Nanomechanics and Nanoscale Adhesion in Biomaterials and Biocomposites: Elucidation of the Underlying Mechanism

Youssefian, Sina

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Nanomechanics and Nanoscale Adhesion in Biomaterials and Biocomposites: Elucidation of the Underlying Mechanism

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Cellulose nanocrystals, one of the most abundant materials in nature, have attracted great attention in the biomedical community due to qualities such as supreme mechanical properties, biodegradability, biocompatibility and low density. In this research, we are interested in developing a bio-inspired material-by-design approach for cellulose-based composites with tailored interfaces and programmed microstructures that could provide an outstanding strength-to-weight ratio. After a preliminary study on some of the existing biomaterials, we have focused our research on studying the nanostructure and nanomechanics of the bamboo fiber, a cellulose-based biocomposite, designed by nature with remarkable strength-to-weight ratio (higher than steel and concrete). We have utilized atomistic simulations to investigate the mechanical properties and mechanisms of interactions between cellulose nanofibrils and the bamboo fiber matrix which is an intertwined hemicellulose and lignin called lignin-carbohydrate complex (LCC). Our results suggest that the molecular origin of the rigidity of bamboo fibers comes from the carbon-carbon or carbon-oxygen covalent bonds in the main chain of cellulose. In the matrix of bamboo fiber, hemicellulose exhibits larger elastic modulus and glass transition temperature than lignin whereas lignin shows greater tendency to adhere to cellulose nanofibrils. Consequently, the role of hemicellulose is found to enhance the thermodynamic properties and transverse rigidity of the matrix by forming dense hydrogen bond networks, and lignin is found to provide the strength of bamboo fibers by creating strong van der Waals forces between nanofibrils and the matrix. Our results show that the amorphous region of cellulose nanofibrils is the weakest interface in bamboo microfibrils. We also found out that water molecules enhance the mechanical properties of lignin (up to 10%) by filling voids in the system and creating hydrogen bond bridges between polymer chains. For hemicellulose, however, the effect is always regressive due to the destructive effect of water molecules on the hydrogen bond in hemicellulose dense structure. Therefore, the porous structure of lignin supports the matrix to have higher rigidity in the presence of water molecules.

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