MOLECULAR DYNAMICS SIMULATION ON VIERATIONAL SPECTROSCOPIC FEATURES OF HYDROGEN BONDS IN CRYSTALLINE POLYMERS

Introduction The molecular dynamics simulation technique has recently proved to be a suitable alternative approachfor simulation of vibrational spectroscopy. In this study, molecular dynamics was utilized to understandlow frequency vibrations in highly ordered poly(ρ-phenylene terephthalmide) (PPTA). A key structuralfeature of this polymer is the presence of hydrogen bonds. There is little question that this strong localized

Nano-scale Interfacial Friction Behavior between Two Kinds of Materials in MEMS Based on Molecular Dynamics Simulations

The aim of this article was to provide a systematic method to perform molecular dynamics simulotion or evaluation for nano-scale interfacial friction behavior between two kinds of materials in MEMS design. Friction is an important factor affecting the performance and reliability of MEMS. The model of the nano-scale interracial friction behavior between two kinds of materials was presented based on the Newton＇ s equations of motion. The Morse potential function was selected for the model. The improved Verlet algorithm was employed to resolve the model, the atom trajectories and the law of the interfacial friction behavior. Comparisons with experimental data in other paper confirm the validity of the model. Using the model it is possible to simulate or evaluate the importance of different factors for designing of the nano-scale interfacial friction behavior between two kinds of materials in MEMS.

Molecular dynamics simulations of mechanical properties of epoxy-amine:Cross-linker type and degree of conversion effects

Molecular dynamics(MD)simulations are conducted to study the thermo-mechanical properties of a family of thermosetting epoxy-amines.The crosslinked epoxy resin EPON862 with a series of cross-linkers is built and simulated under the polymer consistent force field(PCFF).Three types of curing agents(rigidity1,3-phenylenediamine(1,3-P),4,4-diaminodiphenylmethane(DDM),and phenol-formaldehyde-ethylenediamine(PFE))with different numbers of active sites are selected in the simulations.We focus on the effects of the cross-linkers on thermo-mechanical properties such as density,glass transition temperature(T_(g)),elastic constants,and strength.Our simulations show a significant increase in the Tg,Young’s modulus and yield stress with the increase in the degree of conversion.The simulation results reveal that the mechanical properties of thermosetting polymers are strongly dependent on the molecular structures of the cross-linker and network topological properties,such as end-to-end distance,crosslinking density and degree of conversion.

Towards larger spatiotemporal scales in polymer simulations

Molecular dynamics simulations are useful tools to unveil molecular mechanisms of polymer phase separation,self-assembly,adsorption,and so on.Due to large molecular size and slow relaxation of the polymer chains,a great amount of issues related to large-distance chain displacement cannot be tackled easily with conventional molecular dynamic simulations.Systematic coarse-graining and enhanced sampling methods are two types of improvements that can boost spatiotemporal scales in polymer simulations.We present two typical ways to obtain the coarse-graining potential either by fitting to correct liquid structures or by fitting to available thermodynamic properties of polymer systems.The newly proposed anisotropic coarse-grained particle model can be used to describe aggregation and assembly of polymeric building blocks from disk-like micelles to Janus particles.We also present a stochastic polymerization model combined with coarse-grained simulations to investigate the problems strongly influenced by the coupling of polymerization and excluded volume effects.Finally,a facile implementation of integrated tempering sampling method is illustrated to be very efficient on bypassing local energy minima and having access to true equilibrium polymer structures.

THE UNIQUE PROPERTIES OF THE SOLID-LIKE CONFINED LIQUID FILMS:A LARGE SCALE MOLECULAR DYNAMICS SIMULATION APPROACH

The properties of the confined liquid are dramatically different from those of the bulk state, which were reviewed in the present work. We performed large-scale molecular dynamics simulations and full-atom nonequilibrium molecular dynamics simulations to investigate the shear response of the confined simple liquid as well as the n-hexadecane ultrathin films. The shear viscosity of the confined simple liquid increases with the decrease of the film thickness. Apart from the well-known ordered structure, the confined n-hexaxiecane exhibited a transition from 7 layers to 6 in our simulations while undergoing an increasing shear velocity. Various slip regimes of the confined n-hexadecane were obtained. Viscosity coefficients of individual layers were examined and the results revealed that the local viscosity＇coefficient varies with the distance from the wall. The individual n-hexadecane layers showed the shear-thinning behaviors which can be correlated with the occurrence of the slip. This study aimed at elucidating the detailed shear response of the confined liquid and may be used in the design and application of microand nano-devices.

Molecular dynamics simulation and microscopic observation of compatibility and interphase of composited polymer modified asphalt with carbon nanotubes

Interfacing and compatibility are the most challenging issues that affect the performance of polymer modified asphalt.Mechanisms of interfacial enhancement among four base asphalt components(asphaltenes,resins,aromatics,and saturate),styrene-butadiene-styrene(SBS),and carbon nanotubes(CNTs)were investigated by molecular dynamics simulation,with the aim of understanding the key parameters that control the compatibility of CNTs and interphase behavior on the molecular scale.The compatibility of SBS-modified asphalt(SBSMA)was simulated based on self-assembly theory using indexes of binding energy,mean square displacement,diffusion coefficient,and relative concentration distribution.The interphase behavior and microstructure were observed by fluorescence microscopy and scanning electron microscopy.In addition,a rutting experiment was used to verify the molecular dynamics simulation based on macroscopic performance.The results showed that after adding CNTs,the binding energy of the SBS and aromatics increased from 301.8343 to 327.1102 kcal/mol.The diffusion coefficient of the SBS and asphaltenes decreased more than 3.2×10-11 m2/s,and the correlation coefficients between the diffusion coefficient and the molecular weight,surface area and volume were all lower than 0.3.Relative concentration distribution curves indicated that CNTs promote the ability of SBS to swell.Microscopic observations demonstrated that the swelling ability of SBS was increased by CNTs.Overall,the interphase of SBSMA was improved by the additional reinforcement,swelling,and diffusion provided by CNTs.Finally,the rutting experiment found that no matter what the temperature,the rutting factor of CNT/SBSMA is higher than that of SBSMA,which corroborates the findings from the molecular dynamics simulations.

Molecular dynamics simulations of atomic-scale friction in diamond-silver sliding system

Molecular dynamics simulations have been performed to explore the atomic-scale sliding friction, especially the stick-slip friction, in a system consisting of a diamond slider and a silver substrate. The mechanisms of the stick-slip behavior are investigated by considering sliding speeds between 10 m/s and 200 m/s.The analyses of the shear distance between the upmost layer and the downmost layer and displacements of a column of atoms in the slider show that shearing deformation of the slider is the main cause of the stick-slip phenomenon. Our simulations also present that a commensurate fit between the two contact surfaces is unimportant for the stick-slip friction.

Molecular Dynamics Simulations of Thermal Properties of Polymer Composites Enhanced by Cross-Linked Graphene Sheets

Molecular models of pristine, functionalized and cross-linked graphene sheet/polymer composites are developed. Temperature cooling processes are conducted to examine the improve-ment of glass transition temperature of cross-linked graphene sheet/polymer composites using molecular dynamics simulations. The results show that increases of about 12.2% and 8.9% in the glass transition temperature of cross-linked graphene sheet/polymer composites are obtained, respectively, than those of the pristine and functionalized graphene sheet/polymer composites. In order to reveal the enhanced thermal properties from atomic views, the interfacial interaction energy and radius distribution function between the graphene sheets and the polymer matrix, the mean square displacement variations and the free volume of polymer composites are examined and discussed.

Molecular dynamics simulation of a single polymer in hydrophilic nano-slits

The behavior of a single polyethylene polymer in aqueous solution confined between two hydrophilic walls is studied with molecular dynamics (MD) simulations. The thickness of the nano-slit ranges from 1.26 to 3.15 nm, which is comparative to the polymer dimension. A monotonic transition from 3D- to 2D-like configurations is observed as the distance between the two walls narrows. Monomers are compressed into several layers and the preferred bond orientations alternate between parallel and normal to the walls accordingly. The diffusivity in the direction parallel to the wall is always larger than the one perpendicular to it. Calculation of the entropy and enthalpy changes during the folding of the polymer chain alone cannot explain the spontaneous process. The corresponding increase in water entropy due to volume expansion may be large enough to result in the overall free energy decrease.

The application of nonlocal theory method in the coarse-grained molecular dynamics simulations of long-chain polylactic acid

The micro-capsules used for drug delivery are fabricated using polylactic acid（PLA）,which is a biomedical material approved by the FDA.A coarse-grained model of long-chain PLA was built,and molecular dynamics（MD）simulations of the model were performed using a MARTINI force field.Based on the nonlocal theory,the formula for the initial elastic modulus of polymers considering the nonlocal effect was derived,and the scaling law of internal characteristic length of polymers was proposed,which was used to adjust the cut-off radius in the MD simulations of PLA.The results show that the elastic modulus should be computed using nonlinear regression.The nonlocal effect has a certain influence on the simulation results of PLA.According to the scaling law,the cut-off radius was determined and applied to the MD simulations,the results of which reflect the influence of the molecular weight change on the elastic moduli of PLA,and are in agreement with the experimental outcome.

Influence of Solid-liquid Interaction and Temperature on the Dynamic Dewetting of a Thin Polymer Film:A Molecular Dynamics Study

Coarse-grained molecular dynamics simulations were carried out to investigate the dewetting behavior of a polymer thin film on partial wetting solid surface at the early stage of the dewetting process. Spontaneous dewetting is initiated by removing a band of strip from both the ends of the liquid polymer film which has achieved equilibrium. The solid-liquid interaction and temperature were varied to show their influence on the dewetting dynamics during dewetting as well as the shape evolution of the liquid polymer film. As is consistent with the results obtained in previous researches, the liquid film recedes at a constant speed initially with different solid-liquid couplings and tempe- ratures. Furthermore, smaller coupling parameters or higher temperatures tend to accelerate the recession speed of the liquid film and shorten the constant-speed recession duration. Obvious rims were not always observed. Both coupling parameter and temperature can influence the emergence of the rims.

Crystallization of polymer chains induced by graphene: Molecular dynamics study

The present work is devoted to a study of the molecular mechanisms of the crystallization of polymer chains induced by graphene by using molecular dynamics （MD） simulations. From the atomic configuration translation, the number distri- bution of the atoms, and the order parameter S, the crystallization process can be summarized in two steps, the adsorption and the orientation. By analyzing the diffusion properties of the polymer chains, we find that a graphene substrate has a great adsorption for the polymer molecules and the polymer molecules need more time to adjust their configurations. Therefore, the adsorption step and the orientation step are highly cooperative.

Molecular Simulation of Oxygen Sorption and Diffusion in the Poly （lactic acid）

Grand canonical Monte Carlo and molecular dynamics simulation methods are used to simulate oxygen sorption and diffusion in amorphous poly（lactic acid） （PLA）. The simulated solubility coefficient of oxygen is close to experimental data obtained from the quartz crystal microbalance but much higher than those from the time-lag method. This discrepancy is explained by using the dual-mode sorption model. It is found that oxygen sorotion in PLA is predominantly Langmuir type controlled, i.e., through the process of filling holes. The time--lag method only takes into account oxygen molecules that participate the diffusion process whereas a large proportion of oxygen molecules trapped in the void have little chance to execute hopping due to the glassy nature of PLA at room temperature. The simulated diffusion coefficient of oxygen is reasonably close to the data obtained from the time-lag method. The solubility coefficient of oxygen decreases linearly with increasing relative humidity while its diffusion coefficient firstly decreases and then increases as a function of relative humidity.

A mini review of the recent progress in coarse-grained simulation of polymer systems

Molecular dynamics simulation is a powerful tool in the study of polymeric systems.Among various simulation methods,coarse-grained(CG)model is particularly impactful because it effectively reduces the computational complexity and enables the simulation of large-scale polymer systems.In this review,we briefly summarize recent progresses in our group on the development of CG simulation methods,models,as well as in the software development.By compiling the CG models and various simulation methods,we have successfully developed a GPU-accelerated large-scale molecular simulation toolkit(GALAMOST),which provides an efficient platform for polymer simulations.We further developed the new-generation PyGAMD(Python GPU-Accelerated MD Software,website:)software based on the Python platform,which makes the polymer simulation more powerful,flexible and user-friendly.In addition,some recent application cases in different polymer systems are also introduced.The aspiration of this review is to assist researchers in understanding the role of molecular simulations in the design and development of advanced polymer materials not only for academic researches,but also for possible industrial applications.

Stress Transfer in Polymer Nanocomposites:A Coarse-grained Molecular Dynamics Study

In this work,we used coarse-grained molecular dynamics simulation methods to investigate the dispersion and percolation behavior of nanoparticles in polymer nanocomposite.Our aim was to investigate the correlation between particle arrangement in nearby layers and the stretching performance in composite systems by exploring the stress transfer processes during different stages of the stretching process.The machine learning technique of linear regression was used to quantitatively measure the efficiency of stress transfer between particles nearby.According to our research,increasing the strength of attraction can significantly enhance the particle dispersion and affect the percolation threshold.We also noticed a non-monotonic relationship between the interaction strength and the tensile stress.Additionally,we quantified the efficiency of nanoparticles and polymers at transferring stress to nearby nanoparticles.As a result,the stress value provided by each particle in the aggregation body is significantly increased by the aggregation behavior of nanoparticles.The non-monotonic behavior is caused by two variables:the rapid disintegration of aggregates and the improved stress transfer efficiency from polymers to nanoparticles.Significantly,it was discovered that the structural rearrangement of nanoparticles during stretching is the main reason that causes the yield-like behavior seen in poorly dispersed systems.

双羟基染料改性有色水性聚氨酯的制备与性能

为了探究双羟基染料不同引入方式对水性聚氨酯(WPU)乳液及其成膜性能的影响,采用异佛尔酮二异氰酸酯、聚四氢呋喃(PTMG)、2,2-二羟甲基丙酸和1,4-丁二醇(BDO)合成了WPU乳液。将酸性红87作为扩链剂部分替代BDO制备了有色化学共聚聚氨酯(ARWPUA-1~3)乳液;将酸性红87作为软段部分替代PTMG制备了有色化学共聚聚氨酯(ARWPUB-1~3)乳液;将酸性红87与WPU共混制备了WPU/AR。采用FTIR、1HNMR和UV-Vis对样品的结构进行了表征;考察了不同有色WPU的贮存稳定性、耐溶剂性、颜色特征值、耐干/湿摩擦色牢度以及力学性能,并对其进行了分子动力学模拟。结果表明,由n(酸性红87)∶n(BDO)=1∶4制备的ARWPUA-2具有优异的贮存稳定性;在不同溶剂中均不会发生溶解脱色现象;具有良好的耐干/湿摩擦色牢度和较高的力学性能,其拉伸强度22.6MPa,断裂伸长率为810%。

Si-GDP结构和力学性能的反应分子动力学模拟

构建了硅掺杂辉光放电聚合物(Si-GDP)模型,采用反应力场分子动力学模拟(ReaxFF MD)探讨了硅含量、碳氢比及密度对其杂化碳键合与力学性能的影响。研究结果表明:随着硅含量增加,聚合物中sp^(3)C含量增加,趋向于形成一个大分子,同时小分子种类和数目减少,促进了碳硅原子成键并抑制端基CH_(3)生成,进而提高材料力学性能;随着氢含量的增加,sp^(3)C和端基CH_(3)比例增加,生成的端基CH_(3)降低了分子间交联程度,进而降低了材料力学性能,而分子基团数目变化不明显;随着密度的提升,聚合物中sp^(2)C比例提升明显,sp^(3)C比例有少量提升,分子基团数目变化不大,密度主要通过提升sp^(2)C比例提升材料力学性能。研究结果为评估和理解硅掺杂辉光放电聚合物的结构和力学性能提供了新的视角和方法。

Molecular Simulations in Macromolecular Science

Molecular simulations are now an essential part of modern chemistry and physics,especially for the investigation of macromolecules.They have evolved into mature approaches that can be used effectively to understand the structure-to-property relationships of diverse macromolecular systems.In this article,we provide a tutorial on molecular simulations,focusing on the technical and practical aspects.Several prominent and classical simulation methods and software are introduced.The applications of molecular simulations in various directions of macromolecular science are thenfeatured by representative systems,including self-assembly,crystallization,chemical reaction,and some typical non-equilibrium systems.This tutorial paper provides a useful overview of molecular simulations in the rapid progress of macromolecular science,and suggests guidance for researchers who start exploiting molecular simulations in their study.

模拟研究链间非共价键作用对聚合物自修复行为的影响

利用分子动力学模拟方法研究链间非共价键作用对聚合物材料自修复性能的影响.研究结果表明,非共价键作用强度的增加会提升材料的玻璃化转变温度以及力学性能;而对体系的自修复性能而言,非共价键作用的强度大小存在一个阈值,随着链间非共价键作用的增强,体系的自修复效率呈现先上升后下降的趋势.此外,体系的自修复效率与修复温度和修复时间均呈正相关.模拟研究结果为设计和制备自修复聚合物材料提供思路与理论指导.

高分子交联网络中纳米棒的扩散行为模拟研究

采用分子动力学模拟研究了纳米棒在末端交联高分子网络中的扩散行为,建立了末端交联高分子网络纳米棒复合体系的粗粒化模型。模拟平衡后,通过均方位移表征了纳米棒的平动扩散行为,发现纳米棒的运动存在很强的各向异性特点,平行于纳米棒长轴方向上平动扩散的平行分量随着纳米棒长度的增加线性下降,D_(‖)~L^(-1),其垂直分量的下降行为存在2个标度区间,纳米棒较短时,D_(⊥)~L^(-2),纳米棒较长时,D_(⊥)~L^(-3)。通过表征纳米棒的重取向自相关函数,获得纳米棒的转动扩散系数,发现其同平动扩散的垂直分量一样,也存在2个标度区间,分别为D_(R)~L^(-4)和D_(R)~L^(-5),且总体满足D_(R)~D_(⊥)L^(-2),表明纳米棒的垂直运动和转动扩散是相互耦合的。上述研究为高性能橡胶纳米复合材料的设计和开发奠定了坚实的科学基础。