PA6/PPS共混物介观形貌的DPD模拟

应用耗散粒子动力学(DPD)模拟方法研究了PA6/PPS共混物的介观形貌及动力学演变过程.详细分析了不同比例下PA6/PPS共混物的介观形貌、密度、扩散系数以及界面张力等变化情况,同时还考察了不同剪切速率对体系介观形貌的影响.结果表明,PA6/PPS共混物中随PA6含量的增加,PA6的介观形貌依次出现球状、柱状、层状以及连续相等结构,PA6的扩散系数大于PPS,说明PA6的加入可以改善共混物的加工流动性,这与文献报道的实验结果相一致.同时剪切速率的大小对PA6/PPS体系形貌有着重要影响.     

端羟基聚丁二烯/增塑剂共混物相容性的分子动力学模拟和介观模拟

应用分子动力学(MD)和介观动力学(MesoDyn)模拟方法对固体推进剂中端羟基聚丁二烯(HTPB)与增塑剂癸二酸二辛酯(DOS)、硝化甘油(NG)的相容性进行了研究. 采用MD模拟方法在COMPASS力场下, 对纯物质、HTPB/增塑剂共混物的密度、内聚能密度、溶度参数和共混物分子间的Flory-Huggins作用参数及结合能等进行了模拟计算, 通过比较溶度参数差值(Δδ)的大小、模拟前后体系密度变化情况均可以预测HTPB与增塑剂的相容性, 结合能的分析揭示了HTPB/增塑剂共混物组分间的相互作用及本质. 将Flory-Huggins作用参数转化为MesoDyn模拟的输入参数, 采用MesoDyn模拟方法对HTPB/增塑剂共混体系的介观形貌与动力学演变过程进行了研究, 通过模拟得到的等密度图、自由能密度和有序度参数等可以判断共混体系的相容性. MD和MesoDyn模拟结果均表明: HTPB/DOS属于相容体系, 而HTPB/NG属于不相容体系, 其结论与实验结果一致.     

介观模拟方法研究高分子表面活性剂在水介质中的聚集行为

高分子表面活性剂已广泛应用于许多领域, 其构型复杂、分子量大等特点使其聚集行为不同于小分子表面活性剂. 从微观上认识其聚集行为可为应用提供指导, 因而此方面的研究倍受关注. 计算机模拟技术的发展使我们能成功地在微观或介观水平上获得高分子表面活性剂聚集行为的信息. 本文综述了耗散粒子动力学(DPD)和介观动力学(MesoDyn)在高分子表面活性剂聚集行为研究中的应用. 着重介绍了这两种介观模拟方法研究单一高分子表面活性剂溶液的相行为及其与低分子表面活性剂之间的相互作用, 揭示了实验中难以观测的微观相分离及聚集体结构形态的变化规律. 这些信息可以为实验研究提供指导和补充.     

计算机模拟技术在表面活性剂研究中的应用

根据表面活性剂溶液行为的模拟所需的时间和空间尺度,介绍了三种主要的计算机模拟方法:原子模拟、粗粒模拟和介观模拟.综述了这些模拟方法在表面活性剂单体、缔合体系及与聚合物相互作用等研究中的应用.指出了用计算机模拟方法研究表面活性剂体系的发展前景.     

水溶液中表面活性剂月桂醇聚氧乙烯醚硫酸钠性能的介观模拟

采用介观动力学模拟方法MesoDyn研究比较了表面活性剂月桂醇聚氧乙烯醚硫酸钠(AES)和月桂醇硫酸钠(SDS)在水溶液里的聚集行为及相互间作用, 从介观层面上研究了分子结构差异所导致聚集行为的一系列差别. 同时模拟研究了AES/苯体系中各组分对体系形貌的影响. 以苯、正辛醇为油污代表, 通过对密度切片图的研究, 模拟比较了AES对这两种油污去除机理的差异. 选择AES/苯为研究体系, 从介观的层次考察其胶束形成的影响因素. 结果表明, 由于极性头的结构差别, AES和SDS在临界胶束浓度和聚集数方面都有差异, 同时两种表面活性剂之间又存在着较强的协同效应. 除疏水性作用、氢键作用外, 亲水性作用也在胶束形成中起重要作用. 对密度切片图的观察得出, 由于所选择油污的结构差异导致了AES对其增溶方式的差异.     

对pluronic水溶液介观相分离的理论模拟研究

用一种新的动力学密度泛函方法(介观动力学)对pluronics(P85)水溶液的介观相分离动力学进行了模拟研究。该方法可以直接给出水溶液体系中不规则三维微观形貌的动力学形成过程。在动力学模拟中热力学参数通过平均场密度泛函方法计算得到,共聚物分子则用高斯链作为模型进行模拟研究。高斯链的最小结构单元是被抽象为键连的株子体,代表实际体系高聚物分子中的一个单体或几个单体。研究了pluronuc水溶液体系的动力学演变历程并讨论了嵌段共聚物中不同组分长短以及共聚物浓度等因素对溶液微观形貌和体系性质的影响。和其它平衡态模拟方法相比较,介观动力学方法可以给出介观相分离的时间演变过程,有助于加深对很多工业加工处理过程和生理过程机理的理解。     

介观化学体系中的动力学尺度效应

以生命和表面催化体系为对象,研究了介观化学体系中内涨落对体系非线性动力学行为的调控作用.内涨落可以诱导随机振荡,其强度在体系处于最佳尺度时会出现一个甚至多个极大值,并且在耦合体系中会得到进一步增强,表现为尺度共振效应、尺度选择效应和双重尺度效应,揭示了介观化学体系中尺度效应的新机制.     

介观层次上的计算机模拟和应用*

本文综述了近年发展起来的介观层次上的计算机模拟和应用。介绍了两种较为成熟的模拟方法: 介观动力学和耗散颗粒动力学。还介绍了介观模拟方法在胶束形成、胶体絮状物构造、乳化剂、流变学、共聚物和高分子混合形态以及通过多孔介质的流动研究中的应用。     

球壳粒子悬浮系模型化研究——内部相参数对介电谱影响的数值模拟

在Hanai理论基础上对球壳粒子悬浮系的介电模型进行了模拟研究,通过用C++的复数类对理论公式的程序化,建立了介电谱的介电参数与体系内部相参数的关系.所得的解析解可方便地模拟介电弛豫谱依不同相参数的变化曲线,计算并分析了内部参数对介电谱的模式以及介电参数的影响因素.     

用介观动力学模拟Pluronic L64/水/p-Xylene体系的相分离

用介观动力学在介观层次上对不同组分的PluronicL64/水/p-xylene三元体系的相分离进行了研究,得到了和实验相吻合的结果。计算表明对于纯p-xylene溶剂和有含少量水的p-xylene溶剂,体系没有发生相分离,随着水的含量增加,体系发生了明显的相分离,产生了不同形态的胶团。本研究还通过对比不同溶剂组分下的体系介观形貌,讨论了水在体系相分离中的作用。同时通过分析模拟了1000步后体系中水的分布,证实在胶团核中存在自由水(freewater)的猜想。     

Polymer-clay nanocomposites: a multiscale molecular modeling approach

A hierarchical procedure bridging the gap between atomistic and mesoscopic simulation for polymer-clay nanocomposite (PCN) design is presented. The dissipative particle dynamics (DPD) is adopted as the mesoscopic simulation technique, and the interaction parameters of the mesoscopic model are estimated by mapping the corresponding energy values obtained from atomistic molecular dynamics (MD) simulations. The predicted structure of the nylon 6 PCN system considered is in excellent agreement with previous experimental and atomistic simulation results.     

Morphology transformation of hybrid micelles self-assembled from rod-coil block copolymer and nanoparticles

Hybrid polymeric micelles self-assembled from a mixture containing poly(γ-benzyl-L-glutamate)-block-poly(ethylene glycol) (PBLG-b-PEG) block copolymer and gold nanoparticles (AuNPs) were prepared. The effect of AuNPs on the self-assembly behavior of PBLG-b-PEG was studied both experimentally by transmission electron microscopy, scanning electron microscopy, and laser light scattering and computationally using dissipative particle dynamics (DPD) simulations. It was found that, the pure PBLG-b-PEG block copolymer self-assembles into long cylindrical micelles. By introducing AuNPs to the stock block copolymer solution, the formed aggregate morphology transforms to spherical micelles. The DPD simulation results well reproduced the morphological transformations observed in the experiments. And the simulation revealed that the main reason for the aggregate morphology transformation is the breakage of ordered packing of PBLG rods in micelle core by the added nanoparticles. Moreover, from the DPD simulations, the distribution information on nanoparticles was obtained. The nanoparticles were found to prefer to locate near the core/shell interface as well as in the core center of the micelles. The combination of experimental and simulation methods lead to a comprehensive understanding of such a complex self-assembly system.     

A new type of microemulsion containing task-special ionic liquid [NH2ebim][BF4]

The formation of microemulsions in the presence of cyclohexane, Triton X-100, n-butanol, water, and task-special ionic liquid (TSIL) (1-2-aminoethyl-3-butylimidazolium tetrafluoroborate) was studied at 25°C. The phase behavior of this ternary system was investigated. Three subregions (namely, water-in-oil phase, bicontinuous phase, and oil-in-water phase) were identified in the single-phase region by dynamic light scattering (DLS) technique and electrical conductivity measurement. Microstructures of microemulsions with different water contents have been predicted by using dissipative particle dynamics (DPD) simulation. It was found that the DPD simulations successfully reproduce the experimental results in the article. The location of TSIL in the microemulsions was predicted by DPD simulation further. The result indicates that TSIL is more easy to locate in the surfactant and cosurfactant layer and has amphiphilicity, which provides us new insights into the potential applications of TSIL-based microemulsions in separation and new nano-scale material preparation because of the interaction of TSIL with some special components at the interface of oil and water.     

Understanding Multicompartment Micelles Using Dissipative Particle Dynamics Simulation

Multicompartment micelles are a new class of nanomaterials that may find wide applications in the fields of drug delivery, nanotechnology and catalysis. Due to their structural complexity, as well as the wide parameter space to explore, experimental investigations are a difficult task, to which molecular simulation may contribute greatly. In this paper, the application of the dissipative particle dynamics simulation technique to the understanding of multicompartment micelles is introduced, illustrating that DPD is a powerful tool for identifying new morphologies by varying block length, block ratio and solvent quality in a systematic way. The formation process of multicompartment micelles, as well as shear effects and the self-assembly of nanoparticle mixtures in multicompartment micelles, can also be studied well by DPD simulation. The present work shows that DPD, as well as other simulation techniques and theories, can complement experiments greatly, not only in exploring properties in a wider parameter space, but also by giving a preview of phenomena prior to experiments. DPD, as a mesoscopic dynamic simulation technique, is particularly useful for understanding the dynamic processes of multicompartment micelles at a microscopic level.

     

Investigations of Water Morphology in Reverse Micelles: Mesoscopic Simulation and IR Spectral Analysis

Abstract

A simple model, i.e., sodium bis(2‐ethylhexyl) sulfosuccinate [Aerosol OT (AOT)] represented by one‐head and two‐tail beads tied together by a harmonic spring and water or isooctane by one bead, was put forward via dissipative particle dynamics (DPD) simulation method. According to the experimental AOT/water/isooctane system, the aggregates of simulated reverse micelle can be obtained in the three‐dimensional cell. Three types of water morphology, such as bound water, trapped water, and bulky water, were distinguished using the water isodensity slice in DPD simulation. The IR spectra experiment also showed three types of water in the same system. One conclusion is that DPD simulation can be considered as an adjunct to experiments and provide other valuable information for the experiment.     

Constant-pressure simulations with dissipative particle dynamics

Dissipative particle dynamics (DPD) is a mesoscopic simulation method for studying hydrodynamic behavior of complex fluids. Ideally, a mesoscopic model should correctly represent the thermodynamic and hydrodynamic properties of a real system beyond certain length and time scales. Traditionally defined DPD quite successfully mimics hydrodynamics but is not flexible enough to accurately describe the thermodynamics of a real system. The so-called multibody DPD (MDPD) is a pragmatic extension of the classical DPD that allows one to prescribe the thermodynamic behavior of a system with only a small performance impact. In an earlier paper [S. Y. Trofimov, E. L. F. Nies, and M. A. J. Michels, J. Chem. Phys. 117, 9383 (2002)] we much improved the accuracy of the MDPD model for strongly nonideal systems, which are of most practical interest. The ability to correctly reproduce the equation of state of realistic systems in turn makes simulations at constant pressure sensible and useful. This situation of constant-pressure conditions is very common in experimental studies of (soft) condensed matter but has so far remained unexplored with the traditional DPD. Here, as a proof of concept, we integrate a modified version of the Andersen barostat into our improved MDPD model and make an evaluation of the performance of the new model on a set of single- and multicomponent systems. The modification of the barostat suppresses the "unphysical" volume oscillations after a sudden pressure change and simplifies the equilibration of the system.     

The effects of Lowe-Andersen temperature controlling method on the polymer properties in mesoscopic simulations

Lowe-Andersen (LA) temperature controlling method [C. P. Lowe, Europhys. Lett. 47, 145 (1999)] is applied in a series of mesoscopic polymer simulations to test its validity and efficiency. The method is an alternative for dissipative particle dynamics simulation (DPD) technique which is also Galilean invariant. It shows excellent temperature control and gives correct radial distribution function as that from DPD simulation. The efficiency of LA method is compared with other typical DPD integration schemes and is proved to be moderately efficient. Moreover, we apply this approach to diblock copolymer microphase separation simulations. With LA method, we are able to reproduce all the results from the conventional DPD simulations. The calculated structure factors of the microphases are consistent with the experiments. We also study the microphase evolution dynamics with increasing chiN and find that the bath collision frequency Gamma does not affect the order of appearing phases. Although the thermostat does not affect the surface tension, the order-disorder transition (ODT) is somewhat sensitive to the values of Gamma, i.e., the ODT is nonmonotonic with increasing Gamma. The dynamic scaling law is also tested, showing that the relation obeys the Rouse theory with various Gamma.     

Mesoscale simulation of polymer reaction equilibrium: combining dissipative particle dynamics with reaction ensemble Monte Carlo. I. Polydispersed polymer systems

We present a mesoscale simulation technique, called the reaction ensemble dissipative particle dynamics (RxDPD) method, for studying reaction equilibrium of polymer systems. The RxDPD method combines elements of dissipative particle dynamics (DPD) and reaction ensemble Monte Carlo (RxMC), allowing for the determination of both static and dynamical properties of a polymer system. The RxDPD method is demonstrated by considering several simple polydispersed homopolymer systems. RxDPD can be used to predict the polydispersity due to various effects, including solvents, additives, temperature, pressure, shear, and confinement. Extensions of the method to other polymer systems are straightforward, including grafted, cross-linked polymers, and block copolymers. To simulate polydispersity, the system contains full polymer chains and a single fractional polymer chain, i.e., a polymer chain with a single fractional DPD particle. The fractional particle is coupled to the system via a coupling parameter that varies between zero (no interaction between the fractional particle and the other particles in the system) and one (full interaction between the fractional particle and the other particles in the system). The time evolution of the system is governed by the DPD equations of motion, accompanied by changes in the coupling parameter. The coupling-parameter changes are either accepted with a probability derived from the grand canonical partition function or governed by an equation of motion derived from the extended Lagrangian. The coupling-parameter changes mimic forward and reverse reaction steps, as in RxMC simulations.     

表面活性剂与聚合物相互作用的动力学模拟

用扩散颗粒动力学模拟方法（Dissipative Particle Dynamics，DPD）模拟了 中性聚合物与离子型表面活性剂的相互作用。在分子水平上研究了介于微观和宏观 上的一些性质，直观地用三维图形描绘了聚合物在表面活性剂溶液中的聚集形成， 并通过聚合物的末端的变化表征了聚集过程。结果发现：随着表面活性剂浓度的增 加，聚合物呈现自由伸缩→形成松散的棒状结构→再出现胶束状珍珠链结构→最终 在六角状和层状相中分布的过程。DPD模拟方法能够直观地得到聚合物在表面活性 剂溶液中的聚集形态。