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Cellulose: the plant kingdom's strong material

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There is much interest in protein-based strong materials from animals as inspiration for the design of new synthetic polymers and composites. Cellulose is equally inspirational, but different. Microfibrils of pure crystalline cellulose, which are a few nanometers thick, have tensile properties comparable with Kevlar and a strength: weight ratio better than steel. The tensile stiffness of crystalline cellulose is based on a remarkable form of co-operation between hydrogen bonds and the parallel covalent bonding that links the glucose units within each polymer chain. The mechanism of this co-operative elastic stiffness was elucidated by a combination of crystallography under tension and vibrational bandshift analysis. The underlying principle may be termed ‘molecular leverage’, a concept with scope for application to other biopolymers. In plant materials like wood, cotton and the walls of growing plant cells, cellulose is not found in pure crystalline form. Its hydrogen-bonding pattern is partially disordered, particularly at the surfaces of the microfibrils. It is associated with other polysaccharides, loosely termed hemicelluloses, that have similar structures but less capacity for intramolecular hydrogen bonding. Compared to cellulose, hemicellulose chains associate less with one another and more with water. In materials like wood there is a disorder gradient from the crystalline centres of the microfibrils, through their surfaces and bound hemicelluloses, to the viscous, hydrated matrix between. When such materials are under tension they extend by a combination of elastic stretching of cellulose and shear between the cellulose microfibrils, in a ratio that depends on the cellulose orientation. How the shear component dissipates energy to retard fracture is not well understood, although it clearly differs from energy dissipation in strong animal materials like spider silk. Stress-relaxation experiments will be described that make use of vapour-phase deuteration, combined with FTIR bandshift analysis and neutron diffraction, to distinguish crystalline cellulose from the hydrated, less ordered, domains. Cellulose orientation is under tight developmental and genetic control: it is the principal factor that underlies both plant morphogenesis and the mechanical properties of plants. Understanding the mechanical function of cellulosic nanostructures in vivo will give us new insights into the design of new, strong composites and the performance of traditional, sustaina

This talk is part of the Seminars at the Department of Biochemistry series.

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