記述言語：日本語   出版者・発行元：一般社団法人 日本レオロジー学会  

We have been intensively studying the network structure formation and rheology of thermosetting polymer epoxy resins by molecular dynamics simulations. Experiments indicated the existence of network structure heterogeneity in epoxy resins depending on the curing temperature, and a detailed rheological molecular picture of this heterogeneity was revealed. We have been also studying the effects of different chemical properties of the adherend surfaces on the curing shrinkage process and adhesive strength. The mechanism of the formation of nanoscale voids (free spaces) where water molecules can enter during the curing shrinkage process has been elucidated. Furthermore, it was found that the absorbed waters diffuse between the free spaces in a hopping manner. In addition, the effects of aggregation state and adsorbed water at the interface with amorphous silica and alumina substrates on the curing process were also investigated.

DOI： 10.1678/rheology.52.275

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CiNii Research

出版者・発行元：Polymer Chemistry  

The aggregation states of polymer chains at solid interfaces are strongly related to their adhesion properties. In this study, we focused on sum-frequency generation (SFG) vibrational spectroscopy, which offers the best depth resolution at the sub-nanometer level among available techniques, and explored its potential as an imaging method. A poly(methyl methacrylate) (PMMA) thin film with a line-and-space pattern was prepared on a quartz substrate. Imaging of the interfacial line-and-space structure in the film was successfully achieved based on SFG signals arising from ester methyl groups as well as carbonyl groups. Once SFG imaging was established, it was applied to blend films of polystyrene (PS) and PMMA with different compositions on the quartz substrate, enabling the direct and non-destructive observation of wetting layers and phase-separated structures buried at the substrate interface for the first time. The interfacial adhesion strength between the blend films and the quartz substrate, evaluated using the surface and interfacial cutting analysis system, showed a clear correlation with the interfacial structure of the blend. This study, which enables the analysis of the relationship between the local orientation of polymer chains at the interface and adhesion strength, is expected to contribute greatly to the design of next-generation adhesives and adhesion technologies.

DOI： 10.1039/d4py00951g

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出版者・発行元：Macromolecules  

Improving the heat resistance of epoxy resins remains an important and challenging issue across a wide variety of applications. One strategy to address this challenge is the design of cured resins based on multifunctional epoxy. Compared to conventional difunctional epoxy resins, multifunctional epoxy resins are expected to easily form a highly cross-linking structure, which is anticipated to contribute to enhanced heat resistance. However, there is a lack of information about the detailed mechanisms of the formation of such cross-linking structures and their effects on the physical properties. We herein tracked the kinetics of curing reactions of difunctional and trifunctional epoxies with an amine hardener. Despite the identical reactivity of epoxy groups in both base monomers, the trifunctional epoxy system cured faster. In addition, in the difunctional epoxy system, gelation was followed by vitrification, while in the trifunctional epoxy system, gelation and vitrification occurred simultaneously. The accelerated curing reaction observed in the trifunctional epoxy system could be explained in terms of the localized temperature increase from the reaction heat, which subsequently accelerated the following reactions. The resulting post-cured trifunctional epoxy resin did not exhibit a clear glass transition. This, so-called Tg-less behavior was due to the glass transition temperature of the trifunctional epoxy unit being higher than its decomposition temperature. This mechanism for the Tg-less behavior, combined with the accelerated curing reactions, provides valuable insights for the design of thermosetting resins with enhanced thermal stability.

DOI： 10.1021/acs.macromol.4c02178

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出版者・発行元：Macromolecules  

A better understanding of the dynamic behavior of polymer chains on solid surfaces is indispensable for the design and construction of high-performance polymer composites. We herein visualized the in-plane movement of isolated poly(methyl methacrylate) (PMMA) and poly(tert-butyl methacrylate) (PtBMA) single chains on hydrophilic silicon wafers under ambient conditions by atomic force microscopy. Isolated PMMA chains adsorbed to the substrate, whereas PtBMA chains diffused, the degree of which was dependent on the humidity. Neutron reflectivity revealed the formation of a layer of condensed water on the substrate. All-atomistic molecular dynamics simulations implied that the diffusivity difference of the two polymers was based on the submerged depth in which a part of a chain existed. That is, the interaction of a polymer with the surface of the hydrophilic substrate primarily governs its lateral movement, or adsorption behavior, facilitated by the presence of water.

DOI： 10.1021/acs.macromol.4c00724

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記述言語：英語   出版者・発行元：Langmuir  

The adhesion of epoxy adhesives to aluminum materials is an important issue in assembling parts for lightweight mobility. Aluminum surfaces typically possess an oxide layer, which readily adsorbs water. In this study, the aggregation states of water and its effect on the curing reaction were examined by placing a water layer between an amorphous alumina surface and a mixture of epoxy and amine components. This study used molecular dynamics simulations and density functional theory calculations. Before the reaction, water molecules strongly adsorbed onto the alumina surface, aggregating excess water. Some water diffused into the epoxy/amine mixture, accelerating the diffusion of unreacted substances. This led to faster reaction kinetics, particularly in proximity to the alumina surface. The adsorption of water molecules onto the alumina surface and the aggregation of excess water were similarly observed even after the curing process. Subsequently, the interaction between the alumina surface and various functional groups of the epoxy/amine mixture was evaluated before and after the reaction. Epoxy monomers had little interaction with the alumina surface before the reaction, whereas hydroxy groups formed by the ring-opening reaction of epoxy groups exhibited notable interaction. Conversely, sulfonyl and amino groups in amine compounds formed hydrogen bonds with OH groups on the alumina surface before the reaction. However, after the reaction, amino groups weakened their interaction with the alumina OH groups as they transformed from primary to tertiary during the curing reaction. Both epoxy and amine monomers/fragments similarly interacted with water molecules, both before and after the reaction. The insights gained from this study are expected to contribute to a better understanding of the impact of moisture absorption on the application of epoxy resins.

DOI： 10.1021/acs.langmuir.4c01081

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PubMed

記述言語：英語   出版者・発行元：Journal of Physical Chemistry B  

Epoxy resins are essential for various applications, and their properties depend on the curing reactions during which epoxy and amine compounds form the network structure. We here focus on how the presence or absence of two methyl groups in common epoxy bases, diglycidyl ether of bisphenol A and F (4,4′-DGEBA and 4,4′-DGEBF), affects the curing kinetics. The chemical reactions of both 4,4′-DGEBA and 4,4′-DGEBF, when cured with the same amine, were monitored by Fourier-transform infrared (FT-IR) spectroscopy and differential scanning calorimetry (DSC). Despite no difference in the reactivity of epoxy groups between 4,4′-DGEBA and 4,4′-DGEBF, the initial curing reaction was slower for the latter. This delay for the 4,4′-DGEBF system was attributed to intermolecular stacking, which hindered the approach of unreacted epoxy groups to amino groups and vice versa. This conclusion was drawn from the results obtained through ultraviolet (UV) spectroscopy, wide-angle X-ray scattering (WAXS), density functional theory (DFT) calculation, and all-atom molecular dynamics (MD) simulation.

DOI： 10.1021/acs.jpcb.4c01152

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記述言語：英語   出版者・発行元：Langmuir  

A better understanding of the aggregation states of adhesive molecules in the interfacial region with an adherend is crucial for controlling the adhesion strength and is of great inherent academic interest. The adhesion mechanism has been described through four theories: adsorption, mechanical, diffusion, and electronic. While interfacial characterization techniques have been developed to validate the aforementioned theories, that related to the electronic theory has not yet been thoroughly studied. We here directly detected the electronic interaction between a commonly used thermosetting adhesive, cured epoxy of diglycidyl ether of bisphenol A (DGEBA) and 4,4′-diaminodiphenylmethane (DDM), and copper (Cu). This study used a combination of density functional theory (DFT) calculations and femtosecond transient absorption spectroscopic (TAS) measurements as this epoxy adhesive-Cu pairing is extensively used in electronic device packaging. The DFT calculations predicted that π electrons in a DDM molecule adsorbed onto the Cu surface flowed out onto the Cu surface, resulting in a positive charge on the DDM. TAS measurements for the Cu/epoxy multilayer film, a model sample containing many metal/adhesive interfaces, revealed that the electronic states of excited DDM moieties at the Cu interface were different from those in the bulk region. These results were in good accordance with the prediction by DFT calculations. Thus, it can be concluded that TAS is applicable to characterize the electronic interaction of adhesives with metal adherends in a nondestructive manner.

DOI： 10.1021/acs.langmuir.4c00711

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記述言語：日本語   出版者・発行元：一般社団法人 日本レオロジー学会  

DOI： 10.1678/rheology.52.75

CiNii Research

記述言語：英語   出版者・発行元：一般社団法人 日本レオロジー学会  

In silica-filled epoxy resins, amorphous silica particles are generally covered with silanol groups on their surfaces, rendering them susceptible to moisture adsorption. Thus, the importance lies in better understanding the aggregation states of epoxy resins on the surface of amorphous silica and the influence of adsorbed water on the silica surface on the curing reaction. Here, we propose a method to generate amorphous silica by inserting oxygen atoms into the Si-Si bonds of amorphous silicon, serving as a model construction for molecular dynamics (MD) simulations. The surface structure of the silica was then extracted, and a mixture of an epoxy base (diglycidyl ether of bisphenol-A, DGEBA) and an amine hardener (4,4’-diaminodiphenyl sulfone, DDS) was placed on top of it. SO2 groups of DDS strongly adsorbed on the silica surface through hydrogen bonding and remained after curing, although this interaction was weakened due to the reduced degree of freedom caused by cross-linking. On the other hand, DGEBA did not exhibit such a strong interaction with the silica surface. After curing, OH groups generated by the ring-opening of epoxy groups formed slight hydrogen bonds with the silica surface. When a small amount of water was adsorbed onto the silica surface, it diffused slightly into the epoxy resin, enhancing the mobility of the reactants in the system, and consequently accelerating the curing reaction.

DOI： 10.1678/rheology.52.91

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CiNii Research

出版者・発行元：Polymer Journal  

Using X-ray absorption spectroscopy (XAS) with linearly polarized soft X-rays, we investigated the local conformation of poly(methyl methacrylate) (PMMA) adsorbed to a SiOx/Si(111) surface. The preedge intensity of the O K-edge XAS for PMMA, originating from the O 1s → π* transition at a C=O group in the side chain, was stronger for vertically polarized incident X-rays than for horizontally polarized ones. Conversely, the XAS intensity originating from the O 1s → σ* transition showed the opposite trend. These findings suggest that the C=O group in the side chain of PMMA exhibited preferential orientation rather than an amorphous arrangement. To gain further insights, we conducted a depth profile analysis of the local conformation of PMMA using XAS combined with an argon gas cluster ion beam (GCIB). GCIB-XAS analysis revealed that the orientation of the C=O group in the side chain of PMMA differs between the region from the SiOx interface to a distance on the order of 1 nanometer and the bulk PMMA region.

DOI： 10.1038/s41428-023-00864-8

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DOI： 10.1080/27660400.2023.2270529

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記述言語：英語   出版者・発行元：Journal of Chemical Physics  

Polymer chains at a buried interface with an inorganic solid play a critical role in the performance of polymer nanocomposites and adhesives. Sum frequency generation (SFG) vibrational spectroscopy with a sub-nanometer depth resolution provides valuable information regarding the orientation angle of functional groups at interfaces. However, in the case of conventional SFG, since the signal intensity is proportional to the square of the second-order nonlinear optical susceptibility and thereby loses phase information, it cannot be unambiguously determined whether the functional groups face upward or downward. This problem can be solved by phase-sensitive SFG (ps-SFG). We here applied ps-SFG to poly(methyl methacrylate) (PMMA) chains in direct contact with a quartz surface, shedding light on the local conformation of chains adsorbed onto the solid surface. The measurements made it possible to determine the absolute orientation of the ester methyl groups of PMMA, which were oriented toward the quartz interface. Combining ps-SFG with all-atomistic molecular dynamics simulation, the distribution of the local conformation and the driving force are also discussed.

DOI： 10.1063/5.0184315

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記述言語：英語   出版者・発行元：AIP Publishing  

Polymer chains at a buried interface with an inorganic solid play a critical role in the performance of polymer nanocomposites and adhesives. Sum frequency generation (SFG) vibrational spectroscopy with a sub-nanometer depth resolution provides valuable information regarding the orientation angle of functional groups at interfaces. However, in the case of conventional SFG, since the signal intensity is proportional to the square of the second-order nonlinear optical susceptibility and thereby loses phase information, it cannot be unambiguously determined whether the functional groups face upward or downward. This problem can be solved by phase-sensitive SFG (ps-SFG). We here applied ps-SFG to poly(methyl methacrylate) (PMMA) chains in direct contact with a quartz surface, shedding light on the local conformation of chains adsorbed onto the solid surface. The measurements made it possible to determine the absolute orientation of the ester methyl groups of PMMA, which were oriented toward the quartz interface. Combining ps-SFG with all-atomistic molecular dynamics simulation, the distribution of the local conformation and the driving force are also discussed.

CiNii Research

記述言語：英語   出版者・発行元：ACS Applied Materials and Interfaces  

Interfacial polymer layers with nanoscale size play critical roles in dissipating the strain energy around cracks and defects in structural nanocomposites, thereby enhancing the material’s fracture toughness. However, understanding how the intrinsic mechanical dynamics of the interfacial layer determine the toughening and reinforcement mechanisms in various polymer nanocomposites remains a major challenge. Here, by means of a recently developed nanorheology atomic force microscopy method, also known as nanoscale dynamic mechanical analysis (nDMA), we report direct mapping of dynamic mechanical responses at the interface of a model epoxy nanocomposite under the transition from a glassy to a rubbery state. We demonstrate a significant deviation in the dynamic moduli of the interface from matrix behavior. Interestingly, the sign of the deviation is observed to be reversed when the polymer changes from a glassy to a rubbery state, which provides an excellent explanation for the difference in the modulus reinforcement between glassy and rubbery epoxy nanocomposites. More importantly, nDMA loss tangent images unambiguously show an enhanced viscoelastic response at the interface compared to the bulk matrix in the glassy state. This observation can therefore provide important insights into the nanoscale toughening mechanism that occurs in epoxy nanocomposites due to viscoelastic energy dissipation at the interface.

DOI： 10.1021/acsami.3c06123

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1021/acs.macromol.3c00341

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Macromolecules  

In general, the network structure formed in epoxy resins is heterogeneous, and its extent is dependent on the curing temperature. Based on Fourier-transform infrared spectroscopy in conjunction with coarse-grained molecular dynamics simulations, we herein discuss the origin of mesoscopic heterogeneity in an epoxy resin composed of a typical epoxy base and diamine hardener, which react with each other in two steps. The rate balance of the first and second step reactions, which led to chain extension and branching, respectively, depended on the curing temperature; at a higher temperature, the second-step reaction became faster. Since the reaction proceeded more slowly at lower temperatures, a homogeneous domain was formed as unreacted substances were incorporated into the network. At higher temperatures, on the other hand, the reaction proceeded rapidly before unreacted substances were incorporated into the network, leaving isolated small fragments and nanoscale voids or free spaces in the domain. We also found that an actively reacting domain with a length scale of several tens of nanometers was formed even at a conversion lower than the gel point, regardless of the curing temperature. The connection of domains was also key to better understanding the temperature dependence of the hierarchical heterogeneity. The final network was found to be denser and more homogeneous at a lower temperature, while heterogeneous with many free spaces at a higher temperature. Our molecular picture of the network formation drawn by combining experimental and simulation techniques advances our current understanding of the heterogeneity formed in epoxy resins.

DOI： 10.1021/acs.macromol.3c00411

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Polymer Journal  

The relaxation of the local conformation of ethylene-propylene-diene terpolymer (EPDM) rubber at the quartz interface was examined by sum frequency generation (SFG) spectroscopy in conjunction with all-atom molecular dynamics (MD) simulations. At room temperature, SFG peaks due to the CH symmetric stretching vibration of methyl (CH3s) and methylene (CH2s) groups were observed, and the intensity was stronger for CH3s than for CH2s. The CH3s to CH2s intensity ratio was reversed during the heating process, meaning that the local conformation of EPDM at the interface changed. MD simulations revealed that the fraction of ethylene units in the trans conformation on the substrate surface decreased once the temperature was greater the interfacial glass transition temperature (Tg), which was determined based on the temperature dependence of the mass density. Moreover, the temperature-induced change in the fraction of propylene units in the trans conformation was less remarkable than that of ethylene units. Both SFG spectroscopy and MD simulation confirmed that the Tg was higher in the interfacial region than in the bulk.

DOI： 10.1038/s41428-023-00764-x

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：American Association for the Advancement of Science (AAAS)  

Natural systems, composite materials, and thin-film devices adsorb macromolecules in different phases onto their surfaces. In general, polymer chains form interfacial layers where their aggregation states and thermal molecular motions differ from the bulk. Here, we visualize well-defined double-stranded DNAs (dsDNAs) using atomic force microscopy and molecular dynamics simulations to clarify the adsorption mechanism of polymer chains onto solid surfaces. Initially, short and long dsDNAs are individually and cooperatively adsorbed, respectively. Cooperative adsorption involves intertwining of multiple chains. The dependence of adsorption on the chain affects the formation of the interfacial layer, realizing different mechanical properties of DNA/filler bulk composites. These findings will contribute to the development of light and durable polymer composites and films for various industrial, biomedical, and environmental applications.

DOI： 10.1126/sciadv.abn6349

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CiNii Research

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：ACS Applied Materials and Interfaces  

The addition of a small fraction of solid nanoparticles to thermosetting polymers can substantially improve their fracture toughness, while maintaining various intrinsic thermomechanical properties. The underlying mechanism is largely related to the debonding process and subsequent formation of nanovoids at a nanoscale nanoparticle/epoxy interface, which is thought to be associated with a change in the structural and mechanical properties of the formed epoxy network at the interface compared with the matrix region. However, a direct characterization of the local physical properties at this nanoscale interface remains significantly challenging. Here, we employ a recently developed bimodal atomic force microscopy technique for the direct mapping of nanoscale elastic and adhesive responses of an amine-cured epoxy resin filled with ∼50 nm diameter silica nanoparticles. The obtained elastic modulus and dissipated energy maps with high spatial resolution evidence the existence of a ∼20-nm-thick interfacial epoxy layer surrounding the nanoparticles, which exhibits a reduced modulus and weaker adhesive response in comparison with the matrix properties. While the presence of such a soft and weak-adhesive interfacial layer is found not to affect the architecture of structural heterogeneities in the epoxy matrix, it conceivably supports the toughening mechanism related to the debonding and plastic nanovoid growth at the silica/epoxy interface. The incorporation of this soft interfacial layer into the Halpin-Tsai model also provides a good explanation for the effect of the silica fraction on the tensile modulus of cured epoxy nanocomposites.

DOI： 10.1021/acsami.2c12335

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Langmuir  

We have succeeded in visualizing the spatial heterogeneity of the reaction ratio in epoxy resins by combining medium-angle X-ray scattering (MAXS) and computed tomography (CT). The reaction ratio is proportional to the degree of cross-linking between epoxy and amine in epoxy resins. The reaction ratio and its spatial inhomogeneity affect the toughness of epoxy resins. However, there has been no non-destructive method to measure the spatial inhomogeneity of the reaction ratio, although we can measure only the spatially averaged reaction ratio by Fourier-transform infrared spectroscopy (FT-IR). We found that the scattering peak reflected the cross-linking structures in the q region of MAXS and that the peak intensity is proportional to the reaction ratio. By reconstructing CT images from this peak intensity, we visualized the spatial heterogeneity of the reaction ratio. The application of this method may not be limited to epoxy resins but may extend to studying the heterogeneity of cross-linked structures in other materials.

DOI： 10.1021/acs.langmuir.2c01741

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physical Chemistry Chemical Physics  

A better understanding of the chemical reaction between epoxy and amine compounds at a solid interface is crucial for the design and fabrication of materials with appropriate adhesive strength. Here, we examined the curing reaction kinetics of epoxy phenol novolac and 4,4′-diaminodiphenyl sulfone at the outermost interface using sum-frequency generation spectroscopy, and X-ray and neutron reflectivity in conjunction with a full atomistic molecular dynamics simulation. The reaction rate constant was much larger at the quartz interface than in the bulk. While the apparent activation energy at the quartz interface obtained from an Arrhenius plot was almost identical to the bulk value, the frequency factor at the quartz interface was greater than that in the bulk. These results could be explained in terms of the densification and orientation of reactants at the interface, facilitating the encounter of the reactants present.

DOI： 10.1039/d2cp03394a

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ACS Applied Polymer Materials  

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.

DOI： 10.1021/acsapm.2c00855

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記述言語：英語   出版者・発行元：JACS Au  

Epoxy resins are used in various fields in a wide range of applications such as coatings, adhesives, modeling compounds, impregnation materials, high-performance composites, insulating materials, and encapsulating and packaging materials for electronic devices. To achieve the desired properties, it is necessary to obtain a better understanding of how the network formation and physical state change involved in the curing reaction affect the resultant network architecture and physical properties. However, this is not necessarily easy because of their infusibility at higher temperatures and insolubility in organic solvents. In this paper, we summarize the knowledge related to these issues which has been gathered using various experimental techniques in conjunction with molecular dynamics simulations. This should provide useful ideas for researchers who aim to design and construct various thermosetting polymer systems including currently popular materials such as vitrimers over epoxy resins.

DOI： 10.1021/jacsau.2c00120

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記述言語：英語   出版者・発行元：Langmuir  

To ensure the quality and reliability of products bonded by epoxy resin adhesives, elucidation of the microscopic adhesion mechanism is essential. The adhesive interaction and bonding strength between epoxy resins and hydroxylated γ-alumina (001) surfaces were investigated by using a combined molecular dynamics (MD) and density functional theory (DFT) study. The curing reaction of an epoxy resin consisting of diglycidyl ether of bisphenol A (DGEBA) and 4,4′-diaminodiphenyl sulfone (DDS) was simulated. The resin structure was divided into fragmentary structures to study the interaction of each functional group with the alumina surface using DFT calculations. From the characteristics of the adhesive structures and the calculated adhesion energies, it was found that the fragments forming hydrogen bonds with hydroxy groups on the alumina surface resulted in large adhesion energies. On the other hand, the fragments adsorbed on the alumina surface via dispersion interactions resulted in small adhesion energies. The adhesion forces evaluated from the Hellmann-Feynman force calculations indicated the significant contribution of the hydroxy groups and benzene ether moieties derived from DGEBA to the adhesive stress of the DGEBA/DDS epoxy resin. The direction of hydrogen bonding between the epoxy resin and the surface and the difference in geometry at the interface between the donor and acceptor of hydrogen bonding played a central role in maintaining the adhesive strength during the failure process of the adhesive interface.

DOI： 10.1021/acs.langmuir.2c00529

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記述言語：英語   出版者・発行元：Soft Matter  

Adhesion is a molecular event where polymer chains contact with a material surface to form an interfacial layer. To obtain a better understanding of the adhesion on a molecular scale, we herein examined the conformational change of polystyrene (PS) chains at the film surface after contacting with hydrophobic or hydrophilic surfaces using sum-frequency generation (SFG) spectroscopy. Chains altered their local conformations with a quartz surface more quickly than a hydrophobic alkyl-functionalized one. A full-atomistic molecular dynamics simulation showed that these results, which were coupled with the contact process of PS chains with the solid surface, could be explained in terms of the Coulomb interaction between them.

DOI： 10.1039/d1sm01833g

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：ACS Macro Letters  

The interfacial strength of polystyrene (PS) with and without PS oligomers in contact with a glass substrate was examined to determine the relationship between the interfacial aggregation state and adhesion. The shear bond strength and adsorbed layer thickness of neat PS exhibited a similar dependence on the thermal annealing time: they increased to constant values within almost the same time. This implies that the adhesion of the polymer is closely related to the formation of an adsorbed layer at the adhesion interface. Nevertheless, in the case of PS with a small amount of oligomer, the shear bond strength decreased, while the adsorbed layer thickness was almost the same as that of neat PS. Based on the results of interfacial analyses, we propose that the interfacial segregation of the oligomer reduced the entanglement between the interfacial free chains in the adsorbed layer and the bulk chains.

DOI： 10.1021/acsmacrolett.2c00062

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記述言語：英語   出版者・発行元：Soft Matter  

Herein, we report the in situ transmission electron microscopy observation of the deformation and fracture processes of an epoxy resin thin film containing silica nanoparticles under tensile strain. Under tensile strain, the dispersed silica nanoparticles in the composite arrest the progress of the crack tip and prevent crack propagation. Concomitantly, the generation and growth of nanovoids at the epoxy matrix/nanoparticle interfaces were clearly observed, particularly in the region near the crack tip. These nanovoids contribute to the dissipation of fracture energy, thereby enhancing the fracture toughness. We also analyzed the local distributions of the true strain and strain rate in the nanocomposite film during tensile testing using the digital image correlation method. In the region around the crack tip, the strain rate increased by 3 to 10 times compared to the average of the entire test specimen. However, the presence of large filler particles in the growing crack suppressed the generation of strain, potentially contributing to hindering crack growth.

DOI： 10.1039/d1sm01452h

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記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1021/acs.macromol.1c01293

記述言語：その他   掲載種別：研究論文（学術雑誌）  

The preferential orientation of the crystalline lamellae for isotactic polypropylene in spin-coated films was examined by grazing incidence of wide-angle X-ray diffraction in conjunction with sum frequency generation vibrational spectroscopy.

DOI： 10.1039/D1CP03959H

記述言語：その他   掲載種別：研究論文（学術雑誌）  

Epoxy resins with a network structure, which are obtained by curing reactions of epoxy and amine compounds, are an important class of thermosetting resins. We here report the segmental dynamics of epoxy resins in which the three-dimensional network was well-defined and systematically varied. What we found by experiments in conjunction with atomistic molecular dynamics simulation was that when the cross-linking density in the epoxy resins increased, the glass transition temperature increased with an associated reduction in the spatial scale of the cooperative rearranging region and the increase in the dynamic heterogeneity. The knowledge here obtained should be useful for a better understanding of the physical properties of thermosetting polymers.

DOI： 10.1021/acs.macromol.1c00513

記述言語：英語   掲載種別：研究論文（学術雑誌）  

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.

DOI： 10.1039/D1SM00529D

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1038/s41428-021-00528-5

記述言語：英語   掲載種別：研究論文（学術雑誌）  

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.

DOI： 10.1039/D0SM01600D

担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： https://doi.org/10.1678/rheology.49.55

記述言語：英語   掲載種別：研究論文（学術雑誌）  

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.

DOI： 10.1038/s41598-020-77761-0

記述言語：英語   掲載種別：研究論文（学術雑誌）  

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.

DOI： 10.1021/acsmacrolett.0c00638

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1039/D0SM00625D

記述言語：英語   掲載種別：研究論文（学術雑誌）  

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.

DOI： 10.1021/acsapm.9b01154

記述言語：英語   掲載種別：研究論文（学術雑誌）  

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.

DOI： 10.1021/acs.jpcb.9b00680

記述言語：英語   掲載種別：研究論文（学術雑誌）  

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.

DOI： 10.1021/acs.macromol.8b02416

記述言語：その他   掲載種別：研究論文（学術雑誌）  

© 2018, The Society of Polymer Science, Japan. 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.

DOI： 10.1038/s41428-018-0134-7

記述言語：英語   掲載種別：研究論文（学術雑誌）  

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]

DOI： 10.1021/acs.jpcb.7b07062

記述言語：英語  

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.

DOI： 10.1007/978-981-10-0815-3_25

記述言語：英語  

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.

DOI： 10.1007/978-981-10-0815-3_26

記述言語：英語  

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.

DOI： 10.1007/978-981-10-0815-3_28

記述言語：英語   掲載種別：研究論文（学術雑誌）  

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.

DOI： 10.1021/jp307058s

記述言語：英語   掲載種別：研究論文（学術雑誌）  

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.

DOI： 10.1252/kakoronbunshu.38.221

記述言語：英語   掲載種別：研究論文（学術雑誌）  

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.

DOI： 10.1063/1.1914767

担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： https://doi.org/10.1063/1.1563613

担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： https://doi.org/10.1063/1.1456031

担当区分：筆頭著者   記述言語：英語  

DOI： https://doi.org/10.1063/1.474681

担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： https://doi.org/10.1002/pen.10638

担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： https://doi.org/10.1002/pen.760351210

記述言語：英語  

DOI： https://doi.org/10.1063/1.468746

担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： https://doi.org/10.1063/1.466423

担当区分：筆頭著者   記述言語：英語  

DOI： https://doi.org/10.1063/1.464607