relatively small numbers of transitions it is important to establish that the heterogeneity does evolve in time in the manner expected from the local site picture and is not an artifact of short simulations or small numbers. This is undertaken here using a polyethylene system that has been much studied previously. Long trajectories are generated where the time evolution of heterogeneity can be studied. It is found that both the standard deviation and the mean value of the transitions over the bonds evolve linearly in time. This is consistent with the local fixed site picture and not with a random process involving relatively small numbers of transitions.     

单晶金属平面微裂纹扩展模拟及其软件开发

以分子动力学为基础,开发了一个用于模拟单晶金属裂纹扩展现象的仿真软件包（CSMD100）。软件包括模型建立、模型计算、结果处理等三个模块。通过仿真可以对实验中很难观察到的现象进行研究并探索其机理。模拟结果揭示了裂纹在扩展过程中的形状变化特征及扩展路径。为预测在一定条件下单晶金属中微裂纹的扩展行为提供了一个有效的可视化的手段。     

The crystallization of low-density polyethylene: a molecular dynamics simulation

Three models (star-shaped, H-shaped, and comb-shaped polyethylenes) are used to study the crystallization behavior of low-density polyethylene at the molecular level by means of molecular dynamics simulation. It is shown that, for the three types of polyethylene corresponding to the models, the neighboring sequences of trans bonds firstly aggregate together to form local ordered domains, and then they coalesce to a lamellar structure. In the process, the branching sites are rejected to the fold surface gradually. The driving force for the relaxation process is the attractive van der Waals interaction between the chain segments. Furthermore, it is found that the number of the branch sites and the length of the branch play an important role in determining the formation of the lamellar structure. The longer the length of the branch and the fewer the number of the branch sites, the more perfect lamellar structure can be formed.     

Morphological features and mechanical behavior of one- and two-phase polymeric materials simulated by molecular dynamics

Single phase amorphous polymeric materials and two-phase polymer liquid crystals (PLCs) have been created on the computer and their behavior simulated using molecular dynamics. An external force was applied on the material and its response computed along time. The influence of several parameters was investigated, such as the concentration of the rigid LC second phase and the existence of regions of different orientation across the thickness of the material.A simplified 3-region model, such as that used to model the skin-core structure resulting from injection molding, was used. The influence of the relative size of each region with different properties was determined. Thicker skin regions increase the rigidity of the material, due to their higher orientation in the direction of force application. The concentration of the reinforcing LC second phase has a similar effect, also resulting in a more brittle behavior. The simulations have provided a better understanding of these phenomena.A method for calculating the true stress during simulation of computer-generated materials (CGMs) is proposed. The true stress behavior was found to differ qualitatively from the engineering stress when the structure of the material allows for considerable changes in cross-sectional area at large-scale deformation.     

Molecular dynamics simulation of diffusion of O2 and CO2 in blends of amorphous poly(ethylene terephthalate) and related polyesters

The diffusion of small molecules through polymers is important in many areas of polymer science, such as gas barrier and separation membrane materials, polymeric foams, and in the processing and properties of polymers. Molecular dynamics simulation techniques have been applied to study the diffusion of oxygen and carbon dioxide as small molecule penetrants in models polyester blends of bulk amorphous poly(ethylene terephthalate) and related aromatic polyesters. A bulk amorphous configuration with periodic boundary conditions was generated into a unit cell whose dimensions were determined for each of the simulated polyester blends in the cell having the experimental density. The diffusion coefficients for O₂ and CO₂ were determined via NVE molecular dynamics simulations using the Dreiding 2.21 molecular mechanics force field over a range of temperatures (300, 500 and 600 K) using up to 40 ns simulation time. We have focussed on the influence of the temperature, polymer dynamics, density and free volume distribution on the diffusion properties. Correlation of diffusion coefficients with free volume distribution was found.     

Molecular simulations of gas transport in nitrile rubber and styrene butadiene rubber

Nitrile rubber (NBR, 39:61 wt% of acrylonitrile:butadiene) and styrene butadiene rubber (SBR, 50:50 wt% of styrene:butadiene) matrices have been equilibrated by molecular dynamics (MD) simulations. Transition-state approach is used to calculate the diffusion and solubility coefficients of small penetrants in these matrices, indicating quite low values in NBR and reasonable agreement with experimental results. MD simulations have been performed to analyze water diffusion in these matrices. Aggregation of water molecules is observed in the hydrophobic matrix SBR. MD simulations with fictitious nonpolar water molecules inhibit aggregation and lead to enhanced diffusion in SBR. In NBR there is a slight increase in diffusion for fictitious water molecules. The lower diffusion constants in NBR result from slower local relaxation of the matrix due to tighter intermolecular packing and higher cohesive energy density. The free volume distribution that affects solubility coefficients is not a major determining factor for the diffusion coefficients in these matrices.     

PRO残油加氢裂化的催化剂研究

对采用A、B、C和D型4种分子筛制备的催化剂载体,进行镍和钴改性,并且使用各催化剂进行PRO残油的加氢裂化反应实验。分析表明,稠环芳烃的总转化率和单环化合物的选择性与催化剂的孔径相关,金属的含量对它们也有影响,而双环化合物的选择性与这些因素关系不明显。     

Motional coherency in chain dynamics of polybutadiene studied by molecular dynamics simulations

Molecular dynamics simulations of 1,4-polybutadiene in bulk amorphous state were performed. Results were compared with the recent neutron spin-echo measurements. To investigate motional coherency the relaxation rates for the collective and self-motions, the collective and self-relaxation rates, were evaluated for the short and long time regimes of the normalized intermediate scattering functions. The scattering vector dependence of the collective relaxation rates estimated for both fast and slow processes indicated a minimum at scattering vector q = 1.5 Å⁻¹, corresponding to the position of a peak in the static structure factor. The self-relaxation rates increased monotonously with q. A phenomenon known as de Gennes narrowing was reproduced well in the simulation and found to be originated from the inter-molecular correlation. The collective relaxation rate evaluated for fast process appeared to modulate around a peak of q = 2.9 Å⁻¹, corresponding to the intra-molecular correlation.