This research explores the effect of the cell size, cross-linking ratio, and the force fields used in the molecular dynamic simulation for determining the mechanical and thermal properties of cross-linked epoxy formed with a heuristic cross-linking procedure. The effects of the abovementioned variables on density, Young’s modulus, shear modulus, bulk modulus, and glass transition temperature values by molecular dynamics (MD) simulation were evaluated. Epoxy resin diglycidyl ether of bisphenol A and hardener diethyl toluene diamine were used in modeling the epoxy. A Heuristic method for reactive molecular dynamics (REACTER) protocol was used as the cross-linking procedure. Firstly, six structure cells were prepared in different cell sizes with a crosslinking ratio of 75%, and a mechanical analysis of all cells was performed. Then, the largest cell was prepared for three different crosslink ratios and its mechanical and thermal properties were calculated. Finally, the mechanical properties of the largest cell were calculated using the three different force fields namely the COMPASS, DREIDING, and UNIVERSAL. The results were also compared with the molecular dynamic simulation results performed using the other crosslinking procedures, and experimental results available in the literature. In comparison, it was observed that the results obtained with MD simulations coincided with the experimental data. It has been concluded that using the largest cell gives closer results to the experimental data but the processing time is also increasing rapidly. Moreover, it was also observed that the increase in the crosslinking ratio caused an increase in the mechanical properties of the epoxy and a significant increase in the glass transition temperature. Finally, compared to other force fields, it is seen that the mechanical analysis results obtained with the COMPASS force field comply more with the experimental data.
Experimental studies have shown that adding graphene to epoxy at even a low weight ratio increases strength, temperature resistance, and electrical conductivity. This study examines the effects of combining graphene with epoxy and assesses Young's modulus of the resulting nano‐composite materials with various defective graphene types. Molecular dynamics (MD) simulation is are utilized to predict the mechanical properties of nano‐composite materials, and a heuristic protocol is implemented to avoid incorrect bond structures during crosslinking reactions. The study demonstrates that adding graphene to epoxy increases Young's modulus, but the effect declines when the graphene is defective. These results significantly advance the field's knowledge by providing a new understanding of the effects of pristine and defective graphene reinforcement on epoxy Young's modulus. This research stands out for its evaluation of the impact of various graphene defects on epoxy properties using MD simulations. The employed heuristic protocol in this study, which includes thorough updates of both bonded and non‐bonded terms as well as partial charges, serves as a more reliable algorithm than iterative multi‐step protocols by reducing the risk of incorrect molecular linkages resulting from the neglect of angle‐based covalent terms. Additionally, this study evaluates the constant strain minimization method's suitability for determining the mechanical properties of nano‐composite materials. Finally, a thorough comparison between the simulation results and the existing experimental findings reported in scholarly sources was made to guarantee the full disclosure of both qualitative and quantitative results within the scope of the investigation.
Using a new heuristic procedure, the influence of graphene reinforcement on Young's modulus of crosslinked epoxy was analyzed. Graphene reinforcement was investigated for 1%, 2%, 3%, and 4% weight ratios. Graphene sheet edges were functionalized with hydrogen atoms and were placed middle of simulation cells. Simulation cell sizes were determined such that the graphene sheets were non-periodic. Thus, the edge effects of graphene sheets could be observed in dynamic simulations. The heuristic protocol is used for the crosslinking process of epoxy. It is less sophisticated than the multi-step iterative approach and is utilized for various components. It also updates higher-order covalent bond and partial charge terms in real-time to prevent inaccurate chemical couplings caused by ignoring angle-based covalent terms. Crosslinked epoxy structures were created by 80% with this new heuristic protocol as a matrix structure. To analyze the multiple variations with the same amount of molecules in each weight ratio, each simulation cell was built as three individual samples, and the standard deviation values were calculated. Young's modulus of the nanocomposite system was then calculated using a constant-strain energy minimization approach. The inter-atomic and intra-atomic interactions were described using the Molecular Potentials for Atomistic Simulation Studies (COMPASS) force field. As expected, the Young Modulus of epoxy resin increased with the addition of graphene reinforcement. This increase in Young's modulus was calculated by 6% for 1% graphene reinforcement, 10% for 2% graphene reinforcement, 14% for 3% graphene reinforcement, and 16% for 4% graphene reinforcement. As the graphene reinforcement ratio increases, the increase in Young's modulus tends to diminish. It's also worth noting that the MD simulation results in this work were in close agreement with the experimental results published in the literature. Both qualitative and quantitative numerical results show the effect of the abovementioned parameters. They will provide gain energy and time for prior synthesizing the new materials and serve as benchmark solutions for future comparisons of numerical and experimental results.
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