Multiscale modeling of the hierarchical structure of cellulose nanocrystals

Cellulose constitutes the most abundant renewable polymeric resource available today. It considered an almost inexhaustible source of raw material, and holds great promise in meeting increasing demands for environmentally friendly and biocompatible products. Key future applications are currently under development for the automotive, aerospace and textile industries. When cellulose fibers are subjected to acid hydrolysis, the fibers yield rod-like, highly crystalline residues called cellulose nanocrystals (CNCs). These particles show remarkable mechanical and chemical properties (e.g. Young Modulus ∼200 GPa) within the range of other synthetically-developed reinforcement materials. Critical to the design of these materials are fundamental material properties, many of which are unavailable in the existing literature. A multiscale framework has been developed to predict and describe the thermo-mechanical characteristics of cellulose nanocrystals using state-of-the-art computational tools capable of connecting atomistic based simulations to experiments through continuum based modeling techniques. ^ First-principle density functional theory and molecular dynamic simulations were utilized at the atomistic level. Longstanding issues regarding the elastic and thermal expansion anisotropies for crystalline cellulose have been studied in terms of the single-crystal elasticity tensor and the thermal expansion tensor components. ^ First-principles phonon calculations via van der Waals density functionals as well as reverse non-equilibrium molecular dynamics simulations were used to gain a fundamental understanding of defect-free, crystalline cellulose thermo-mechanical properties. Entropy, enthalpy, constant pressure heat capacity, thermal expansion tensor, thermal conductivity, Young\u27s modulus, and Poisson\u27s ratio, were computed over a wide range of temperatures (0 to 500 K). A comprehensive study of the hydrogen bond structure that characterizes crystalline cellulose has been conducted in an attempt to ascertain the roles that inter- and intra- molecular hydrogen bonds play in determining the mechanical properties of CNCs. Five different force fields/parameter sets were compared with experimental results and first-principles simulations in terms of their ability to predict the following properties: lattice parameters and angles, linear elasticity tensor and linear thermal expansion tensor. Continuum based modeling techniques were used to answer fundamental questions regarding the role of hydrogen bonding in the mechanical response of CNCs. A variety of finite element-based continuum models were specifically developed for cellulose chains and non-bonding interactions (van der Waals, Coulomb and hydrogen bonds). As a result, a complete multiscale framework capable of reproducing the mechanical behavior of cellulose nanocrystals has been developed.