Effects of Curing Agents on the Adhesion of Epoxy Resin to Copper: A Density Functional Theory Study

Epoxy resins are widely used adhesives in industrial fields. To use epoxy resin as an adhesive, it is necessary to mix the epoxy resin with a hardener. Hardeners have various functional groups and skeletons, and the properties of epoxy resins vary depending on the hardener. Although the adhesion of epoxy resins has been extensively studied using density functional theory (DFT) calculations, few studies have evaluated the effect of hardener molecules. Therefore, in this study, DFT calculations of adhesion energies and bonding structures on Cu (111) and Cu2O (111) surfaces are performed for model molecules of adducts of epoxy resin with hardeners having various functional groups and skeletons to evaluate the influence of the hardeners on the adhesion of epoxy resin to the metal surface. The adhesion energy to the Cu (111) surface is governed by the energy due to dispersion forces. Hardeners of the thiol type, which contain relatively heavy sulfur atoms, and hardeners with aromatic rings, displaying high planarity, enable the entire molecule to approach the metal surface, resulting in a relatively high adhesion strength. The calculations for the Cu2O (111) surface show the adhesion strength is more strongly influenced by interactions such as hydrogen bonds between the surface and adhesive molecules than by dispersion forces. Therefore, in adhesion to Cu2O (111), the benzylamine-epoxy adduct with hydrogen bonding and OH-π interactions with the surface, in addition to having a relatively flexible framework, shows a high adhesion strength.

Elucidating the Effects of Chemisorbed Water Molecules on the Adhesive Interactions of Epoxy Resin to γ-Alumina Surfaces

The adhesion mechanism of epoxy resin to the γ-alumina (110) surface was investigated using first-principles density functional theory (DFT). Aluminum materials are lightweight and are used in a wide range of industrial fields. Its surface is oxidized to alumina, and the stable surface is known as the γ-alumina (110) surface. The coverage of hydroxy groups by chemisorbed water molecules on this surface varied depending on the pretreatment temperature. In this study, we investigated the adhesive interactions of epoxy resin on four alumina surfaces with different densities of surface hydroxy groups (0, 3, 6, and 9 OH/nm2) and have discussed their effects. At each interface, the energy curves of the vertically displaced epoxy resin were calculated and the adhesive forces were estimated by differentiating these curves. As the coverage of the surface hydroxy groups increased from 0 to 6 OH/nm2, the adhesive strength gradually decreased. However, the adhesive strength at 9 OH/nm2 was relatively large and almost equal to that at 3 OH/nm2. This inverse volcano-type behavior was analyzed via the decomposition of adhesive forces and the crystal orbital Hamilton population (COHP). The decomposition of adhesive forces into DFT and dispersion components revealed that the inverse volcano-type behavior is derived from the DFT component, and the interfacial interactions owing to the DFT component are accompanied by charge transfer. These were investigated using a COHP analysis, which revealed that this behavior was caused by changes in the activity of the aluminum atoms on the surface and surface reconstruction by chemisorbed water molecules. It is noteworthy that the adhesive strength for 9 OH/nm2 was only 6.9% lower than that for 0 OH/nm2 wherein the chemisorbed water molecules were completely removed from the surface. These results are expected to provide a guideline for the adhesion of epoxy resin to aluminum materials.

Molecular Understanding of Adhesion of Epoxy Resin to Graphene and Graphene Oxide Surfaces in Terms of Orbital Interactions

The adhesion mechanism of epoxy resin (ER) cured material consisting of diglycidyl ether of bisphenol A (DGEBA) and 4,4'-diaminodiphenyl sulfone (DDS) to pristine graphene and graphene oxide (GO) surfaces is investigated on the basis of first-principles density functional theory (DFT) with dispersion correction. Graphene is often used as a reinforcing filler incorporated into ER polymer matrices. The adhesion strength is significantly improved by using GO obtained by the oxidation of graphene. The interfacial interactions at the ER/graphene and ER/GO interfaces were analyzed to clarify the origin of this adhesion. The contribution of dispersion interaction to the adhesive stress at the two interfaces is almost identical. In contrast, the DFT energy contribution is found to be more significant at the ER/GO interface. Crystal orbital Hamiltonian population (COHP) analysis suggests the existence of hydrogen bonding (H-bonding) between the hydroxyl, epoxide, amine, and sulfonyl groups of the ER cured with DDS and the hydroxyl groups of the GO surface, in addition to the OH-π interaction between the benzene rings of ER and the hydroxyl groups of the GO surface. The H-bond has a large orbital interaction energy, which is found to contribute significantly to the adhesive strength at the ER/GO interface. The overall interaction at the ER/graphene is much weaker due to antibonding type interactions just below the Fermi level. This finding indicates that only dispersion interaction is significant when ER is adsorbed on the graphene surface.

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.

Shear adhesive strength between epoxy resin and copper surfaces: a density functional theory study

Shear adhesive strengths of epoxy resin for copper and copper oxide surfaces are estimated based on quantum chemical calculations. Shear adhesion has periodicity, and its origin is revealed.

Augmented Self-Association by Electrostatic Forces in Thienopyrrole-Fused Thiadiazoles that Contain an Ester instead of an Ether Linker

During the self-assembly of π-conjugated molecules, linkers and substituents can potentially add supportive noncovalent intermolecular interactions to π-stacking interactions. Here, we report the self-assembly behavior of thienopyrrole-fused thiadiazole (TPT) fluorescent dyes that possess ester or ether linkers and dodecyloxy side chains in solution and the condensed phase. A comparison of the self-association behavior of the ester- and ether-bridged compounds in solution using detailed UV-vis, fluorescence, and NMR spectroscopic studies revealed that the subtle replacement of the ether linkers by ester linkers leads to a distinct increase in the association constant (ca. 3-4 fold) and the enthalpic contribution (ca. 3 kcal mol-1). Theoretical calculations suggest that the ester linkers, which are in close proximity to one another due to the π-stacking interactions, induce attractive electrostatic forces and augment self-association. The self-assembly of TPT dyes into well-defined 1D clusters with high aspect ratios was observed, and their morphologies and crystallinity were investigated using SEM and X-ray diffraction analyses. TPTs with ester linkers exhibit a columnar liquid crystalline mesophase in the condensed phase.

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.

Elucidation of Adhesive Interaction between the Epoxy Molding Compound and Cu Lead Frames

Clarification of adhesive interactions in semiconductor packages can improve reliability of power electronics. In this study, the adhesion interfaces between the epoxy molding compound and Cu-based lead frames were analyzed using the density functional theory. A resin fragment was prepared based on the polymer framework formed in the curing reaction of epoxy cresol novolac (ECN) and phenol novolac (PN), which are typical molding materials. The resin fragment was optimized on the surfaces of Cu and Cu2O. We calculated the charge density differences for adhesion structures and discussed the origin of adhesive interactions. The ECN-PN fragment's adhesion to the Cu surface relied mainly on dispersion forces, whereas in the case of Cu2O, the resin bonded chemically to the surface via (1) σ-bonds formed between the ECN-PN's OH group oxygen and coordinatively unsaturated copper (CuCUS) and (2) hydrogen bonds between resin's OH groups and coordinatively unsaturated oxygen (OCUS) located close to to CuCUS, resulting in a stable adhesive structure. The energy required to detach the resin fragment from the optimized structure was determined using the nudged elastic band method in each model of the adhesive interface. Morse potential curve was used to approximate the obtained energy, and the energy differentiation by detachment distance yielded the theoretical adhesive force. The maximum adhesive stress was 1.6 and 2.2 GPa for the Cu and Cu2O surfaces, respectively. The extent to which the ECN-PN fragment bonded to the Cu2O surface stabilized was 0.5 eV higher than in the case of the Cu surface.

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.

Energy Decomposition Analysis of the Adhesive Interaction between an Epoxy Resin Layer and a Silica Surface

We investigate the adhesive interaction energy (ΔE_int) between an epoxy resin and a silica surface using pair interaction energy decomposition analysis (PIEDA), which decomposes ΔE_int into four components: electrostatic (ΔE_es), exchange repulsion (ΔE_ex), charge-transfer (ΔE_ct), and dispersion (ΔE_disp) energies based on quantum chemistry. Our previous study with PIEDA showed that synergistic effects of ΔE_es and ΔE_disp are critical at the interface between an epoxy resin fragment and a hydrophilic surface. The present study is designed to show in detail that the synergistic effects are significant at the interface between an epoxy layer model consisting of 20 epoxy monomers and a hydrophilic silica surface. The ratio of the dispersion energies to the total interaction energies of the layer model shows good agreement with experimental values, that is, the dispersion ratio of the work of adhesion (W_ad). The 20 epoxy molecules in the layer model are investigated individually to closely correlate the four decomposed energies with their structural features. Our energy-decomposition analyses show that H-bonding and OH-π interactions play important roles at the interface between an epoxy resin and a silica surface. PIEDA calculations for the epoxy layer model also show that the region 3.6 Å from the silica surface accounts for more than 99% of the total interaction energies.

Role of Hydrogen-Bonding and OH-π Interactions in the Adhesion of Epoxy Resin on Hydrophilic Surfaces

Epoxy resin adhesives are widely used for joining metal alloys in various industrial fields. To elucidate the adhesion mechanism microscopically, we investigated the interfacial interactions of epoxy resin with hydroxylated silica (0 0 1) and γ-alumina (0 0 1) surfaces using periodic density functional theory calculations as well as density of states (DOS) and crystal orbital Hamilton population (COHP) analyses. To better understand the interfacial interactions, we employed and analyzed water and benzene molecules as hydrophilic and hydrophobic adsorbates, respectively. Structural features and calculated adhesion energies reveal that these small adsorbates have a higher affinity for the γ-alumina surface than that for the silica surface, while a fragmentary model for the epoxy resin exhibits a strong interaction with the silica surface. This discrepancy suggests that the structural features of the hydroxylated silica surface dictate its affinity to a specific species. Partial DOS and COHP curves provide evidence for the presence of OH-π interactions between the OH groups on the surfaces and the benzene rings of the epoxy resin fragments. The orbital interaction energies of the H-bonding and OH-π interactions evaluated from the integrated COHP indicate that the OH-π interaction is a nonnegligible origin of the adhesion interaction, even when polymers with hydrophobic benzene rings are adsorbed on hydroxylated surfaces.

Adhesion of Epoxy Resin with Hexagonal Boron Nitride and Graphite

Adhesion interaction of epoxy resin with the basal surfaces of h-BN and graphite is investigated with the first-principles density functional theory calculations in conjunction with the dispersion correction. The h-BN/epoxy and graphite/epoxy interfaces play an important role in producing nanocomposite materials with excellent thermal dissipation properties. The epoxy resin structure is simulated by using four kinds of fragmentary models. Their structures are optimized on the h-BN and graphite surfaces after an annealing simulation. The distance between the epoxy fragment and the surface is about 3 Å. At the interface between h-BN and epoxy resin, no H-bonding formation is observed, though one could expect that the active functional groups of epoxy resin, such as hydroxyl (-OH) group, would be involved in a hydrogen-bonding interaction with nitrogen atoms of the h-BN surface. The adhesion energies for the two interfaces are calculated, showing that these two interfaces are characterized by almost the same strength of adhesion interaction. To obtain the adhesion force-separation curve for the two interfaces, the potential energy surface associated with the detachment of the epoxy fragment from the surface is calculated with the help of the nudged elastic band method and then the adhesion force is obtained by using either the Morse-potential approximation or the Hellmann-Feynman force calculation. The results from both methods agree with each other. The maximum adhesion force for the h-BN/epoxy interface is as high as that for the graphite/epoxy interface. To better understand this result, a force-decomposition analysis is carried out, and it has been disclosed that the adhesion forces working at both interfaces mainly come from the dispersion force. The trend of increase in the C 6 parameters used for the dispersion correction for the atoms included in the h-BN or graphite surface is in the order: N < C < B, which reasonably explains why the strengths of the dispersion forces operating at the two interfaces are similar. Also, the electron localization function analysis can explain why the h-BN surface cannot form an H bond with the hydroxyl group in epoxy resin.

Density-Functional Tight-Binding Study on the Effects of Interfacial Water in the Adhesion Force between Epoxy Resin and Alumina Surface

Adhesion is one of the most interesting subjects in interface phenomena from the viewpoint of wide-range applications as well as basic science. Interfacial water has significant effects on coatings, adhesives, and fiber-reinforced polymer composites, often causing adhesion loss. The way of thinking based on quantum mechanics is essential for a better understanding of physical and chemical properties of adhesive interfaces. In this work, the molecular mechanism of the adhesion interaction between epoxy resin and hydroxylated alumina surface in the presence of interfacial water molecules is investigated by using density-functional tight-binding calculations. Periodic slab model calculations demonstrate that hydrogen bond is an important factor at the adhesion interface. Effects of interfacial water molecules located between epoxy resin and hydroxylated alumina surface are assessed by using a dry model without interfacial water and wet models with water layers of 3, 6, and 9 Å thicknesses. Interesting first- and second-layer structures are observed in the distribution of interfacial water molecules in the tight space between the adhesive and adherend. Energy plots with respect to the displacement of epoxy resin from the alumina surface are nicely approximated by the Morse potential. The adhesion force and stress are theoretically obtained by differentiating the potential curve with respect to the displacement of epoxy resin. Computational results show that the adhesion force and stress are significantly weakened with an increase in the thickness of interfacial water layer. Thus, interfacial water molecules have a clue as to the role of water in the loss of adhesion.