Potential of Anisotropic Cellulose Aerogels

Assembly of (bio)polymers into long-range anisotropic nanostructured materials is a naturally occurring building principle. It can be found, for example, in tobacco mosaic and tomato bushy stunt viruses, clay minerals, iridescent iron oxide Schiller layers, or bentonite sols. Inspired by nature, the exploitation of specific self-assembly phenomena in solution or dispersion state is of great interest in advanced materials engineering, too. It enables directional tuning of properties, including diffusivity, light, heat and sound propagation, cell proliferation, stiffness, toughness, elasticity, or resilience. Among the wide variety of biopolymers that have been hitherto explored in this regard, cellulose is probably the most appealing one from the viewpoint of availability, accessibility, purity, inherent properties, and economic considerations and also in the context of large-scale applications. This is also due to the particular self-assembling capabilities of cellulose nanoparticles in aqueous dispersion state which can be set and preserved throughout the preparation of aerogels. This book chapter gives an overview of the different strategies toward the manufacture of anisotropic cellulose aerogels, presents some of their intriguing properties, and adumbrates their great application potential. Following an introduction to specific aspects of cellulose nanoparticles and derived aerogels, the first main part of the chapter discusses comprehensively isolation, properties, and self-assembly of the two principal types of cellulose nanoparticles, i.e., cellulose nanocrystals (CNC) and cellulose nanofibrils (CNFs). The second main part reviews the current state of anisotropic nanocellulose aerogel research. Next to chiral-nematic CNC templates and aerogels, novel approaches toward CNF aerogels are presented. This chapter concludes with a critical discussion of the pros and cons of cellulose nanoparticle aerogels and gives an outlook on potential further applications.