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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1. Introduction In recent years, carbon fiber reinforced composites (CFRP), which achieve excellent specific strength and specific stiffness through the combination of carbon fibers and matrix polymers, have been expanding their applications, especially as structural materials for automobiles. However, conventional CFRP has been pointed out to have poor toughness and environmental adaptability, requiring a new materials design. Considering environmental adaptability, bio-based materials have attracted significant attention for new material design because of their low environmental impact and excellent recyclability (Dobos et al. 2012) (Thakur, Singha and Mehta 2010) (Necula et al. 2010). Among such bio-based materials, lignin, one of the main components of plant cell walls, has advantageous not only in economic benefits but also in mechanical properties; lignin, which has been disposed of as industrial waste, can react with epoxy resin which is a conventional thermosetting polymer constituting the matrix part of CFRP. Vijay Kumar et al. examined the potential of lignin in polymer composites. Based on recent advances and challenges in the use of lignin in composite material development, they discussed the structure and function of lignin polymers and introduce research on lignin-based polymer composites for engineering applications (Thakur et al. 2014). Lignin-based composites are of growing interest, and significant efforts are devoted to the preparation of high performance composites with lignin as a low-cost and low environmentally impact material. Although lignin is a promising candidate for new green composite materials, its complex natures make it difficult to process. Therefore, to improve the processibility, the chemical modification of lignin is important. Recently, “glycol lignin”, a polyethylene glycol PEG)-modified lignin with improved processibility over pure lignin, has attracted attentions. Nge et al. have industrially produced glycol lignin to meet commercial and industrial needs for advanced lignin-based material production. Thus, glycol lignin has potential to be structural material with excellent environmental adaptability in addition to its material properties (Nge et al. 2018). On the other hand, its applicability has not been fully investigated. This study considers the applicability of glycol lignin to CFRP matrices. In particular, the purpose of this study is to elucidate the mechanism of material property of glycol lignin from a molecular point of view. Therefore, material characteristics are obtained using all-atom molecular dynamics (MD) simulations (Jun et al. 2019) (Iwamoto, Oya and Koyanagi 2022), which are time- and cost-effective and can link microscopic chemical structures to macroscopic thermodynamic properties (Takase et al. 2021) (Sakai et al. 2022) (Oya et al. 2018). For example, there are studies that evaluated the interface stability between different materials in a composite materials using molecular dynamics (MD) simulation (Morita et al. 2022) (Kasahara et al. 2020), and that developed an evaluation method that considers dissociation of covalent bonds in molecules (Yamada et al. 2023) and so on. In this study, we investigate the mechanical properties of (glycol) lignin/epoxy polymers in both bulk and interface systems. It is well known that epoxy and lignin (with and without PEG) chemically reacted each other. This study considers a polymer model in which (glycol) lignin and epoxy (Bisohenol A diglycidyl ether, DGEBA) are linearly coupled. This polymerization model is justified by the following two previous findings. First, both glycol-lignin and DGEBA have two reactive functional groups per molecule. Second is the relative abundance of linear polymers in the vicinity of the interface. In order to study the effect of the side chains (PEGs) attached to the lignin on the mechanical properties, we prepare two polymerization models of matrix polymers with and without PEG. In following, epoxy/lignin and Epoxy/Glycol Lignin are referred to “EL” and “EGL”, respec tively. Our simulations are roughly classified into following two. First, uniaxial tensile simulations for EL and EGL are performed to evaluate bulk mechanical properties. Second, interface models between matrix polymers and reinforcement are constructed to obtain interfacial stability that is strongly correlated with interface strength. The performance of composite materials depends not only on the properties of reinforcing fibers and matrix polymers, but also on their interfacial properties. The force applied to the composite material is transmitted through the interface between the fiber reinforcement and polymers, and the composite material easily breaks when the interfacial adhesiveness is weak. Surface modification of carbon fibers by oxidation treatment is known to improve interfacial adhesion. This study considers carbon fibers with and without functional groups as reinforcement. Thus, the interfacial energies are obtained by MD simulations for six different composite models consisting of two different matrix polymers (Epoxy-Lignin (EL), Epoxy-Glycol Lignin (EGL)) and three different graphene with and without functional groups, including oxygen (-O-) and hydroxyl groups (-OH).