Kai Jin1 Markus Buehler1

1, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States

Wood is a biological material with outstanding mechanical properties resulting from its hierarchical structure across different scales. While the cellular structure and the winding angle between the stiff cellulose fibrils and the wood cell axis have been reported to be key factors for its high stiffness and strength at light weight, the role of the molecular organization of its chemical components: cellulose, hemicellulose and lignin, is still to be fully understood. At the molecular scale, most of the hemicellulose molecules attach on the surface of the stiff crystalline cellulose fibrils, while lignin molecules fill the rest space. By applying a recently developed full atomistic model of the wood cell wall material, we studied how wood benefits from this intriguing molecular organization by intentionally varying the material distribution so that hemicellulose and lignin are randomly mixed or lignin molecules attach on the cellulose surface. It is found that better adhesion between the cellulose fibrils are achieved with the material distribution found in natural wood. Further detailed studies on individual molecules reveal that the hemicellulose show higher adhesion than lignin with cellulose and the three-dimensional structure of lignin originates its higher rigidity. Moreover, the covalent crosslinks between hemicellulose and lignin molecules enhance the load transferring between them. Therefore, the material distribution found in natural wood forms a pathway for load transferring that materials with stronger interactions are paired together. Our study reveals the molecular principles that wood adapts to achieve the outstanding mechanical properties at the macro-scale, which could shed light on the design of composite materials.