PSI - Issue 52

Mayu Morita et al. / Procedia Structural Integrity 52 (2024) 195–202 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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found near the graphene surface, which cannot be found for graphene modified with oxygen atoms. Further, EGL has a higher intensity of this peak than the EL. These results are consistent with the order of material combinations in interface stability, indicating hydrogen bonds between oxygen atoms of matrix polymer and hydrogen atoms of the hydroxyl groups in graphene significantly contribute to interfacial adhesion. Due to such an atomistic scale interaction, EGL with many oxygen atoms per molecule are strongly attracted to the functionalized graphene. 4. Bond dissociation We are currently conducting MD simulations considering the dissociation of covalent bonds. Epoxy lignin (EL) forms a cross-linking structure by reacting with curing agents such as epoxy resin. This cross link structure contributes to the mechanical properties of lignin. Therefore, it is necessary to perform MD simulations taking into account the dissociation of covalent bonds in the epoxy lignin molecule. As criteria for breaking covalent bonds, we set conditions such as breaking the bond when the interatomic distance exceeds a certain threshold or breaking the bond when the potential energy is lower comparing the potential energies before and after the dissociation of the covalent bond. By using this MD simulation evaluation method, we plan to investigate the covalent bonds in epoxy lignin in more details.

Fig.5 Schematic diagram of the dissociation of covalent bonds

5. Conclusions Elucidating the atomic-scale mechanisms for the mechanical properties and interfacial stability of lignin-based polymers is important for the development of new designs of biobased materials that combine excellent strength and recyclability. In this study, molecular dynamics (MD) simulations were performed to investigate the material characteristics of epoxy/lignin polymers with and without PEG side chains, referred to EL and EGL, respectively. First, stress-strain curves were obtained from uniaxial tensile simulations of these polymers in bulk. In the result, the strength, Young's modulus, and toughness were improved in EGL compared to EL, suggesting that PEG sidechains enhance the bulk mechanical properties due to increasing entanglement points between different molecules. Second, interface energy was evaluated for six interface models by combining two types of polymers (EL and EGL) with three types of graphene sheets (with and without functional groups, including -O, -OH). Our results showed that EGL improved the interface energy with functionalized graphene more than pure lignin. For both polymers, the interfacial stability increased in order of -O and -OH. Therefore, the interface between EGL and functionalized graphene with - OH is most stable among various combinations, which is attributed to the hydrogen bonds between oxygen atoms of EGL and hydrogen atoms of -OH.