基于分子动力学模拟的轮胎橡胶催化热解制氢机理

采用分子动力学模拟方法,选取Ni、ZSM-5以及Ni/ZSM-5催化剂,对轮胎橡胶热解制氢的机理进行探究,并同时与前人做过的实验研究进行对比验证模拟计算。文中利用Material Studio建立轮胎橡胶模型,DMol3模块对生成氢气路径进行过渡态搜索,CULP模块对其加入Ni催化剂的热解过程进行模拟。模拟结果表明,制氢催化效果顺序为Ni>Ni/ZSM-5>ZSM-5。催化热解大致分为两个阶段：①低温阶段长链裂解成单体化合物,单体主要是异戊二烯、苯乙烯以及1,3-丁二烯;②高温阶段自由基攻击单体生成小分子物质。加入Ni催化剂后降低了热解终止温度。催化剂的加入在低温阶段主要表现在加快热解进程,增加低温阶段时单体数量。在高温阶段主要表现在改变了气体产物分布,Ni的加入降低了轮胎热解温度,并且使氢比例增加。     

ReaxFF动力学分析煤热反应机理

为了从微观角度研究煤的热解机理,找出热解过程中反应路径、中间产物等反应信息,用Reax FF动力学的方法分析煤的热反应机理。阐述了模拟过程中煤分子体系的构建、模拟结果的处理方法,介绍了如何通过监测自由基中间体种类,获得反应过程中常见产物的分布和基元反应,进而解释煤热解、氧化和水热处理等反应过程的机理,并对Reax FF动力学在煤热反应体系中复杂化学反应的研究前景进行展望。通过Reax FF方法可模拟煤的热反应过程,可以获得主要产物的生成路径、产物种类、产物的生成顺序等信息,还可以研究煤热解过程中煤分子间或与其他分子间的相互作用,进而分析热解过程中煤的热反应机理。后续可通过时温等效原理建立模拟温度与实际温度的定量关系,解决Reax FF方法模拟温度与实际温度不对应的问题;通过编写相应的后台程序缩短模拟计算结束后数据的处理时间。     

A reactive molecular dynamics model of thermal decomposition in polymers: I. Poly(methyl methacrylate)

The theory and implementation of reactive molecular dynamics (RMD) are presented. The capabilities of RMD and its potential use as a tool for investigating the mechanisms of thermal transformations in materials are demonstrated by presenting results from simulations of the thermal degradation of poly(methyl methacrylate) (PMMA). While it is known that depolymerization must be the major decomposition channel for PMMA, there are unanswered questions about the nature of the initiation reaction and the relative reactivities of the tertiary and primary radicals formed in the degradation process. The results of our RMD simulations, performed directly in the condensed phase, are consistent with available experimental information. They also provide new insights into the mechanism of the thermally induced conversion of this polymer into its constituent monomers.     

基于分子动力学模拟的轮胎橡胶气相热解产物反应机理

运用分子动力学的方法,对轮胎橡胶的热解过程进行了模拟,并结合模拟结果和密度泛函数对其气相产物的反应路径进行推测计算。模拟结果表明,热解过程主要分为两个阶段,低温热解阶段发生的主要反应是橡胶长链断裂形成单体,主要产物为异戊二烯、苯乙烯和1,3-丁二烯;高温热解阶段发生的主要反应是单体进一步生成小分子气体,产物中CH₄、H₂、C₂H₄占大部分,还有少量C₂H₆、C₃H₆。其中CH₃·攻击异戊二烯和苯乙烯单体夺取特定位置的H·是生成CH₄的最优路径,H·攻击苯乙烯单体夺取特定位置的H·是生成H₂的最优路径,CH₂CH·攻击1,3-丁二烯单体夺取特定位置的H·是生成CH₂CH₂的最优路径。本文还将热解产物分别跟天然橡胶单独热解和天然橡胶与丁苯橡胶共热解的热解产物做对比,为废旧轮胎橡胶热解得到特定的气相产物和催化热解提供理论依据。     

基于分子反应动力学模拟的六甲基二硅氧烷热解机理研究

六甲基二硅氧烷是燃烧合成高纯二氧化硅纳米颗粒的重要前体,采用ReaxFF分子动力学模拟方法研究其高温热解过程,讨论了三种不同反应力场对模拟的影响并分析其可靠性,选择其中最合适的力场开展不同温度与压力下的热解产物分析,结合气相色谱实验,揭示六甲基二硅氧烷的热解路径和机理。结果表明,反应力场对ReaxFF分子动力学模拟有重要影响,通过比较分析获得了最佳反应力场,六甲基二硅氧烷的初始热解反应为Si—C键断裂导致的CH₃脱离,温度升高会加剧解热反应的发生且使产物趋向于碎片化,热解的主要产物为CH₃、CH₄、C₂烃、H₂、CH₂O等小分子以及SiH₄、SiH₂、CH₄Si等含硅化合物。压力的改变会造成热解体系浓度的改变,从而影响分子间相互碰撞概率和反应的发生,压力越大则越容易形成稳定的热解产物。     

基于分子反应动力学模拟的六甲基二硅氧烷热解机理研究

六甲基二硅氧烷是燃烧合成高纯二氧化硅纳米颗粒的重要前体,采用ReaxFF分子动力学模拟方法研究其高温热解过程,讨论了三种不同反应力场对模拟的影响并分析其可靠性,选择其中最合适的力场开展不同温度与压力下的热解产物分析,结合气相色谱实验,揭示六甲基二硅氧烷的热解路径和机理。结果表明,反应力场对ReaxFF分子动力学模拟有重要影响,通过比较分析获得了最佳反应力场,六甲基二硅氧烷的初始热解反应为Si—C键断裂导致的CH₃脱离,温度升高会加剧解热反应的发生且使产物趋向于碎片化,热解的主要产物为CH₃、CH₄、C₂烃、H₂、CH₂O等小分子以及SiH₄、SiH₂、CH₄Si等含硅化合物。压力的改变会造成热解体系浓度的改变,从而影响分子间相互碰撞概率和反应的发生,压力越大则越容易形成稳定的热解产物。     

A reactive molecular dynamics model of thermal decomposition in polymers. II. Polyisobutylene

An improved version of the reactive molecular dynamics method is presented. The method, which extends conventional (force-field-based) molecular dynamics to modeling chemical reactions, is used to simulate the thermal decomposition of polyisobutylene. The results of the simulations are generally consistent with experimental observations. A quantitative analysis of the results shows that the rate constant of the key initiation reaction, backbone scission, depends on the size of the molecular model of the polymer. This implies that the kinetics of some elementary reactions that take place in a polymer melt are affected by the macromolecular nature of the environment.     

纳米狭缝中水流动非平衡分子动力学模拟

采用非平衡分子动力学模拟（non-equilibrium molecular dynamics simulation）方法研究了不同间距纳米狭缝之间水的流动行为。研究了纳米狭缝间距、壁面性质和外部压力对水流动速度径向分布、有效黏度、壁面速度和滑移长度的影响,讨论了Navier-Stoke（N-S）方程的适用性。研究结果表明,N-S方程仅适用于3 nm以上的孔道;狭缝尺寸的增加和施加压力的增加均会使得管内流速增加,而造成表观黏度降低以及滑移长度增加。壁面亲水性的增加仅使得滑移长度降低,表观黏度并没有发生较大变化。     

用分子动力学模拟研究丁二烯微观结构对丁腈橡胶物理机械性能的影响

采用分子动力学模拟的方法对基于3种不同丁二烯键合结构建立的丁腈橡胶模型(A、B、C模型)进行了模拟计算,以考察丁二烯微观结构对丁腈橡胶物理机械性能的影响.采用恒定应变法计算模型的物理机械性能,然后将模型与铁原子层结合建立摩擦副模型,利用剪切模拟分析丁腈橡胶摩擦过程的磨损机理.结果表明,与A模型和C模型相比,B模型分子链...     

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343～543 K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(T_g)。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T_g处发生转折。模拟计算得到的壳聚糖的T_g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T_g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。     

客体分子数对甲烷水合物导热性能影响的分子动力学模拟

本文通过采用EMD方法Green-Kubo理论计算263.15 K 晶穴占有率0-100% sI甲烷水合物导热系数,研究客体分子数对甲烷水合物导热性能的影响。模拟结果显示,甲烷水合物的低导热性能由主体分子构建的笼型结构决定。而在相同温压条件下,随着客体分子甲烷进入晶胞数目增多,晶穴占有率增大后,密度增大,同时客体分子对声子的散射也增强,二者均导致导热性能增强。     

丙炔基双酚A醚硼聚合物热解过程的ReaxFF分子动力学模拟

聚合物的热解过程涉及的化学反应较为复杂,难以通过测试表征的手段深入探究其机理。本研究在实验表征的基础上,结合ReaxFF分子动力学(ReaxFF-MD)模拟方法,研究了丙炔基双酚A醚硼聚合物(PB)的热解过程。通过观察升温过程中PB的结构变化,可得到其热解过程中的断键顺序。此外,采用ReaxFF-MD模拟,其研究结果不仅验证了实验中热重-红外光谱联用(TG-FTIR)分析所得的PB热解生成小分子的主要组成为CH₄、H₂O、H₂ 和 CO,并且通过追踪上述小分子的生成过程可得到其主要的生成途径。以上研究结果表明,ReaxFF-MD模拟方法不仅有助于理解PB聚合物的热解机理,直观地反映出其热解产物生成途径,而且对聚合物耐热性能的研究有所借鉴。     

A study on hygrothermal degradation and recovery of an epoxy adhesive using molecular dynamics simulation

For application of epoxy adhesive to joining similar or dissimilar materials in vehicle bodies, its hygrothermal degradation (HTD) caused by severe environmental conditions of service will always be an issue until the relevant mechanism is clearly addressed and the remedy is found. This study provides experimental observations of an epoxy adhesive in terms of HTD and recoverability of the mechanical performance, and in the meantime, molecular dynamics (MD) simulations are performed to analyse the underlying mechanism. Comparing experimental results of the adhesive among states of the initial, HTD and dried manifests that the glass transition temperature (T_g) and the uniaxial tensile properties both reduced after HTD but partially recovered when dried. In the MD simulations, both of the dominant HTD factors, plasticization and hydrolysis, are accounted for via characterizations of water inclusion and bond scission. The simulation results reveal that both of the HTD factors reduce T_g, while only hydrolysis weakens the tensile properties. A quantitative comparison between the influences of plasticization and hydrolysis implies that hydrolysis is reversible for this specific epoxy adhesive.     

Investigation of packaging adhesive properties by molecular dynamics and experiments

Adhesive polymer is a common and important material used for packaging of microelectronics and microsystem by attaching dies onto packaging shell, and its mechanical property plays a vital role in isolating dies from the thermal stress of substrate. Therefore, it is extremely significant to evaluate the polymer property in a specific packaging process. The molecular dynamics (MD) simulation is conducted in this article to investigate the material properties of the cross-linked epoxy resin formed by epoxy resin component diglycidyl ether bisphenol A (DGEBA) and curing agent 1,6-Diaminohexane. The polymer network with conversion up to 87.5% is successfully generated and simulated by constant pressure-constant temperature ensemble (NPT) and canonical ensemble (NVT) at different temperatures of curing process. Glass transition temperature (T_g) and Young's modulus are extracted and the predicted material properties are in great agreement with the experimental data. The conclusion provides a guideline to design the special curing process for different adhesive requirements.     

Inelastic relaxation in silica via reactive molecular dynamics

Silica glass exhibits rate-dependent and irreversible processes during deformation and failure, resulting in inelastic effects. To explore this phenomena, molecular dynamics simulations of structural relaxation surrounding a crack tip in silica glass were performed at four different temperatures (100, 300, 600, 900 K) using a reactive force field. Per-atom stresses were found to relax during the simulation, with the highest stress relaxation occurring at 900 K. Stress relaxation was radially dependent relative to the crack tip, with stress dissipation occurring primarily within a 25–30 Å inelastic region. Within 10 Å of the crack tip, the defect concentration decreased from 0.18 to 0.09 #/nm² during inelastic relaxation at 900 K. Conversely, the defect concentration 20 Å from the crack tip increased from 0.105 to 0.118 #/nm² at 300 K, and from 0.113 to 0.126 #/nm² at 600 K, which formed a defect-enriched region ahead of the crack tip. The difference in defect concentrations suggests the possibility of a stress mediated defect migration mechanism, where defects move away from the crack tip during inelastic relaxation. Additionally, defect speciation indicated that undercoordinated silica defects, such as non-bridging oxygen, were removed through the formation of higher coordination defects during relaxation. Overall, stress relaxation causes changes in the defect concentration profile near the crack tip, which has the potential to alter the properties of silica glass in the inelastic region during relaxation.     

醇类有机物热传导的分子动力学模拟及微观机理研究

传热是化工生产的基本问题之一,热导率是化工产品生产工艺设计中一类重要的热力学数据。通过非平衡分子动力学方法模拟了8种液态醇类有机物在不同温度下的导热过程。热导率计算值与实验值的平均相对偏差为3.77%。通过对热流的分解发现,分子动能、分子间库仑相互作用和分子内的二面角对醇类有机物的热传导影响较大。同时随着分子链增长,通过分子内相互作用进行的热传导逐渐占主导作用,表明醇类有机物的热能传输机理与分子结构有显著关系。此外,随着温度的升高,通过分子的动能、分子间库仑作用和分子内键角、键伸缩作用项传输的热流增大,表明温度对液态醇类有机物的热传导也有一定影响。本工作从微观分子间和分子内作用分析了液态醇类有机物结构和温度对热导率的影响,为液态有机物的热传导研究提供了微观依据。     

分子模拟在聚合物膜研究中的应用

简要回顾了近15年来分子模拟在各种聚合物膜研究中的应用,分析了它为宏观实验现象提供的新解释以及给聚合物膜的改性和设计研究工作带来的新思路,同时也对当前计算条件下模拟中存在的问题进行了探讨。     

全原子分子动力学模拟反相色谱分离细胞色素C的过程

采用全原子分子动力学方法模拟了反相色谱分离蛋白质的吸附和洗脱过程。采用表面键合C4烷基链的硅胶基质作为反相色谱介质和细胞色素C作为蛋白质模型,模拟蛋白质在反相色谱分离过程中的构象变化。结果显示:在吸附过程中,蛋白质在释放出表面结合水分子的同时置换出色谱介质表面的水分子。与其在溶液中的天然构象相比,吸附态蛋白质的构象发生改变。在洗脱过程中,随着溶剂从水切换到甲醇,甲醇取代水分子包覆在介质和蛋白质表面,将蛋白质从介质表面置换下来。分子模拟的结果再现了有关反相色谱"优先水化"的吸附机制,并从分子水平上展现了吸附和洗脱过程中蛋白质、色谱介质和溶剂之间相互作用,对反相色谱介质设计和过程优化提供了参考。     

Molecular dynamics simulation and mechanism study on thermal conductivity of alcohols

Heat transfer is one of the basic issues in chemical production, and thermal conductivity is an important thermodynamic data in the design of chemical product production processes. In this paper, nonequilibrium molecular dynamics methods are used to simulate the heat transfer of liquid alcohols at four different temperatures, and the thermal conductivity of the corresponding conditions is obtained. The average relative deviation between the calculated value and the experimental value was 3.77%. Through the decomposition of heat flux, it was found that molecular kinetic energy, Coulomb interaction and intramolecular dihedral angle contribute the most to the heat conduction of alcohols. At the same time, as the molecular volume increases, the thermal conduction pathway of the intramolecular interaction term gradually dominates, indicating that the thermal conduction mechanism of alcohols has a significant relationship with the molecular structure. In addition, as the temperature elevates, the heat flux transmitted through the molecular kinetic energy, intermolecular Coulomb interaction, and the intramolecular angle bending and bond stretching term increases, while the heat flux transmitted through the molecular potential energy decreases significantly. This work provides a microscopic explanation for the effects of the structure and temperature of liquid alcohols on thermal conductivity, and provides a micro foundation for the study of heat conduction of liquid alcohols.     

Mechanism of the odor-adsorption effect of zinc ricinoleate. A molecular dynamics computer simulation

Zinc ricinoleate [Zn(Ri)₂] is widely used in surfactant and detergent mixtures for the adsorption of odor-active compounds. The mechanism of this process is not known. In this initial study, we discuss the results of molecular dynamics computer simulations that were performed to get more information and detailed insights into the interaction mechanism between Zn(Ri)₂ and odor-active substances. The calculations, based on simple molecular mechanics approximations, simulated the dynamic features of the molecular structures of Zn(Ri)₂ in vacuum, in the oil phase, and in aqueous solution. We determined actual molecular conformations and simulated an adduct of ammonia with Zn(Ri)₂. On close inspection, in the vacuum and oil phase structures, the Zn²⁺ ion is almost completely shielded by the oxygen ligands. Calculated structural transitions caused by the interaction of Zn(Ri)₂ with water-solvent molecules resulted in a weakening of the electrostatic shield. Nucleophilic attack of odor-active compounds to the relatively unprotected Zn²⁺ atom is easy to achieve in aqueous solution. Simulation of the addition product of Zn(Ri)₂ with ammonia revealed an elementary structural change, resulting in an increase of the solubility and adsorption activity of Zn(Ri)₂. Molecular dynamics simulations showed that the results coincide with experimental observations.