The Crystallinity of Nanocellulose: Dispersion-Induced Disordering of the Grain Boundary in Biologically Structured Cellulose

In trees, a-few-nanometers-wide crystalline fibrils of cellulose are tightly bundled with other biopolymers, such as lignin and hemicelluloses, to form robust cell walls. Cellulose nanofibers (CNFs) are obtained by successive treatments including the purification, modification, and disintegration of cell-wall cellulose. Herein, we report that the crystallinity of CNFs is governed by the interface between the bundled cellulose fibrils. The cellulose molecules at the interface or at the surface of the individual fibrils are ordered when the fibrils are more densely bundled upon removal of hemicelluloses. In turn, the interfacial molecules become disordered when bundled fibrils disperse upon disintegration as CNFs. Morphological analysis of the dispersed fibrils supports a model where single-cellulose fibrils in trees are composed of 18 molecular chains. Considering the 18-chain model, the surface molecules of the dispersed fibrils are disordered in the conformation of carbon atoms as analyzed by NMR, while all of the 18 chains are fully ordered in molecular sheet stacking to diffract X-rays.