The quintessential form of cellulose in wood consists of microfibrils that have high aspect ratio crystalline domains embedded within an amorphous cellulose domain. In this study, we apply unitedatom molecular dynamics simulations to quantify changes in different morphologies of cellulose. We compare the structure of crystalline cellulose with paracrystalline and amorphous phases that are both obtained by high temperature equilibration followed by quenching at room temperature. Our study reveals that the paracrystalline phase may be an intermediate, kinetically arrested phase formed upon amorphisation of crystalline cellulose. The quenched structures yield isotropic amorphous polymer domains consistent with experimental results, thereby validating a new computational protocol for achieving amorphous cellulose structure. The non-crystalline cellulose compared to crystalline structure is characterized by a dramatic decrease in elastic modulus, thermal expansion coefficient, bond energies, and number of hydrogen bonds. Analysis of the lattice parameters shows that Ib cellulose undergoes a phase transition into hightemperature phase in the range of 450-550 K. The mechanisms of the phase transition elucidated here present an atomistic view of the temperature dependent dynamic structure and mechanical properties of cellulose. The paracrystalline state of cellulose exhibits intermediate mechanical properties, between crystalline and amorphous phases, that can be assigned to the physical properties of the interphase regions between crystalline and amorphous cellulose in wood microfibrils. Our results suggest an atomistic structural view of amorphous cellulose which is consistent with Electronic supplementary material The online version of this article (
The interaction of water with many biopolymers is known to rearrange their internal structure, make them moisture sensitive, and influence their physical properties. We study amorphous cellulose and hemicellulose, two hydrophilic biopolymers, using molecular dynamics simulations, and we analyze their structural and physical properties over the full range of moisture content. We find a quasi-linear dependence of volumetric strain on moisture content, and a linear scaling between volumetric strain and porosity, showing that swelling is directly related to the space created by adsorbed water molecules. The interaction of water with the polymer structure results in a weakening of the mechanical properties, leading to rubberlike behavior at high moisture content. Weakening is caused by a decrease in the number of hydrogen bonds that follow exponential scaling. Breaking of the hydrogen bonds system is found to control not only the mechanical response but also the evolution of porosity and the volumetric strain.
A two-phase model of a wood microfibril consisting of crystalline cellulose and amorphous hemicellulose is investigated with molecular dynamics in full range of sorption to understand the molecular origin of swelling and weakening of wood. Water is adsorbed in hemicellulose, and an excess of sorption is found at the interface, while no sorption occurs within cellulose. Water molecules adsorbed on the interface push away polymer chains, forcing the two phases to separate and causing breaking of h-bonds, particularly pronounced on the interface. Existence of two different regions in moisture response is demonstrated. At low moisture content, water is uniformly adsorbed within hemicellulose, breaking a small amount of hydrogen bonds. Microfibril does not swell, and the porosity does not change. As moisture content increases, water is adsorbed preferentially at the interface, which leads to additional swelling and porosity increase at the interface. Young's and shear moduli decrease importantly due to breaking of h-bonds and screening of the long-range interactions.
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