| 1. |
Atsuomi Shundo, Mika Aoki, Pangpang Wang, Taiki Hoshino, Satoru Yamamoto, Sunao Yamada, Keiji Tanaka, Effect of a Heterogeneous Network on the Fracture Behavior of Epoxy Resins, Macromolecules, 10.1021/acs.macromol.3c00341, 56, 11, 3884-3890, 2023.05. |
| 2. |
Satoru Yamamoto, Ryoya Ida, Mika Aoki, Riichi Kuwahara, Atsuomi Shundo, Keiji Tanaka, Formation Mechanism of a Heterogeneous Network in Epoxy Resins, Macromolecules, 10.1021/acs.macromol.3c00411, 56, 11, 3913-3921, 2023.05. |
| 3. |
Yuma Morimitsu, Hisao Matsuno, Yukari Oda, Satoru Yamamoto, Keiji Tanaka, Direct Visualization of Cooperative Adsorption of a String-like Molecule onto a Solid, Science Advances, 10.1126/sciadv.abn6349, 8, 41, abn634, 2022.10. |
| 4. |
Ko Yamaguchi, Daisuke Kawaguchi, Noboru Miyata, Tsukasa Miyazaki, Hiroyuki Aoki, Satoru Yamamoto, Keiji Tanaka, Kinetics of the Interfacial Curing Reaction for an Epoxy–Amine Mixture, Physical Chemistry Chemical Physics, 10.1039/d2cp03394a, 24, 21578, 2022.09. |
| 5. |
Satoru Yamamoto, Riichi Kuwahara, Keiji Tanaka, Effects of Chemistry of Silicon Surfaces on the Curing Process and Adhesive Strength for Epoxy Resin, ACS APPLIED POLYMER MATERIALS, 10.1021/acsapm.2c00855, 4, 8, 6038-6046, 2022.08, The adhesive strength of epoxy resins is generally dependent on the surface chemistry of an adherend. Although the free space, or the nanoscopic void space, formed at the adhered interface due to the curing shrinkage is expected to have a significant impact on the adhesive strength, the molecular picture is not yet well understood. In this study, all-atom molecular dynamics simulations were used to investigate how the curing reaction and thereby adhesive strength of an epoxy resin differed on hydrophilic and hydrophobic silicon substrates. Before the reaction, a hardener of amine with a smaller molecular size was segregated at the silicon surface, and the extent became more remarkable on the hydrophilic surface with hydroxy groups that formed hydrogen bonds with amine. The epoxy resin shrank as the curing reaction proceeded, forming the overall 5-10% free space. The resin remained attached to the hydrophilic substrate, but was partly separated from the hydrophobic surface, resulting in the 15% free space in the 0.2 nm adhered interfacial region and thus a lesser contact area. Reflecting this, under tensile deformation, cohesive failure and interfacial delamination occurred for the hydrophilic and hydrophobic surfaces, respectively, under a yield stress of 200 MPa and a strain of 0.1. Our findings make it clear that the surface chemistry of an adherend was crucial for the adhesive strength of the epoxy resins via the microstructure formation at the interface.. |
| 6. |
Satoru Yamamoto, Riichi Kuwahara, Keiji Tanaka, Dynamic behaviour of water molecules in heterogeneous free space formed in an epoxy resin, Soft Matter, 10.1039/D1SM00529D, 17, 25, 6073-6080, 2021.07, Although an epoxy resin is a stable material, it absorbs moisture over a long period of time, causing deterioration of its material properties. We here applied a full-atomistic molecular dynamics (MD) simulation to study where water molecules exist in an epoxy resin and how they dynamically behave. First, the curing reaction was simulated to obtain a network structure so that the time course of the density, and thereby the free space, in the resin were obtained. The results made it possible to discuss the formation and size distribution of the free spaces which were not connected to each other. Then, a few percent of water were inserted into the free space of the cured epoxy resin to examine the location and dynamics of their molecules. We found that several water molecules were clustered at a preferred site, where hydrogen bonds can be formed with hydroxy, ether and amino groups of the network, in the free space, and they heterogeneously moved from there to other sites.. |
| 7. |
Satoru Yamamoto, Keiji Tanaka, Entropy-driven segregation in epoxy-amine systems at a copper interface, Soft Matter, 10.1039/D0SM01600D, 17, 5, 1359-1367, 2021.06, The composition of an epoxy resin at the interface with the adherend is usually different from that in the bulk due to the enrichment of a specific constituent, a characteristic called interfacial segregation. For better adhesion, it should be precisely understood how epoxy and amine molecules exist on the adherend surface and react with each other to form a three-dimensional network. In this study, the entropic factor of the segregation in a mixture of epoxy and amine at the copper interface before and after the curing reaction is discussed on the basis of a full-atomistic molecular dynamics (MD) simulation. Smaller molecules were preferentially segregated at the interface regardless of the epoxy and amine, and this segregation remained after the curing process. No segregation occurred at the interface for a combination composed of epoxy and amine molecules with a similar size. These findings make it clear that the size disparity between constituents affects the interfacial segregation via the packing and/or translational entropy. The curing reaction was slower near the interface than in the bulk, and a large amount of unreacted molecules remained there. Finally, the effect of molecular shape was also examined. Linear molecules were more likely to segregate than round-shaped ones even though they were similar in volume. We believe that these findings, which are difficult to obtain experimentally, contribute to the understanding of the interfacial adhesion phenomena on a molecular scale.. |
| 8. |
Yukari Oda, Daisuke Kawaguchi, Yuma Morimitsu, Satoru Yamamoto, Keiji Tanaka, Direct observation of morphological transition for an adsorbed single polymer chain, SCIENTIFIC REPORTS, 10.1038/s41598-020-77761-0, 10, 1, 2020.12, A better understanding of the structure of polymers at solid interfaces is crucial for designing various polymer nano-composite materials from structural materials to nanomaterials for use in industry. To this end, the first step is to obtain information on how synthetic polymer chains adsorb onto a solid surface. We closely followed the trajectory of a single polymer chain on the surface as a function of temperature using atomic force microscopy. Combining the results with a full-atomistic molecular dynamics simulation revealed that the chain became more rigid on the way to reaching a pseudo-equilibrium state, accompanied by a change in its local conformation from mainly loops to trains. This information will be useful for regulating the physical properties of polymers at the interface.. |
| 9. |
Kiminori Uchida, Kazuki Mita, Satoru Yamamoto, Keiji Tanaka, Local Orientation of Polystyrene at the Interface with Poly(methyl methacrylate) in Block Copolymer, ACS Macro Letters, 10.1021/acsmacrolett.0c00638, 9, 11, 1576-1581, 2020.11, The local conformation of polystyrene (PS) at the phase-separated lamellar interface with poly(methyl methacrylate) (PMMA) in their diblock copolymer (BCP) was examined by sum-frequency generation spectroscopy in conjunction with a full-atomistic molecular dynamics simulation. While PS phenyl groups of BCP were oriented in the interfacial region, they were random in the bulk. Such an interfacial orientation of phenyl groups was not clear for the corresponding blend of PS and PMMA. The PS backbone of BCP was in-plane oriented and folded near to the chemical junction point located in the interfacial region and the orientation became random at several nanometers distant. No evidence for the chain folding at the interface was found for the blend system.. |
| 10. |
Satoru Yamamoto, Riichi Kuwahara, Mika Aoki, Atsuomi Shundo, Keiji Tanaka, Molecular Events for an Epoxy-Amine System at a Copper Interface, ACS APPLIED POLYMER MATERIALS, 10.1021/acsapm.9b01154, 2, 4, 1474-1481, 2020.04, Epoxy is a class of thermosetting resins and has been widely used as a representative example of structural adhesives. Nevertheless, it remains unclear how the epoxy resin and curing agent are present on the adherend surface and how they move around dynamically and react with each other to form a three-dimensional network. We here adopt a fully atomistic molecular dynamics (MD) simulation to study molecular events of an epoxy resin composed of hydrogenated bisphenol A diglycidyl ether and 1,4-cyclohexanebis(methylamine) at the interface using a narrow gap, which was sandwiched between copper surfaces. The depth profiles of the density, molecular orientation, and concentration in addition to molecular diffusivity at the interface are addressed. These are finally combined with the kinetics for the curing reactions at the interface. Although some of the information here obtained is accessible by experimentation, most is not. We believe that the findings of this study will lead to a better understanding of the adhesion phenomenon.. |
| 11. |
Yoshitake Suganuma, Takuya Mitsuoka, Satoru Yamamoto, Tomoyuki Kinjo, Hiroaki Yoneyama, Kazuhiko Umemoto, Wettability of Primer-Treated Al2O3 Surfaces by Bisphenol A Diglycidyl Ether
Determination of the Mechanism from Molecular Dynamics Simulations and Experiments, Journal of Physical Chemistry B, 10.1021/acs.jpcb.9b00680, 123, 20, 4434-4442, 2019.05, This study aims to develop a molecular dynamics (MD) simulation procedure to investigate the wettability of primer-treated Al2O3 surfaces by bisphenol A diglycidyl ether (BADGE) and to understand the interaction between the surface and the liquid. The MD simulation results were compared with those obtained by contact angle measurements, time-of-flight secondary ion mass spectrometry (TOF-SIMS), and atomic force microscopy (AFM) and were found to be in agreement with the experimental evaluations. The results obtained from both the MD simulations and the experiments suggest that the configuration of the primers on the surface affect its wettability. In other words, silanes lying flat on the surface, such as mercapto silane, make it easy for BADGE to access any polar functional groups of the silane, thereby leading to a strong interaction and good wettability. For amino silane, although the configuration is similar to that of mercapto silane, its amino groups are bound to the surface owing to their high polarity, which results in a reduced accessibility for BADGE and a relatively poor wettability in comparison with mercapto silane. On the contrary, for silanes that stand up on the surface, including trifluoroalkyl silane, BADGE is hindered from approaching the silanol groups and interacting with them, and the surface shows poor wettability.. |
| 12. |
Mika Aoki, Atsuomi Shundo, Riichi Kuwahara, Satoru Yamamoto, Keiji Tanaka, Mesoscopic Heterogeneity in the Curing Process of an Epoxy-Amine System, Macromolecules, 10.1021/acs.macromol.8b02416, 52, 5, 2075-2082, 2019.03, Epoxy resins are composed of a three-dimensional network formed by chemical reactions between epoxy and amino compounds, which plays an important role in the mechanical properties. Thus, to use epoxy resins in various applications, it is necessary to gain a better understanding of their network structure. Here, we study the structural heterogeneity evolved in an epoxy-amine mixture during the curing process on the basis of a particle tracking technique, in which the thermal motion of probe particles in the mixture was tracked, small-angle X-ray scattering measurements in conjunction with coarse-grained molecular dynamics simulation. The heterogeneous environment was generated even at the initial stage of the curing process. Notably, the characteristic length scale was on the order of several hundreds of nanometers down to several tens of nanometers, depending on the extent of curing. Once a reaction occurs between a pair of epoxy and amino groups, the temperature at the site is locally elevated due to the heat of formation, accelerating a subsequent reaction nearby. Repeating such a situation, actively and scarcely reacted domains are formed. This is the main origin of the structural heterogeneity in epoxy resins.. |
| 13. |
Kentaro Yamamoto, Daisuke Kawaguchi, Kazuki Sasahara, Manabu Inutsuka, Satoru Yamamoto, Kiminori Uchida, Kazuki Mita, Hiroki Ogawa, Mikihito Takenaka, Keiji Tanaka, Aggregation States of Poly(4-methylpentene-1) at a Solid Interface, Polymer Journal, 10.1038/s41428-018-0134-7, 51, 2, 247-255, 2019.02, A thin film of poly(4-methylpentene-1) (P4MP1) was prepared on a quartz substrate, which was a model system of an interface in filler-reinforced semicrystalline polymer composites. Grazing-incidence wide-angle X-ray diffraction measurements revealed that P4MP1 in the thin film after isothermal crystallization formed a Form I crystal polymorph composed of a tetragonal unit cell with a 72 helix, in which the chain axis was oriented along the direction parallel to the quartz interface. Combining sum-frequency generation vibrational spectroscopy with molecular dynamics simulation enabled us to gain access to the local conformation of P4MP1 chains at the quartz interface and the changes that occurred with isothermal crystallization. Finally, the way in which the initial chain orientation at the substrate interface impacted the crystalline structure in the thin film was discussed.. |
| 14. |
Yoshitake Suganuma, Satoru Yamamoto, Tomoyuki Kinjo, Takuya Mitsuoka, Kazuhiko Umemoto, Wettability of Al2O3 Surface by Organic Molecules
Insights from Molecular Dynamics Simulation, Journal of Physical Chemistry B, 10.1021/acs.jpcb.7b07062, 121, 42, 9929-9935, 2017.10, We use molecular dynamics (MD) simulations to investigate the wettability of Al2O3 (0001) by organic molecules. Diffusion coefficients estimated for organic molecules are clearly correlated with the contact angles observed experimentally. The results of the MD simulations suggest that molecular flexibility influences wettability. In other words, wettability owing to flexible molecules, such as an epoxy tridecamer, improves with increasing temperature because the interaction between the droplet and the surface increases due to changes in molecular conformation. Conversely, for phenylene sulfide tetramer, wettability does not change with temperature because of the molecular rigidity. In addition, for epoxy monomers, we analyze the different molecular structures responsible for modifying the droplet-surface interaction. For hydrogens in aromatic rings and in methyl groups, the interaction with the surface clearly decreases with increasing temperature. [Figure presented]. |
| 15. |
Satoru Yamamoto, Taku Ozawa, Kosuke Ohata, Analysis of relaxation mechanism of thread-like micelle solution, Computer Simulation of Polymeric Materials
Applications of the OCTA System, 10.1007/978-981-10-0815-3_25, 347-358, 2016.01, Dissipative particle dynamics is applied to study the crossing dynamics at an entanglement point of surfactant thread-like micelles in an aqueous solution. This chapter investigates the possibility of a phantom crossing, which is the relaxation mechanism for the pronounced viscoelastic behavior of a surfactant thread-like micellar solution. When two thread-like micelles are encountered at an entanglement point under a condition close to thermal equilibrium, they fuse to form a four-armed branch point. Then, a phantom crossing reaction occurs occasionally, or one micelle is cut down at the branch point. When increasing the repulsive forces between hydrophilic parts of the surfactants, fusion occurs less and the thread-like micelle is frequently broken down at an entanglement point. In these three schemes (i.e., a phantom crossing, a cut at the branch point, and a break at the entanglement point), the breakage occurs somewhere along the thread-like micelle. The breakage is considered as an essential process in the relaxation mechanism, and a phantom crossing can be seen as a special case of these processes. To explain the experimental evidence that a terminal of thread-like micelles is scarcely observed, a mechanism is also proposed where the generated terminal merges into the connected micelle part between two entanglement points owing to thermal motion.. |
| 16. |
Satoru Yamamoto, Taku Ozawa, Electrolyte membranes, Computer Simulation of Polymeric Materials
Applications of the OCTA System, 10.1007/978-981-10-0815-3_28, 369-377, 2016.01, The mesoscale structure of a hydrated electrolyte membrane (Nafion) in polymer electrolyte fuel cells is studied using dissipative particle dynamics (DPD). A polymer is modeled by coarse-grained particles, and interaction parameters for particles are estimated from the cohesive energy for each chemical species according to an atomistic simulation. In the dissipative particle dynamics simulation, a spongelike structure of the hydrophobic phase of the Nafion backbone spontaneously emerges from the initial condition of random dispersion; this structure matches that observed experimentally. The cluster size and its dependence on the water content are also in good agreement with experimental results for small-angle X-ray scattering. In addition, by considering molecular rigidity and shear flow, the current model explains the anisotropy in the swelling behavior observed in hydrocarbon-based electrolyte membranes in which the main chain skeletons are rigid.. |
| 17. |
Satoru Yamamoto, Taku Ozawa, Vesicle formation, Computer Simulation of Polymeric Materials
Applications of the OCTA System, 10.1007/978-981-10-0815-3_26, 359-368, 2016.01, In this chapter, the spontaneous vesicle formation of amphiphilic molecules in an aqueous solution is studied employing dissipative particle dynamics simulation. The amphiphilic molecule is represented by a coarse-grained model, which contains a hydrophilic head group and a hydrophobic tail group. Water is modeled by particles in the same way, with one particle representing a group of several H2O molecules. In the dissipative particle dynamics simulation, starting with either a randomly dispersed system or a bilayer structure of the amphiphile for the initial condition, spontaneous vesicle formation via an intermediate state of an oblate micelle or a bilayer membrane is observed. The membrane fluctuates and encapsulates water particles and then closes to form a vesicle. During the process of vesicle formation, the energy of the hydrophobic interaction between the amphiphile and water reduces. Furthermore, it is found that the aggregation process is faster for two-tailed amphiphiles than for single-tailed amphiphiles.. |
| 18. |
Satoru Fujita, Kazuya Kamazawa, Satoru Yamamoto, Madhusudan Tyagi, Toru Araki, Jun Sugiyama, Naoki Hasegawa, Masaya Kawasumi, Proton conductivity under dry conditions for mesoporous silica with highly dense sulfonic acid groups, Journal of Physical Chemistry C, 10.1021/jp307058s, 117, 17, 8727-8736, 2013.05, Highly dense sulfonic acid-functionalized mesoporous electrolytes with high proton conductivity under dry conditions were prepared using tetramethoxysilane and 3-mercaptopropyltrimethoxysilane in the presence of surfactants. Impedance spectroscopy and quasielastic neutron scattering measurements showed that the proton conductive properties of the mesoporous electrolytes depended significantly on the sulfonic acid densities on the surface of the mesoporous walls. This finding was also supported by molecular dynamics simulations. The proton conductivity of the mesoporous electrolyte with the highest acid density of 3.1 SO3H molecules/nm2 showed a value of 0.3 mS/cm at 433 K even under dry conditions. This value was higher than that for Nafion by about 2 orders of magnitude. Such high proton conductivity is thought to be induced by proton hopping in the hydrogen-bonded networks that were predominantly formed by neighboring sulfonic acid groups.. |
| 19. |
Kazuhiro Fukumoto, Satoru Yamamoto, Kenichirou Suzuki, Adsorption of trimethylamine by mesoporous silica, kagaku kogaku ronbunshu, 10.1252/kakoronbunshu.38.221, 38, 4, 221-225, 2012.08, The capacity of FSM-16 to adsorb trimethylamine and the adsorption mechanism were investigated by experiment and simulation. Highly mesoporous FSM-16 was found to have much higher adsorptivity of trimethylamine than silica gel and activated carbon. The adsorption equilibrium of trimethylamine to these adsorbents was described by a Langmuir-type isotherm. At saturation, the amount of trimethylamine adsorbed by FSM-16 and silica gel increased in proportion to their specific surface area, and all the experimental data for these adsorbents were on the same line. The results indicate that the specific surface area of FSM-16 and silica gel is the most important factor regulating their capacity to adsorb trimethylamine. Further, the adsorption equilibrium constant of the Langmuir parameters suggested that silica-based adsorbents with ca. 2.5-2.7 nm pore size are the most suitable for trimethylamine removal. The results obtained by computer simulation were consistent with experimental ones, and suggested that the adsorption mechanism was dominated by hydrogen bonding.. |
| 20. |
Satoru Yamamoto, Shi Aki Hyodo, Mesoscopic simulation of the crossing dynamics at an entanglement point of surfactant threadlike micelles, Journal of Chemical Physics, 10.1063/1.1914767, 122, 20, 2005.05, The crossing dynamics at an entanglement point of surfactant threadlike micelles in an aqueous solution was studied using a mesoscopic simulation method, dissipative particle dynamics, with a coarse-grained surfactant model. The possibility of a phantom crossing, which is the relaxation mechanism for the pronounced viscoelastic behavior of surfactant threadlike micellar solution, was investigated. When two threadlike micelles were encountered at an entanglement point under the condition close to thermal equilibrium, they fused to form a four-armed branch point. Then, a phantom crossing reaction occurred occasionally, or one micelle was cut down at the branch point. Increasing the repulsive forces between hydrophilic parts of the surfactants, fusion occurred less and the threadlike micelle was frequently broken down at an entanglement point. In these three schemes (a phantom crossing cut down at the branch point, and break down at the entanglement point), the breakage occurs at somewhere along the threadlike micelle. The breakage is considered as an essential process in the relaxation mechanism, and a phantom crossing can be seen as a special case of these processes. To explain the experimental evidence that a terminal of threadlike micelles is scarcely observed, a mechanism was also proposed where the generated terminal merges into the connected micelle part between two entanglement points due to the thermal motion.. |
