Formation of Vesicles from Amphiphilic Random Copolymers in Solution: A Dissipative Particle Dynamics Simulation Study

Five coarse-grained models were built for amphiphilic random copolymers. The self-assembly of amphiphilic random copolymers in selective solvent was investigated via dissipative particle dynamics simulations. The simulation results showed that the content of hydrophilic particles and the repulsive parameter between solvent and copolymer particles were two key factors of the vesicle formation. We report herein on how to control the self-assembled morphology evolution. The two mechanisms of vesicle formation from amphiphilic random copolymers are found through investigating the dynamic processes of vesicle formation, which is in accordance with the experiment and simulation results of amphiphilic block copolymer reported in the literature.      

Micellar polymerization: Computer simulations by dissipative particle dynamics

Nowadays, micellar polymerization is widely used in different fields of industry and research, including modern living polymerization technique. However, this process has many variables and there is no comprehensive model to describe all features. This research presents simulation methodology which describes key properties of such reactions to take a guide through a variety of their modifications. Dissipative particle dynamics is used in addition to Monte Carlo scheme to simulate initiation, propagation, and termination events. Influence of initiation probability and different termination processes on final conversion and molecular‐weight distribution are presented. We demonstrate that prolonged initiation leads to increasing in polymer average molecular weight, and surface termination events play major role in conversion limitation, in comparison with recombination. © 2018 Wiley Periodicals, Inc.     

Dissipative particle dynamics simulation of gold nanoparticles stabilization by PEO–PPO–PEO block copolymer micelles

Dissipative particle dynamics (DPD) was used to simulate the formation and stabilization of gold nanoparticles in poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) block copolymer micelles. Primary gold clusters that were experimentally observed in the early stage of gold nanoparticle formation were modeled as gold bead in DPD simulation. It showed that gold beads were wrapped by the block copolymer and aggregated into spherical particles inside the micelles and forming stable Pluronic–gold colloids with two-layer structures. Increasing Pluronic concentration, molecular weight, and PPO block length led to the formation of more uniform and more stable gold nanoparticles. Density profiles of water beads suggested that the micelles, especially the hydrophobicity of the micellar cores, played an important role in stabilizing gold nanoparticles. Dynamic process indicated that the formation of gold nanoparticles was controlled by the competition between aggregation of primary gold clusters and the stabilization by micelles of block copolymers.. The DPD simulation results of gold–copolymer–water system agree well with previous experiments, while more structure information on microscopic level could be provided.     

Conserving the linear momentum in stochastic dynamics: Dissipative particle dynamics as a general strategy to achieve local thermostatization in molecular dynamics simulations

Stochastic dynamics is a widely employed strategy to achieve local thermostatization in molecular dynamics simulation studies; however, it suffers from an inherent violation of momentum conservation. Although this short‐coming has little impact on structural and short‐time dynamic properties, it can be shown that dynamics in the long‐time limit such as diffusion is strongly dependent on the respective thermostat setting. Application of the methodically similar dissipative particle dynamics (DPD) provides a simple, effective strategy to ensure the advantages of local, stochastic thermostatization while at the same time the linear momentum of the system remains conserved. In this work, the key parameters to employ the DPD thermostats in the framework of periodic boundary conditions are investigated, in particular the dependence of the system properties on the size of the DPD‐region as well as the treatment of forces near the cutoff. Structural and dynamical data for light and heavy water as well as a Lennard–Jones fluid have been compared to simulations executed via stochastic dynamics as well as via use of the widely employed Nose–Hoover chain and Berendsen thermostats. It is demonstrated that a small size of the DPD region is sufficient to achieve local thermalization, while at the same time artifacts in the self‐diffusion characteristic for stochastic dynamics are eliminated. © 2016 Wiley Periodicals, Inc.     

Self-assembly behaviors of zwitterionic heterogemini surfactant at an oil-water interface: A dissipative particle dynamics study

The self-assembly behaviors of a battery of zwitterionic heterogemini surfactants C_mH_2m+1-PO^4--(CH₂)₂-N⁺(CH₃)₂-C_nH_2n+1, abbreviated as C_m-P-N-C_n (m, n?=?9, 9; 9, 12; 9, 15; 9, 18; 12, 12; 12, 15; 12, 18; 15, 15; 15, 18; 18, 18), have been explored at an oil-water interface by means of the dissipative particle dynamics (DPD) method. Regular oil-water contact together with oil-in-water and water-in-oil emulsions has come into being. Compared with the C_m-P-N-C_n concentration and the oil-water ratio, the hydrophobic chain length plays a less important role in the self-assembly morphology of C_m-P-N-C_n molecules at the interface together with the interfacial morphology. The asymmetry in the molecular structure of C_m-P-N-C_n dominates its critical micelle concentration (CMC) and interfacial efficiency. The C_m-P-N-C_n concentration and its hydrophobic chain length work together to affect the interfacial thickness. What’s more, the dependence of CMC on the C_m-P-N-C_n molecular structure is in qualitative agreement with corresponding experimental findings.     

PEO—PPO—PEO三嵌段共聚物自组装行为的耗散粒子动力学模拟

采用耗散粒子动力学方法（DPD）,模拟了聚氧乙烯-聚氧丙烯-聚氧乙烯（PEO—PPO—PEO）三嵌段共聚物在乙醇溶液中的自组装行为,考察了该共聚物的体积分数和聚氧乙烯（PEO）嵌段链长对介观形貌的影响。当F88（PEO104-PPO39-PEO104）体积分数为20％时,胶柬由初始的均衡分散态逐渐聚合,最终形成PPO为核、PEO为壳的平衡态柱状团聚体。改变共聚物的体积分数和PEO链的长度,会形成不同的介观结构,如：球状、柱状、立体网络、层状和穿孔状结构等。结果表明,DPD方法是研究三嵌段共聚物自组装行为和介观结构形成机理的有效工具,对合成具有特定结构性能的材料有一定的指导意义。     

Nanostructure stability and swelling of ternary block copolymer/homopolymer blends: A direct comparison between dissipative particle dynamics and experiment

Ternary block copolymer (BCP)‐homopolymer (HP) blends offer a simple method for tuning nanostructure sizes to meet application‐specific demands. Comprehensive dissipative particle dynamic (DPD) simulations were performed to study the impact of polymer interactions, molecular weight, and HP volume fraction (φ_HP ) on symmetric ternary blend morphological stability and domain spacing. DPD reproduces key features of the experimental phase diagram, including lamellar domain swelling with increasing φ_HP , the formation of an asymmetric bicontinuous microemulsion at a critical HP concentration ${\phi }_{\mathrm{HP}}^{*}$, and macrophase separation with further HP addition. Simulation results matched experimental values for ${\phi }_{\mathrm{HP}}^{*}$ and lamellar swelling as a function of HP to BCP chain length ratio, α = N_HP/N_BCP . Structural analysis of blends with fixed φ_HP but varying α confirmed that ternary blends follow the wet/dry brush model of domain swelling with the miscibility of HPs and BCPs depending on α . Longer HPs concentrate in the center of domains, boosting their swelling efficiencies compared to shorter chains. These results advance our understanding of BCP‐HP blend phase behavior and demonstrate the value of DPD for studying polymeric blends. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 794–803     

Mesoscopic simulation of self-assembly in surfactant oligomers by dissipative particle dynamics

Self-assembling properties of surfactant oligomers in an aqueous medium is simulated by dissipative particle dynamics (DPD). The critical micellar concentration (CMC) of dimeric (oligomerization = 2) and trimeric (oligomerization = 3) surfactant is much lower than their single-chain counterpart. All surfactants form spherical micelles at the concentration not far above their CMC. The transition from spherical to cylindrical micelles exhibits with increasing surfactant concentration. Lamellar micelles will appear with further increasing the surfactant concentration. For dimeric and trimeric surfactants, cylindrical micelles transform into extremely long “wormlike” or “threadlike” micelles before the transition to lamellar micelles. These results are in qualitative agreement with laboratory experiment. Average aggregation numbers (AN) of micelles increase with a power law of AN  c when the surfactant concentration c CMC. The self-diffusion coefficients will drop with a power law of D  c⁻ when wormlike micelles are formed.     

Dissipative Particle Dynamics Simulations on the Self-Assembly of New Segmented Random-Block Copolymers in Selective Solvents

Dissipative particle dynamics simulation was applied to investigate the self-assembly of new segmented random-block copolymers (A-co-B)-b-B in selective solvents. The coarse-grained models of random-block copolymers were built. The effects of the composition of the copolymer, the interaction parameters, the volume fractions, as well as the ratio of hydrophilic particles to hydrophobic particles in the random segment, on the morphology of the aggregations were systematically investigated. The results showed that new segmented random-block copolymers (A-co-B)-b-B could self-assemble into rodlike micelle, spherical micelle and two-compartment micelle. Oval-shaped micelle and spherical micelle could be formed at different volume fractions. The simulation results provide an important reference to the design and synthesis of specific micelles.     

Phase Behavior of Amphiphilic Molecules in the Presence of One Solvent: A Dissipative Particle Dynamics Study

We simulate the phase behavior of amphiphilic molecules in the presence of one solvent by DPD. In general, DPD has successfully captured most of the effects of composition of amphiphilic molecules, solvent selectivity, and solvent amount, on the phase transition behavior obtained by both SCMF calculations and experiments. When a neutral good solvent is added, the solutions undergo a lyotropic transition analogous to the thermotropic transition in the melts. Furthermore, the order‐disorder transition results obtained via DPD are in good agreement with theoretical predictions by including the fluctuation effects, as well as with experiments. In the selective solvents, various transitions from the “normal” phases (i.e., the minority blocks form the minor domains) to even the “inverted” phases (formed by the majority blocks) have been observed by varying solvent selectivity and solvent amount. Since the packing order of the spheres is greatly affected by the finite size of the simulation box, it becomes difficult to examine the most stable packing array of spheres via DPD as has been predicted by SCMF theory. However, DPD reveals a possible spherical order of A15, which has been ignored in current SCMF work but observed in some amphiphilic molecule systems.

     

Self-Assembly Behaviors of Heterogemini Surfactant in Aqueous Solution Investigated by Dissipative Particle Dynamics

As a preliminary study, self-assembly behaviors of heterogemini surfactant in aqueous solution are explored tentatively by means of dissipative particle dynamics simulation. Five kinds of heterogemini molecules are involved, and a variety of novel morphologies have been obtained. Results based on detailed comparisons among themselves and with traditional symmetric gemini surfactant show the proportion of hydrophilic to hydrophobic chain lengths in one monomer is the most important, more difference between proportions in the two monomers can induce more diverse self-assembly morphologies. The second important is the hydrophilic chain length, in which a small change can lead to obvious difference in self-assembly behaviors. While the length of hydrophobic chain has a less important influence, only the concentration for self-assembly morphologies appearing can be affected by its change. The microscopic morphology of heterogemini surfactant in its self-assembly structure can be embodied through its radius of gyration. Our simulation results can undoubtedly provide a theoretical guide to further research towards self-assembly behaviors of heterogemini surfactants and practical applications of these matters.     

嵌段共聚物组成对薄膜形貌影响的多体耗散粒子动力学研究

利用多体耗散粒子动力学(Multibody Dissipative Particle Dynamics, Multibody DPD)方法研究了在溶剂蒸发条件下, 嵌段共聚物在表面自组装形成薄膜的过程, 分别考虑了两嵌段共聚物和三嵌段共聚物及不同组成对薄膜形貌的影响. 模拟得到了无序状薄膜和层状薄膜, 并计算了这些薄膜的序参量和薄膜厚度随时间的演化. 结果表明, 嵌段共聚物的组成对薄膜厚度几乎没有影响, 当某种组分的链段很短时, 只能形成序参量较小的无序薄膜, 相反, 则可以得到序参量较大的层状薄膜.     

Dissipative particle dynamics simulation study on the binary mixture phase separation coupled with polymerization

The influence of polymerization on the phase separation of binary immiscible mixtures has been investigated by the dissipative particle dynamics simulations in two dimensions. During polymerization, the bulk viscosity increases, which consequently slows down the spinodal decomposition process. The domain size growth is monitored in the simulations. The absence of 23 exponent for inertial hydrodynamic mechanism clearly reflects the suppressing effect of polymerization on the phase separation. Due to the increasing viscosity, the individual phase may be trapped in a metastable stage instead of the lamellar morphology identified for symmetric mixtures. Moreover, the polymerization induced phase separation in the binary miscible mixture has been studied. The domain growth is strongly dependent on the polymerization probability, which is naturally related to the activation energy for polymerization. The observed complex phase separation behavior is attributed to the interplay between the increasing thermodynamic driving force for phase separation and the increasing viscosity that suppresses phase separation as the polymerization proceeds.     

梳状-线性共聚物自组装的耗散粒子动力学模拟

梳状-线性共聚物在选择性溶剂中可以自组装形成两种不同类型的聚集体, 其中第I类的自组装发生在亲、疏溶剂链之间, 而第II类发生在线性链和梳状亚结构之间. 本工作利用耗散粒子动力学方法, 分别研究了梳状-线性共聚物在侧链和主链选择性溶剂中形成的这两类聚集体, 探讨了侧链长度和侧链数量等对聚集体类型及形貌的影响. 研究表明, 第II类聚集体在侧链长度较短且侧链数量较多时容易形成. 将模拟结果与文献报道的实验结果相比较, 发现两者能较好地吻合. 此外, 本研究获得了一些在实验中较难得到的信息, 有助于进一步理解梳状-线性共聚物的自组装行为.     

Dissipative Particle Dynamics Simulations of Polymer Brushes: Comparison with Molecular Dynamics Simulations

Summary: The structure of polymer brushes is investigated by dissipative particle dynamics (DPD) simulations that include explicit solvent particles. With an appropriate choice of the DPD interaction parameters , we obtain good agreement with previous molecular dynamics (MD) results where the good solvent behavior has been modeled by an effective Lennard–Jones potential. The present results confirm that DPD simulation techniques can be applied for large length scale simulations of polymer brushes. A relation between the different length scales and is established.

Polymer brush at a solid–liquid interface.     

Dissipative particle dynamic simulation study of lipid membrane

The lipid membrane plays crucial roles in countless biologic processes, ranging from cell motility, endo- and exocytosis, and cell division to protein aggregation and trafficking. To gain a molecular insight in these biologic processes, the recently developed mesoscale simulation technique, dissipative particle dynamics (DPD) simulation, has become an invaluable tool. By providing a brief survey of existing atomistic and popular coarse-grained models used today in studying the dynamics (including vesicle formation and (protein-mediated) vesicle fusion) and phase behavior of lipid bilayers, this review illustrates how mesoscopic DPD models can be used to obtain a better understanding of these biologic processes currently inaccessible to atomistic and most coarse-grained models.     

Coil-to-globule transition by dissipative particle dynamics simulation

The dynamics of a collapsing polymer under a temperature quench in dilute solution is investigated by dissipative particles dynamics. Hydrodynamic interactions and many-body interaction are preserved naturally by incorporating explicit solvent particles in this approach. Our simulation suggests a four-stage collapse pathway: localized clusters formation, cluster coarsening in situ, coarsening involving global backbone conformation change into a crumpled globule, and compaction of the globule. For all the quench depths and chain lengths used in our study, collapse proceeds without the chain getting trapped in a metastable "sausage" configuration, as reported in some earlier studies. We obtain the time scales for each of the first three stages, as well as its scaling with the quench depths ξ and chain lengths N. The total collapse time scales as τ(c) ～ ξ(-0.46 ± 0.04)N(0.98 ± 0.09), with the quench depth and degree of polymerization.     

Molecular dynamics study of polymer crystallization in the presence of a particle

We have used molecular dynamics simulations with a coarse‐grained model to study the effect of a particle on the crystallization of polymer melt. We analyzed in particular a bond order parameter to characterize the nucleation and crystallization process. Our calculations show that the presence of a particle modifies the free energy landscape of polymer melts, locally induces the ordering of polymer melts near the particle surface, and thus enhances the polymer crystallization. Because the interaction between the particle and polymers is repulsive, our results suggest that the origin of the enhancement for polymer crystallization is entropic. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2161–2166, 2007     

Dissipative Particle Dynamics Study of the Formation of Multicompartment Micelles from ABC Star Triblock Copolymers in Water

Summary: Dissipative particle dynamics simulation was performed to study the formation of multicompartment micelles from ABC star triblock copolymers in water. The study revealed some new morphologies that had not been observed before and also provided a direct visualization of the evolution of wormlike multicompartment micelles that follows the fusion process. Thus, this work provides molecular understanding of multicompartment micelles which will be useful for the future rational synthesis of novel micelles.

Multicompartment micelles formed from ABC star triblock copolymers in water by DPD simulations.