Chirality in cellulose and cellulose-based materials

Chirality plays a key role in biochemical and organic chemical reactions and properties at a molecular level. However, molecular chirality may also be expressed in material properties at higher levels of organization. Cellulose and cellulosic materials have provided some of the first recognized examples of such chiral structures and properties. A wide range of cellulose derivatives form chiral nematic phases, both in concentrated solution and in the melt. Solid films and gels retaining the chiral nematic ordering of the polymer chains may be prepared from these liquid crystalline phases. Dilute aqueous suspensions of cellulose crystallites prepared by acid degradation also show chiral nematic order. The order is retained in solid films formed by removing the water from the suspensions. In nature, chiral nematic structures have been observed in natural composites based on cellulose, chitin and collagen. However, the commonest chiral structure is not chiral nematic: it is the helically wound microfibrillar structure in the secondary wall of wood fibres, where the cellulose microfibrils form a steep right-handed helix. This structural chirality at the microscopic level influences macroscopic material properties. For example, paper and wood may display a chiral mechanical response to changes in relative humidity. Cellulosic materials thus form chiral structures at length scales from the molecular to the macroscopic.