Cellulose Nanofibrils as a Functional Material

The recent developments to disintegrate nanocelluloses from plant and wood cell materials have attracted considerable interest in materials science. Nanocelluloses are considered as fascinating building blocks for functional materials, thanks to their high mechanical properties, rod-like or fibrous structure, biocompatibility and sustainability. In addition, they own high surface area with numerous free reactive groups offering great opportunities for chemical or physical functionalization. Among nanocelluloses, native cellulose nanofibrils form an important class of materials, and this thesis explores them as building blocks for functional and responsive materials. The characteristics of enzymatically and mechanically prepared cellulose nanofibrils are studied in Publication I. The cellulose nanofibrils form strong inherent hydrogels at low concentration without chemical cross-linking, and show shear thinning which is useful both in processing and in applicability. Publication II demonstrates that the hydrogel enables vacuum freeze-drying to form light-weight aerogels which are ductile and deformable, even if the porosity totals 98%. The tunable morphology provides multiple length scale structures and porosity, as well as percolative template for conducting polymer, e.g. polyaniline to achieve highly porous conducting materials. The aerogels, made from the long and entangled cellulose nanofibrils, are also versatile templates for titanium dioxide (TiO2) and fluorosilane deposition to tune the wetting, and to enable responsive materials. In Publication III, the TiO2-coated nanocellulose aerogel shows switchable water absorption between nonabsorbent and superabsorbent states upon exposure of UV light. In addition, TiO2-coated aerogel shows improved photo-catalytic activity. The aerogel template not only serves as a support for TiO2 but also enhances the effects due to its multiple scale structures. The superoleophobic and -hydrophobic bio-inspired nanocellulose cargo carrier, reported in Publication IV, is also shown to benefit from multiple length scaled structures and the pores. The TiO2 approach is extended in Publication V, where the selective absorption of TiO2-coated nanocellulose aerogels is applied, and floatable oil-absorbing nanocellulose aerogel is demonstrated for oil spill removal. This thesis contributes to basic research on nanocelluloses, while yielding concrete benefits for the industrial context. It furthers the fundamental understanding of the behavior of cellulose nanofibrils, and suggests novel value-added applications beyond the classic cellulose applications. Nanocellulose can lead to strengthened competitiveness of the forest industry in its changing operational environment.