Molecular dynamics simulation of relationship between local structure and dynamics during glass transition of Mg_7Zn_3 alloy

The rapid solidification process of Mg7Zn3 alloy was simulated by the molecular dynamics method. The relationship between the local structure and the dynamics during the liquid-glass transition was deeply investigated. It was found that the Mg-centered FK polyhedron and the Zn-centered icosahedron play a critical role in the formation of Mg7Zn3 metallic glass. The self-diffusion coefficients of Mg and Zn atoms deviate from the Arrhenius law near the melting temperature and then satisfy the power law. According to the time correlation functions of mean-square displacement, incoherent intermediate scattering function and non-Gaussian parameter, it was found that the β-relaxation in Mg7Zn3 supercooled liquid becomes more and more evident with decreasing temperature, and the α-relaxation time rapidly increases in the VFT law. Moreover, the smaller Zn atom has a faster relaxation behavior than the Mg atom. Some local atomic structures with short-range order have lower mobility, and they play a critical role in the appearance of cage effect in theβ-relaxation regime. The dynamics deviates from the Arrhenius law just at the temperature as the number of local atomic structures begins to rapidly increase. The dynamic glass transition temperature （Tc） is close to the glass transition point in structure （TgStr）.

Molecular Dynamics Simulation of Glass Transition Behavior of Polyimide Ensemble

The effect of chromophores to the glass transition temperature of polyimide ensemble has been investigated by means of molecular dynamics simulation in conjunction with barrier analysis. Simulated Tg results indicated a good agreement with experimental value. This study showed the MD simulation could estimate the effect of chromophores to the Tg of polyimide ensemble conveniently and an estimation approach method had a surprising deviation of Tg from experiment. At the same time, a polyimide structure with higher barrier energy was designed and validated by MD simulation.

Multi-scale Studies of Glass Transition and Uniaxial Tensile Properties of a Commercially Available Epoxy Adhesive Using Experimental Measurements and Molecular Dynamics Simulation

In this study, the glass transition and uniaxial tensile properties of a commercially available epoxy adhesive were investigated using experimental measurements and molecular dynamics （MD） simulation. Differential scanning calorimetry （DSC） was used to study the change of glass transition temperature （Tg） with cross-link density （CLD）. Uniaxial tensile test was performed to measure the Young＇s modulus （E）, Poisson＇s ratio （v） and yielding strength （tyv）. In MD simulation, the complicated epoxy system was simplified as the mixture of two kinds of simple molecules, with the key information well preserved and the less important details omitted. The molecular model of the cross-linked epoxy network was constructed and its mechanical properties were calculated using MD simulation. Overall, the MD simulation results agreed with experimental ones, which proved the validity of the molecular model and justified the simplification method of the industry- level epoxy system.

Molecular Dynamics Simulation of the Glass Transition Temperature of Fullerene Filled Cis-1,4-polybutadiene Nanocomposites

In this work, the effect of the fullerene（C_（60）） weight fraction and PB-C_（60） interaction on the glass transition temperature（T_g） of polymer chains has been systemically investigated by adopting the united atom model of cis-1,4-poly（butadiene）（cis-PB）. Various chain dynamics properties, such as atom translational mobility, bond/segment reorientation dynamics, torsional dynamics, conformational transition rate and dynamic heterogeneity of the cis-PB chains, are analyzed in detail. It is found that T_g could be affected by the C_（60） weight fraction due to its inhibition effect on the mobility of the cis-PB chains. However, T_g is different, which depends on different dynamics scales. Among the chain dynamics properties, T_g is the lowest from atom translational mobility, while it is the highest from the dynamic heterogeneity. In addition, T_g can be more clearly distinguished from the dynamic heterogeneity; however, the conformational transition rate seems to be not very sensitive to the C_（60） weight fraction compared with others. For pure cis-PB chains, T_g and the activation energy in this work can be compared with those of other polymers. In addition, the temperature dependence of the dynamic properties has different Arrhenius behaviors above and below T_g. The activation energy below T_g is lower than that above T_g. This work can help to understand the effect of the C_（60） on the dynamic properties and glass transition temperature of the cis-PB chains from different scales.

Unveiling the early stage evolution of local atomic structures in the crystallization process of a metallic glass

The early stage evolution of local atomic structures in a multicomponent metallic glass during its crystallization process has been investigated via molecular dynamics simulation.It is found that the initial thermal stability and earliest stage evolution of the local atomic clusters show no strong correlation with their initial short-range orders,and this leads to an observation of a novel symmetry convergence phenomenon,which can be understood as an atomic structure manifestation of the ergodicity.Furthermore,in our system we have quantitatively proved that the crucial factor for the thermal stability against crystallization exhibited by the metallic glass is not the total amount of icosahedral clusters,but the degree of global connectivity among them.

Learning physical states of bulk crystalline materials from atomic trajectories in molecular dynamics simulation

Melting of crystalline material is a common physical phenomenon,yet it remains elusive owing to the diversity in physical pictures.In this work,we proposed a deep learning architecture to learn the physical states(solid-or liquidphase)from the atomic trajectories of the bulk crystalline materials with four typical lattice types.The method has ultrahigh accuracy(higher than 95%)for the classification of solid-liquid atoms during the phase transition process and is almost insensitive to temperature.The atomic physical states are identified from atomic behaviors without considering any characteristic threshold parameter,which yet is necessary for the classical methods.The phase transition of bulk crystalline materials can be correctly predicted by learning from the atomic behaviors of different materials,which confirms the close correlation between atomic behaviors and atomic physical states.These evidences forecast that there should be a more general undiscovered physical quantity implicated in the atomic behaviors and elucidate the nature of bulk crystalline melting.

Molecular Dynamic Simulation Study on Glass Transition Temperature of DGEBA-THPA/SWCNTs Composites

Molecular dynamic (MD) simulations were carried out to predict the thermo-mechanical properties of the cured epoxy network composed of diglycidyl ether bisphenol A (DGEBA) epoxy resin and tetrahydrophthalic anhydride (THPA) curing agent and their single-walled carbon nanotubes (SWCNT) reinforced the epoxy matrix composites. Different characters such as the density of the materials and mean square displacements (MSDs) were calculated to estimate the glass transition temperatures (Tgs) of of the materials. 365 K and 423 K of the Tgs were obtained respectively, whereas the latter is much higher than the former. The simulation results indicated that the incorporation of SWCNTs in the epoxy matrix can significantly improve the Tg of the cured epoxy. The approach presented in this study is ready to be applied more widely to a large group of candidate polymers and nanofillers.

Molecular dynamics simulations on specific heat capacity and glass transition temperature of liquid silver

The embedded-atom method is adopted to simulate the specific heat capacity of liquid silver. The relationship between the specific heat capacity and the temperature above and below melting point is derived. The results show that there exists an anormaly of the specific heat capacity of liquid silver near 950 K. Simulated pair distribution functions show that the liquid-to-glass transition takes place at this temperature.

Liquid-to-glass transition of tetrahydrofuran and 2-methyltetrahydrofuran

Both tetrahydrofuran （THF） and 2-methyltetrahydrofuran （MTHF） are studied systematically at desired temperatures using molecular dynamics simulations. The results show that the calculated densities are well consistent with experiment. Their glass transition temperatures are obtained： 115 K - 130 K for THF and 131 K - 142 K for MTHF. The calculated results from the dipolar orientational time correlation functions indicate that the ＂long time＂ behavior is often associated with a glass transition. From the radial and spatial distributions, we also find that the methyl has a direct impact on the structural symmetry of molecules, which leads to the differences of physical properties between THF and MTHF.

Predicting Glass Transition Temperature of Polyethylene/Graphene Nanocomposites by Molecular Dynamic Simulation

The glass transition temperature of polyethylene/graphene nanocomposites was investigated by molecular dynamic simulation. The specific volumes of three systems（polycthylene, polyethylene with a small graphene sheet and two small graphene sheets） were examined as a function of temperature. We found that the glass transition temperature decreases with increasing graphene. Then the van der Waals energy changes obviously with increasing graphene and the torsion energy also plays an important role in the glass transition of polymer. The radial distribution functions of the inter-molecular carbon atoms suggest the interaction between PE and graphene weakens with increasing graphene. These indicate that graphene can prompt the motion of chain segments of polymer and decrease the glass transition temperature （Tg） of polymer.

Computer simulation studies of the influence of side alkyl chain on glass transition behavior of carbazole trimer

In this work,all-atom molecular dynamics simulations were employed to study the influence of the side alkyl chain on glass transition behavior of several carbazole trimers(CT) in a temperature range from 423 to 183 K.The glass transition temperatures were obtained from the break in the slope of the volume-temperature curves and found to agree with the experimental values.The short time dynamics of four CT molecules were probed by usingvelocity autocorrelation functions and mean-square displacements.The current studies showed that the dynamics of CT systems can be easily interpreted through the cage effect.Furthermore,the investigation of the torsional autocorrelation function and P_(2-state)/P_(3-state) functions showed that the rotational barriers of side chains can slow down the conformational relaxation and lead to stronger temperature dependence of conformational relaxation.The relaxation time,characteristic time of P_(2-state)(t) and P_(3-state)(t) functions were all found to have Arrhenius-type temperature dependence.

Effect of Branching Architecture on Glass Transition Behavior of Hyperbranched Copolystyrenes:the Experiment and Simulation Studies

By controlling the feed ratio of CMS/styrene and the polymerization time, a series of hyperbranched copolystyrenes（HBCPS） were synthesized with comparable weight-averaged molecular weights（Mw） but different degree of branching（DB） through atom transfer radical self-condensing vinyl copolymerization（ATR-SCVCP） with Cu Br/2,2？-bipyridyl as the catalyst. The resulting HBCPS samples were used to investigate the effect of branching architecture on their glass transition behavior. With the DB increased, the glass transition temperatures（Tg） of HBCPS samples measured by DMA and DSC both decreased. Their spin-lattice relaxation times（1H T1r） of protons displayed the same downtrend with increasing DB. Besides, a correlation between the Tgs and the DB was well established by all-atom molecular dynamics（MD） simulations. The values of MD-determined Tgs are little higher than the corresponding experimental ones. However, the dependence of Tgs on DB is in good agreement with the experimental results, i.e., Tg decreases both in experiments and simulations with increasing DB.

Research on the nanometric machining of a single crystal nickel via molecular dynamics simulation

Molecular dynamics simulations are employed to study the nanometric machining process of single crystal nickel. Atoms from different machining zones had different atomic crystal structures owing to the differences in the actions of the cutting tool. The stacking fault tetrahedral was formed by a series of dislocation reactions, and it maintained the stable structure after the dislocation reactions. In addition, evidence of crystal transition and recovery was found by analyzing the number variations in different types of atoms in the primary shear zone, amorphous region, and crystalline region. The effects of machining speed on the cutting force, chip and subsurface defects, and temperature of the contact zone between the tool and workpiece were investigated. The results suggest that higher the machining speed, larger is the cutting force. The degree of amorphousness of chip atoms and the depth and extent of subsurface defects increase with the machining speed. The average friction coefficient first decreases and then increases with the machining speed because of the temperature difference between the chip and machining surface.

Structural relaxation and glass transition behavior of binary hard-ellipse mixtures

Structural relaxation and glass transition in binary hard-spherical particle mixtures have been reported to exhibit unusual features depending on the size disparity and composition. However, the mechanism by which the mixing effects lead to these features and whether these features are universal for particles with anisotropic geometries remains unclear. Here, we employ event-driven molecular dynamics simulation to investigate the dynamical and structural properties of binary two-dimensional hard-ellipse mixtures. We find that the relaxation dynamics for translational degrees of freedom exhibit equivalent trends as those observed in binary hard-spherical mixtures. However, the glass transition densities for translational and rotational degrees of freedom present different dependencies on size disparity and composition. Furthermore,we propose a mechanism based on structural properties that explain the observed mixing effects and decoupling behavior between translational and rotational motions in binary hard-ellipse systems.

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 A1 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 {lll}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.

Determining the structural phase transition point from the temperature of ^(40)Ca^+ Coulomb crystal

We observed the linear-to-zigzag structural phase transition of a ^40Ca^＋ crystal in a homemade linear Paul trap. The values of the total temperature of the ion crystals during the phase transition are derived using the molecular-dynamics（MD） simulation method. A series of simulations revealed that the ratio of the radial to axial secular frequencies has a dependence on the total temperature that obeys different functional forms for linear and zigzag structures, and the transition point occurs where these functions intersect; thus, the critical value of the ratio of secular frequencies that drives the structure phase transition can be derived.

基于分子动力学模拟研究交联密度对氢化丁腈橡胶性能的影响

采用分子动力学构建交联密度为0,10%,20%,30%和40%的氢化丁腈橡胶(HNBR)分子模型,研究交联密度对HNBR性能的影响。结果表明:HNBR分子模型的玻璃化转变温度(T_(g))随着交联密度的增大而升高,交联密度为0~10%时,T_(g)升高较快,交联密度大于10%时,T_(g)升高速率趋于平缓;HNBR分子模型的均方位移(MSD)随着交联密度的增大而减小,分子链的柔顺性变差,交联密度达到30%后,MSD变化较小;交联密度为0~30%时,HNBR分子模型的拉伸性能随着交联密度的增大而提高,交联密度为30%时,HNBR分子模型的拉伸性能最好,交联密度大于30%时,晶格内应力集中,HNBR分子模型的拉伸性能随交联密度的增大而降低。

基于分子动力学模拟预测环氧树脂形状记忆性能研究

形状记忆环氧树脂是制备可展开空间结构的重要材料之一,通过分子动力学模拟预测环氧树脂的形状记忆性能是设计与制备形状记忆环氧树脂的有效指导手段。在pcff力场下,通过分子动力学模拟方法,构建了交联度分别为36%、48%、59%、72%和84%的三乙烯四胺固化的双酚A环氧树脂聚合物模型,并模拟计算了其玻璃化转变温度、力学性能及形状记忆性能。结果表明,不同交联度的环氧树脂聚合物模型均表现出较好的形状固定率,为80%~86%;分析聚合物模型的x、y、z三轴回复性可知,36%交联度的环氧树脂不具有形状记忆性能,48%交联度的环氧树脂形状记忆性能较差,而59%、72%、84%交联度的环氧树脂的形状回复率均能达到80%以上,具有良好的形状记忆性能,且交联度越高形状回复速度越慢。

不同构型聚丙烯的玻璃化转变温度的分子模拟

应用分子力学和分子动力学的方法对3种不同构型聚丙烯高分子的玻璃化转变温度进行了模拟.用NPT(等温等压)分子动力学模拟获得聚丙烯(PP)在不同温度下的特征体积,通过对模拟得到的V-T做图,求得玻璃化转变温度,其结果与实验值吻合较好.并分析了聚丙烯主链柔顺性和立构规整度对高分子玻璃化转变的影响.

分子动力学模拟预测壳聚糖的玻璃化转变温度

为了预测壳聚糖的玻璃化转变温度,在COMPASS力场和恒温恒压(NPT)系综条件下,利用分子动力学模拟方法,在343～543K温度范围内研究了壳聚糖的玻璃化转变行为,通过模拟体系在不同温度下的比体积、回转半径和能量参数,获得了壳聚糖的玻璃化转变温度(Tg)。研究结果表明,壳聚糖的比体积、回转半径、内能随温度有规律的变化并在Tg处发生转折。模拟计算得到的壳聚糖的Tg与实验方法获得的值基本相符,分子动力学方法可用于壳聚糖玻璃化转变温度的预测。其中,通过回转半径-温度曲线获得的Tg与实验值最相符,回转半径是影响玻璃化转变的一个重要因素,可用于预测聚合物的玻璃化转变温度。