Molecular Understanding of the Distinction between Adhesive Failure and Cohesive Failure in Adhesive Bonds with Epoxy Resin Adhesives

In the development of adhesives, an understanding of the fracture behavior of the bonded joints is inevitable. Two typical failure modes are known: adhesive failure and cohesive failure. However, a molecular understanding of the cohesive failure process is not as advanced as that of the adhesive failure process. In this study, research was developed to establish a molecular understanding of cohesive failure using the example of a system in which epoxy resin is bonded to a hydroxyl-terminated self-assembled monolayer (SAM) surface. Adhesive failure was modeled as a process in which an epoxy molecule is pulled away from the SAM surface. Cohesive failure, on the other hand, was modeled as the process of an epoxy molecule separating from another epoxy molecule on the SAM surface or breaking of a covalent bond within the epoxy resin. The results of the simulations based on the models described above showed that the results of the calculations using the model of cohesive failure based on the breakdown of intermolecular interactions agreed well with the experimental results in the literature. Therefore, it was suggested that the cohesive failure of epoxy resin adhesives is most likely due to the breakdown of intermolecular interactions between adhesive molecules. We further analyzed the interactions at the adhesive failure and cohesive failure interfaces and found that the interactions at the cohesive failure interface are mainly accounted for by dispersion forces, whereas the interactions at the adhesive failure interface involve not only dispersion forces but also various chemical interactions, including hydrogen bonds. The selectivity between adhesive failure and cohesive failure was explained by the fact that varying the functional group density affected the chemical interactions but not the dispersion forces

Adsorption Site Preference Determined by Triangular Topology: Application of the Method of Moments to Transition Metal Surfaces

The adsorption sites of the top and hollow on the close-packed surfaces of transition metals are well known. In this paper, which site is more preferred for the adsorption of atoms and molecular fragments on the metal surfaces is discussed based on the topology of the adsorption geometry. For this purpose, the method of moments for the electronic density of states is applied to the surface. Adsorption at the hollow site generates a triangular topology, leading to a more negative value of the third moment (μ3) than that at the top site, which generates no triangular topology. When the difference in energy between the two adsorption sites is plotted against the band filling of the metal surface, a characteristic node at around the intermediate band filling can be found. This is a signature that the energy difference curve is controlled by μ3. Roughly speaking, the hollow-site adsorption, which has a more negative μ3 value, takes precedence at low band fillings, while the top site adsorption, which has a less negative μ3 value, takes precedence at high band fillings. One can conclude that an adsorption structure with more three-membered rings on a surface is more stable at low electron counts whereas that with less three-membered rings is more stable at high electron counts. However, if the strength of the metal–adsorbate bond is significantly greater than that of the metal–metal bond, the effect of the second moment (μ2) on the energy difference curve cannot be neglected. The hollow-site adsorption leads to a larger value of μ2 due to the topological feature of a larger coordination number around the adsorbate atom. As a result, the hollow-site adsorption is preferred over the top site at any band filling

Current Rectification through π–π Stacking in Multilayered Donor–Acceptor Cyclophanes

Extended π-stacked molecules have attracted much attention since they play an essential role in both electronic devices and biological systems. In this article electron transport properties of a series of multilayered cyclophanes with the hydroquinone donor and quinone acceptor units in the external positions are theoretically studied with applications to molecular rectifiers in mind. Calculations of electron transport through the π–π stacked structures in the multilayered cyclophanes are performed by using nonequilibrium Green’s function method combined with density functional theory. Calculated transmission spectra show that the conductance decreases exponentially with the length of the molecule with a decay factor of 0.75 Å^–1, which lies for the values between π-conjugated molecules and σ-bonded molecules. Applied bias calculations provide current–voltage curves, which exhibit good rectifying behavior. The rectification mechanism in the coherent transport regime is qualitatively explained by the response of the frontier orbital energy levels, especially LUMO levels, to the applied bias, where the rectifying direction is expected to be opposite to the Aviram–Ratner model. The maximum value of rectification ratio increases with an increase in the number of stacking layers due to the effective separation of the donor and acceptor parts, where effects from the opposite electrodes to the donor and acceptor are negligible. Multilayered donor–acceptor cyclophanes are suitable materials for investigating the relationship among electron transport properties, rectification properties, and molecular length (separation between the donor and acceptor parts)

Competition between Hydrogen Bonding and Dispersion Force in Water Adsorption and Epoxy Adhesion to Boron Nitride: From the Flat to the Curved

Hexagonal boron nitride (h-BN) is a material with excellent thermal conductivity and electrical insulation, used as an additive to various matrices. To increase the affinity of h-BN to them, hydrogen bonds should be formed at the interface. In reality, however, they are not formed; the N atoms are not capable of accepting hydrogen bonds due to the delocalization of their lone pair electrons over the B–N π bonds. To make it form hydrogen bonds, one may need to break the planarity of h-BN so that the orbital overlap in the B–N π bonds can be reduced. This idea is verified with first-principles calculations on the adsorption of a water molecule on hypothetical h-BN surfaces, the planarity of which is broken. One can do it in silico but not in vitro. BN nanotubes (BNNTs) are considered as a more realistic BN surface with nonplanarity. The hydrogen bond is shown to become stronger as the curvature of the tube increases. On the contrary, the strength of the dispersion force acting at the interface becomes weaker. In water adsorption, these two interactions are in competition with each other. However, in epoxy adhesion, the interaction due to dispersion forces is overwhelmingly stronger than that due to hydrogen bonding. The smaller the curvature of the surface, the smaller the distance between more atoms at the interface; thus, the interaction due to dispersion forces maximized

Adsorption and Activation of Methane on the (110) Surface of Rutile-type Metal Dioxides

Methane strongly adsorbs on the (110) surface of IrO₂, a rutile-type metal dioxide. Its C–H bond is facilely dissociated even below room temperature, as predicted in a few theoretical works and actually observed in a recent experimental study. Thence, three questions are posed and answered in this paper: First, why does methane adsorb on the IrO₂ surface so strongly? Second, why is the surface so active for the C–H bond breaking reaction? Third, is there any other rutile-type metal dioxide that is more active than IrO₂? A second-order perturbation theoretic approach is successfully applied to the analysis of the electronic structure of methane, which is found to be significantly distorted on the surface. Regarding the first point, it is clarified that an attractive orbital interaction between the surface Ir 5d_<i>z</i>² orbital and the distorted methane’s highest occupied molecular orbital leads to the strong adsorption. As for the second point, the bond strength between the surface metal atom and the CH₃ fragment generated after the C–H bond scission of methane is correlated well with the activation barrier. A substantial bonding interaction between CH₃’s nonbonding orbital and the d_<i>z</i>² orbital hints at the strong Ir–CH₃ bond and hence high catalytic activity ensues. Last but not least, β-PtO₂, a distorted rutile-type dioxide, is identified as a more active catalyst than IrO₂. Here again, a perturbation theoretic line of explanation is found to be of tremendous help. This paper is at the intersection of theoretical, catalytic, inorganic, and physical chemistry. Also, it should serve as a model for the design and study of metal-oxide catalysts for the C–H bond activation of methane

Conductance through Short DNA Molecules

The conductance through short DNA molecules connected to gold electrodes is studied with density functional theory and nonequilibrium Green’s function method combined with density functional theory. The anchoring of the molecules to the electrodes is investigated, and in addition to the covalent S−Au bond, weak interactions between the aromatic heterocyclic bases and the electrodes are found. These weak interactions are important for the electron transport through DNA molecules. A tunneling mechanism is suggested, and the conductive properties of the nucleotides in a metal−molecule−metal junction are compared. Different four-nucleotide DNA sequences are investigated. A significant value for the current, 20 pA, is calculated for 1.5 V applied bias for a DNA sequence consisting of guanine and cytosine nucleotides. It is shown that adenine-thymine nucleotide pairs introduce potential barriers for the electron transport and therefore significantly decline the conductance. The obtained results are compared with recent experimental observations (Nanotechnology 2009, 20, 115502) and confirm the possibility for electron transport through DNA molecules as well as provide an explanation for the reduced conductance through DNA sequences, which contain adenine-thymine nucleotide pairs. The results are compared with a previous theoretical study, performed with the extended Hückel method (ChemPhysChem 2003, 4, 1256), which reports low conductance for DNA molecules. The difference in the conclusions is due to the applied bias self-consistent field calculations used in the recent study, which take into account the changes of the transmission probabilities with the bias

Hückel Molecular Orbital Analysis for Stability and Instability of Stacked Aromatic and Stacked Antiaromatic Systems

Face-to-face stacking of aromatic compounds leads to stacked antiaromaticity, while that of antiaromatic compounds leads to stacked aromaticity. This is a prediction with a long history; in the late 2000s, the prediction was confirmed by high-precision quantum chemical calculations, and finally, in 2016, a π-conjugated system with stacked aromaticity was synthesized. Several variations have since been reported, but essentially, they are all the same molecule. To realize stacked aromaticity in a completely new and different molecular system and to trigger an extension of the concept of stacked aromaticity, it is important to understand the origin of stacked aromaticity. The Hückel method, which has been successful in giving qualitatively correct results for π-conjugated systems despite its bold assumptions, is well suited for the analysis of stacked aromaticity. We use this method to model the face-to-face stacking systems of benzene and cyclobutadiene molecules and discuss their stacked antiaromaticity and stacked aromaticity on the basis of their π-electron energies. By further developing the discussion, we search for clues to realize stacked aromaticity in synthesizable molecular systems

Molecular Dynamics Study on the Thermal Aspects of the Effect of Water Molecules at the Adhesive Interface on an Adhesive Structure

The presence of adsorbed water on hydrophilic solid surfaces should be taken into account, especially in humid environments. It significantly reduces the adhesive strength between the epoxy resin and the adherend surface. Here, the adhesion structure of interfacial water sandwiched between bisphenol A epoxy resin and a hydroxylated silica (001) surface is investigated with microsecond molecular dynamics simulations. Specifically, interfacial water layers with initial thicknesses of 7.5, 10, and 20 Å are modeled. The density curves of water and the diglycidyl ether of bisphenol A show that at room temperature, the surface of the silica with hydroxyl groups is completely covered with a thick layer of water. For water layers thinner than 10 Å, the density of epoxy resin on the silica surface increases when the system is heated and does not return to the original density when the system is cooled. Furthermore, calculation of the interaction energy revealed that the exclusion of water from the hydroxylated surface by epoxy resin during heating can contribute to the increase in the adhesive interaction between the epoxy resin and the silica surface with hydroxyl groups

Peel Adhesion Strength between Epoxy Resin and Hydrated Silica Surfaces: A Density Functional Theory Study

Adhesive strength is known to change significantly depending on the direction of the force applied. In this study, the peel and tensile adhesive forces between the hydroxylated silica (001) surface and epoxy resin are estimated based on quantum chemical calculations. Here, density functional theory (DFT) with dispersion correction is used. In the peel process, the epoxy resin is pulled off from the terminal part, while in the tensile process, the entire epoxy resin is pulled off vertically. As a result of these calculations, the maximum adhesive force in the peel process is decreased to be about 40% of that in the tensile process. The adhesion force–displacement curve for the peeling process shows two characteristic peaks corresponding to the process where the adhesive molecule horizontally oriented to the surface shifts to a vertical orientation to the surface and the process where the vertical adhesive molecule is dissociated from the surface. Force decomposition analysis is performed to further understand the peel adhesion force; the contribution of the dispersion force is found to be slightly larger than that of the DFT force. This feature is common to the tensile process as well. Each force in the peel process is about 40% smaller than the corresponding force in the tensile process