Native Cellulose Nanofibril-based Functional Materials

Cellulose is the most abundant polymeric source on earth, and it has been used for centuries in different applications ranging from paper making to technological materials. Recently, nanocellulose, cleaved from native cellulose fibers, has extensively been explored because of its excellent mechanical, optical and thermal properties, combined with renewability. It can be used to fabricate composites, transparent films, fibers, porous foams and aerogels for diverse applications. This thesis focuses on utilizing native cellulose nanofibrils as building blocks for selected novel functional materials. Publication I addresses biomimetic nanocomposites, where the reinforcing hard cellulose nanofibrils form the majority of phases as separated by the soft rubbery block copolymeric domains. A facile ionic assembly is used to produce strong films with a promoted work-of-fracture showing a synergistic effect. This concept makes it possible to pursue biomimetic nanocomposites with further increased fracture toughness, while maintaining stiffness and strength. Publication II studies the mechanical and electrical properties of cellulose nanofibril and few-walled carbon nanotube hybrid aerogels. Incorporating carbon nanotubes with cellulose nanofibrils allows us to combine the attractive features of both components: wide availability, easy processing, and the sustainability of nanocellulose and the advanced electrical properties of carbon nanotubes. The concept demonstrates that hybrid aerogels could potentially be used in pressure sensing applications. Publication III further deals with the electrical behaviour of the above native cellulose nanofibril/carbon nanotube hybrid aerogels under repeated cyclic compression to explore "electrical fatigue". The hybrid aerogels can be constructed to allow stable reversible resistance changes in cyclic compression. Publication IV describes a nanocellulose film actuation with reversible bidirectional bending triggered by humidity. The films show a steady-state bending when exposed to humidity and then relaxed when removed from the imposed humidity continuously. The bending is highly sensitive to humidity, as demonstrated by its bending when exposed to a human hand at a distance of several millimeters. Such a film offers a facile route toward biomimetic actuation and novel types of active materials. In conclusion, cellulose nanofibrils are versatile building blocks in hybrid materials with block copolymers to tune the self-assemblies for mechanical properties, as templates for electroactive aerogels, and as films with new properties.