8. Native Crystalline Polysaccharide Nanofibers: Processing and Properties

Native polysaccharide nanocrystals have gained increasing interest as fibrous reinforcement in nanocomposites. Unique mechanical properties combined with biodegradability and renewability have placed them as alternative for designing environmentally friendly materials. The source origin and processing have a large impact on the nanofiber dimensions and properties. Most of the studies have been devoted to cellulose and chitin nanocrystals which are organized into fiber bundles in nature. Cellulose nanofibers can be obtained from animal, bacterial, algal, and plant sources. Chitin fibrils constitute, for example, fungal cell walls and arthropod exoskeletons. Based on processing, one defines two major families of polysaccharide nanofibers (whiskers and nanofibrils of polysaccharide). The preparation of the elementary whisker monocrystals has been achieved by acid hydrolysis, which allows collecting them after cleavage of the amorphous domains of the original substrates. Alternatively, the nanofibrillated material constitutes the other family, which results from the peeling of native microfibrils into a network of nanofibrils. The microfibril delamination is often performed with mechanical devices. Chitosan is the deacetylated derivative of chitin. Nevertheless, the preparation of chitosan crystalline nanofibrils that preserve the native directional packing is challenging. The preparation of chitosan nanofibril networks was recently reported by means of a chitosan mild hydrolysis at the solid state. This chapter reviews the methodologies used to produce crystalline nanofibers of polysaccharide with preserved native structural packing. Nanofibers of polysaccharides cellulose, chitin, and chitosan will be the focus of this review. The methods used to characterize these nanofibers will be revised, and the nanofiber properties will be discussed.