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

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.

Protonation of Strained Epoxy Resin under Wet Conditions via First-Principles Calculations Using the H+-Shift Method

A significant challenge in adhesive bonding is the accelerated breaking of stretched adhesives under wet conditions, which is known as cohesive failure. One group of commonly used adhesives consists of the amine-cured epoxy resins. Based on deprotonation free-energy calculations of the unstrained resin in water, it has recently been proposed that these adhesives can undergo failure through breakage originating at the protonated amine group under neutral or acidic conditions [J. Phys. Chem. B 2021, 125, 8989-8996]. In this study, we comprehensively investigated the degree of protonation of the amine group under both stretched and compressed conditions by devising a robust first-principles protonation calculation method applicable to strained materials. It was found that the amine group was partially protonated in neutral water at 298 K and that the amine group was protonated when the epoxy resin was stretched to a greater extent in water, and vice versa. These findings support the physicochemical cause of cohesive failure due to protonation of the amine group in the stretched amine-cured epoxy resins.

Influence of Water on an Epoxy/Amine-Metal Interphase: A Combined DFT and Mixing Calorimetry Approach

Epoxy-amine systems are ubiquitous in the field of industrial thermosetting polymers, often used in a moist atmosphere. In addition, previous studies showed amine-metal interactions through the formation of an interphase, with the formation of surface complexes that may involve the formation of water molecules. However, to date, the impact of water on an epoxy/amine-metal interphase has not been specifically addressed. In this work, we examined for the first time the role of this potential fourth component by way of a dual experimental/computational approach. The effect of water on the glass-transition temperature of the obtained polymers was quantified. The in situ formation of a DETA-Al-water interphase was followed by mixing calorimetry. The DETA-water interaction was highly exothermic, and the underlying mechanism was discussed on the basis of DETA hydration, which was confirmed by density functional theory (DFT) and Monte Carlo simulations. Taking into account the pre-existing interaction between diethylenetriamine (DETA) molecules allowed us to model all experimental data. Comparison of experimental and calculated IR spectra contributed to validate the simulation parameters used. Our findings indicate that the presence of water may noticeably affect epoxy-amine-based systems. Mixing calorimetry and computational modeling appear as particularly adapted tools for the comprehension of such complex systems.

Directly Using Paraffin as the Toughening Agent of Epoxy Composites: An Experimental and Molecular Dynamics Simulation Study

It is still a challenge in studying the toughening mechanism by well combining the experimental and atomistic molecular dynamics (MD) simulation study. This article directly introduced eicosane (C20, model compound of paraffin) into the epoxy matrix (DGEBA) by using a special epoxy resin with alkyl side chains (D12) as a compatibilizer, which was synthesized through thiol-ene click chemistry. The toughening mechanism of the ternary DGEBA/D12/C20 (EPDA-X) systems was systematically investigated by experimental and MD simulation methods. Though C20 can be well dispersed in the curing mixture, the huge polarity difference between C20 and DGEBA can be the driving force for C20 to stay away from DGEBA, demonstrating the self-assembly effect of C20 around the alkyl side chains of D12 because of the good compatibility of D12 and C20. The soft alkyl chains of D12 and C20 as well as the self-assembly effect of C20 around the D12 molecules can simultaneously improve the strength, modulus, and toughness of the EPDA-2.5 system. This article not only provides a brand new toughening strategy by directly using nonfunctional alkyl derivatives as the toughening agent of epoxy composites with superior mechanical properties but also provides a systematic MD simulation method to evaluate whether there is the interaction or not and the strength of interaction between different molecular chains so as to provide a theoretical basis for the cause of the microphase separation structure and related toughening mechanism in cross-linking networks on the atomic and molecular levels.

Neutron Reflectivity Study on the Suppression of Interfacial Water Accumulation between a Polypropylene Thin Film and Si Substrate Using a Silane-Coupling Agent

We measured the neutron reflectivity (NR) of isotactic polypropylene (PP) thin films deposited on Si substrates modified by hexamethyldisilazane (HMDS) at the saturated vapor pressure of deuterated water at 25 °C and 60 °C/85% RH to investigate the effect of HMDS on the interfacial water accumulation in PP-based polymer/inorganic filler nanocomposites and metal/resin bonding materials. We found that the amount of water accumulated at the PP/Si interface decreased with increasing immersion time of the Si substrate in a solution of HMDS in hexane prior to PP film deposition. During the immersion of the Si substrate, the HMDS molecules were deposited on the Si substrate as a monolayer without aggregation. Furthermore, the coverage of the HMDS monolayer on the Si substrate increased with increasing immersion time. At 60 ° C and 85% RH, only a slight amount of interfacial water was detected after HMDS treatment for 1200 min. As a result, the maximum concentration of interfacial water was reduced to 0.1 from 0.3, where the latter corresponds to the PP film deposited on the untreated substrate.

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.