Published papers

A micelle is the self-assembly of surfactant molecules formed in surfactant aqueous solution. The micelle has various forms such as a threadlike, spherical, discoid, and bilayer. Particularly, the threadlike micelle has the great possibility of the application in the industry, utilizing the uniqueness in its formation. However, an overall understanding of the formation dynamics of the threadlike micelle has not been accomplished, because of the so-called 'mesoscopic problem' in dynamics. In this paper to clarify the multiscale dynamics in the threadlike micelles we adopted the Dissipative Particle Dynamics (DPD) simulations. Moreover, the shear stress was put on a similar system, and it comnpared it without shear stress.

A micelle is the self-assembly of surfactant molecules formed in surfactant aqueous solution. The micelle has various forms such as a threadlike, spherical, discoid, and bilayer. Particularly, the threadlike micelle has the great possibility of the application in the industry, utilizing the uniqueness in its formation. However, an overall understanding of the formation dynamics of the threadlike micelle has not been accomplished, because of the so-called 'mesoscopic problem' in dynamics. In this paper to clarify the multiscale dynamics in the threadlike micelles we adopted the Dissipative Particle Dynamics (DPD) simulations. Moreover, the shear stress was put on a similar system, and it comnpared it without shear stress.)

A coarse-grained model of peptide amphiphiles (PA) dissolved in aqueous solution was presented, where the effects of PA concentration, temperature and shear stress upon the selfassembly of PA were numerically studied by dissipative particle dynamics (DPD) simulation. We technically investigate the repulsion parameter ahw which indicates the repulsion force between the hydrophilic head of PA and water molecules, hence, at the same time, indicating the change in temperature. It was found that ahw played an important role in the self-assembly dynamics and in the resulting micro-structures of PA. By imposing shear strain on the simulation system, the formation of wormlike PA micelles was accelerated. The simulation results were in good agreement with our previous experimental results and the mechanism of shear-induced transition was proposed. © 2009 Materials Research Society.

We used molecular simulation to study the static behavior of polymer droplets in vacuum and on solid surfaces, namely the size of the droplet and the contact angle, respectively. The effects of the polymer chain length and the total number of particles were calculated by the many-body dissipative particle dynamics method. For the spherical droplet containing the same number of particles, we show that its radius depends on the polymer chain length. The radius of the droplet is also proportional to one-third power of the total number of particles for all given chain lengths. For the hemispherical droplet, the contact angle increases with the number of particles in the droplet, and this effect is relatively strong, especially for longer polymer chains. The effect of wettability of the solid surface was also investigated by using polymerphobic (low-affinity) and polymerphilic (high-affinity) surfaces. As the chain length increases, the contact angle on the low-affinity surface decreases, while that on the hydrophilic surface increases. The simulation reveals that there is a critical affinity for the monomer on the solid surface; above and below which the wettability increases and decreases as the molecular length increases, respectively.

© 2014 Taylor and Francis. A telechelic polymer is an amphiphilic polymer that can form micellar structures when dissolved in water. A telechelic polymer solution shows viscoelastic behaviour owing to the formation of characteristic networks, i.e. loops, bridges and dangling chains. For industrial purposes, telechelic polymers have many applications as thickening agents, such as in paints and cosmetics. Thus, it is desirable to predict and control the rheological properties of telechelic polymers. However, detailed studies at the molecular level have not yet been performed. In this study, I use the dissipative particle dynamics (DPD) method to investigate the relationship between the characteristic structural properties and the molecular structure in telechelic polymer solutions. I show that the morphology of telechelic polymer solutions depends on the concentration and chain length, the distribution of the end-to-end distance, the mean square end-to-end distance, the mean square radius of gyration and the time-averaged mean square displacement. Although an effect of entanglement is important for properties of polymer melts, the polymer chain composed of DPD particles cannot reproduce it. Therefore, I compare telechelic polymer solutions with and without the segmental repulsive potential (SRP), which can simulate the effect of entanglement in DPD simulations. The results indicate that it is necessary to include the SRP in DPD simulations to correctly analyse the behaviour of telechelic polymer solutions.

分子シミュレーション研究会発足20周年を記念し，この20年間の研究会に関するいくつかのデータをまとめた．具体的には，研究会，討論会，夏の学校，分子シミュレーションスクール，アンサンブルに関するデータを報告する．なお各種データは，2002年までは河村雄行先生がまとめた分子シミュレーションのあゆみを参照し，その後は事務局（三上益弘先生，岡崎進先生，田中秀樹先生，泰岡）に保管されているデータを参照した．

A telechelic polymer is a prepolymer capable of entering into further polymerization or other reactions through its reactive end-groups. A telechelic polymer solution shows viscoelastic behavior due to the formation of a characteristic network, i.e. loop, bridging chain, and dangling chain. By our personal appearance, it is used for paint, cosmetics, etc. as a thickening agent. Therefore, there is a huge demand for industrial applications. However, detailed studies in the molecular level have not been carried out yet. In this study, we adopted a dissipative particle dynamics simulation to clarify the relation the characteristic properties and the molecular structure in telechelic polymer solutions.

A telechelic polymer is an amphiphilic polymer which can form micellar structures when dissolved in water. A telechelic polymer solution shows viscoelastic behavior due to the formation of a characteristic network, i.e. loop, bridge, and dangling chain. For industrial purposes, it has been utilized for paint, cosmetics, etc. as a thickening agent. It is thus strongly demanding to predict or control its rheological properties. However, detailed studies in the molecular level have not been clarified out yet. In this study, we adopted a dissipative particle dynamics simulation method to investigate the relation between the characteristic rheological properties and the molecular structure in telechelic polymer solutions.

A self-assembly research is currently one of the most studied branches of materials chemistry, therefore, has gained a lot of attention due to a variety of potential applications in electronics, engineering, biomedical, and optical fields. So far, many previous studies have been reported to investigate the self-assembly of surfactant for developing various functional materials. In this report, we focused on the chemical nature of the wall surface as a critical parameter which can drastically change the self-assembled structure. We have studied the rheological properties of surfactant solutions in the distinct chemical nanotubes using a computer simulation. We found that the difference in the viscosity is able to obtain depending on the chemical nature of the wall surface even if systems without the surfactant molecules. With the surfactant molecules, the characteristic viscosity behaviors were observed with rich steady-state morphologies. For example, although the concentration of surfactant is the same (c = 30%), completely different viscosity behaviors were seen in hydrophilic (shear-thinning) and hydrophobic (shear-thickening) nanotubes. Our simulation offers a guide to control the rheological properties of surfactant aqueous solutions by the chemical nature of the wall surface, the effect of confinement, concentration of surfactant aqueous solution and the self-assembled structure.

In recent years, Janus amphiphilic nanotubes with complex surfaces have been synthesized. However, the self-assembly behavior of surfactant solutions confined in a Janus amphiphilic nanotube has not been investigated so far. We performed molecular simulations to investigate the morphologies and phase diagrams of a surfactant confined in Janus amphiphilic nanotube consisting of both hydrophobic and hydrophilic surfaces. We derived a phase diagram of the representative snapshots of equilibrium morphologies. Morphologies that were not observed in the bulk eventuated in confined systems. Moreover, the self-assembled structures were found to be dependent on the spatial confinement. Furthermore, the self-assembled structures confined in hydroneutral and Janus amphiphilic nanotubes were compared. The results suggested that the self-assembled structures confined in the Janus amphiphilic nanotube resembled that confined in the hydroneutral nanotube owing to a strong confinement effect. Our simulation offers a guide to control the rheological properties of surfactant solutions by the chemical patterns on the surfaces of nanochannels, the effect of confinement, and the self-assembled structure.

The morphology of graphene on Si and SiC substrates is investigated using molecular-dynamics simulation. The effects of the size and orientation of graphene on its roughness, distance from the substrate, and periodic structure are examined. The roughness and distance show the size dependency which agrees with the size dependency of the ratio of the periphery length of graphene to its area. It is found there are some cases in which the roughness of graphene can be suppressed. © 2012 Materials Research Society.