Abstract
Cellulose is crystallized by plants and other organisms into fibrous nanocrystals. The mechanical properties of these nanofibers and the formation of helical superstructures with energy dissipating and adaptive optical properties depend on the ordering of polysaccharide chains within these nanocrystals, which is typically measured in bulk average. Direct measurement of the local polysaccharide chain arrangement has been elusive. In this study, we use the emerging technique of scanning electron diffraction to probe the packing of polysaccharide chains across cellulose nanofibers and to reveal local ordering of the chains in twisting sections of the nanofibers. We then use atomic force microscopy to shed light on the size dependence of the inherent driving force for cellulose nanofiber twisting. The direct measurement of crystalline twisted regions in cellulose nanofibers has important implications for understanding single-cellulose-fibril properties that influence the interactions between cellulose nanocrystals in dense assemblies. This understanding may enable cellulose extraction and separation processes to be tailored and optimized.
Highlights
Cellulose is the most abundant biomolecule on Earth, produced in the cell wall of plants and many other organisms by the condensation polymerization of glucose
Twisting and kinking of nanocellulose extracted from both plants and other organisms has been observed by electron microscopy and atomic force microscopy (AFM).[10−12] This twist is associated with the tough, energy dissipating, cross-ply structures in the cell walls of many plants and the ability of CNCs to form cholesteric phases,[3] but the local atomic and molecular arrangement in these twisted regions has remained elusive in the absence of a suitable nanoscale structural probe
This indexation of the diffraction data shows that the long axis of the cellulose nanofibers (CNFs) corresponds to the crystallographic c-axis, which is the direction of the polysaccharide chain axis (Figure 1d−f)
Summary
Cellulose is the most abundant biomolecule on Earth, produced in the cell wall of plants and many other organisms by the condensation polymerization of glucose. The constituent polysaccharide chains typically form nanocrystalline units with dimensions that depend on the organism growing the crystals. Such “nanocellulose”, which may comprise cellulose nanofibers (CNFs) or nanocrystals (CNCs), is lightweight, strong, and chiral, leading to significant interest for sustainable materials applications ranging from biomedical scaffolds to thermal insulation foams, packaging materials, and photonic films.[1−3]. Twisting and kinking of nanocellulose extracted from both plants and other organisms has been observed by electron microscopy and atomic force microscopy (AFM).[10−12] This twist is associated with the tough, energy dissipating, cross-ply structures in the cell walls of many plants and the ability of CNCs to form cholesteric phases,[3] but the local atomic and molecular arrangement in these twisted regions has remained elusive in the absence of a suitable nanoscale structural probe
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