Bamboo, a fast-growing grass, has higher strength-to-weight ratios than steel and concrete. Here we have experimentally and numerically studied mechanical and fracture properties of bamboo at multiple scales from atomistic scale to the culm scale. The unique properties of bamboo come from the natural composite structure of fibers that comprises mainly cellulose nanofibrils in a matrix of intertwined hemicellulose and lignin called lignin-carbohydrate complex (LCC). We have utilized atomistic simulations to investigate the mechanical properties and mechanisms of the interactions of these materials in the structure of bamboo fibers. It is shown that a control hemicellulose model has better thermodynamic and mechanical properties than lignin while lignin exhibits greater tendency to adhere to cellulose nanofibril. Therefore, the role of hemicellulose found to be enhancing the mechanical properties while lignin provides the strength of bamboo fibers. We have studied the effects of water on thermodynamics of bamboo at multiple scales. Elastic modulus of lignin, calculated by molecular dynamics simulations, increases initially with increasing moisture content, and then decreases. In contrast, elastic modulus of hemicellulose decreases constantly with MC. Below 10% MC, water molecules tend to break hydrogen bonds between polymer chains and form new hydrogen bond bridges between the polymer chains, while above 10% MC, water molecules aggregate together and create nano-droplets inside the materials. During the process of bridging, the fractional free volume of lignin decreases. The free volume reduction along with formation of hydrogen bond bridges causes a growth in elastic modulus of lignin at low MC. The constant increase of hemicellulose free volume, however, causes the aggregation of voids in the system and diminution of elastic properties.

Nanostructures, molecular interactions, and anisotropic elastic moduli of lignocellulosic cell walls with moisture

255th National Meeting (2018)

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