Fabrication of nanocellulose-based materials : Liquid crystalline phase formation and design of inorganic–nanocellulose hybrids

Sammanfattning: The increasing need to replace fossil fuels as a source of energy and raw material is resulting in extensive research efforts towards identifying and developing high performance materials and devices based on renewable sources. Cellulose being the most versatile and abundant biopolymer in nature is one of the obvious choices. Cellulose, due to its properties that arise from the hierarchical structure, has been used for millennia by mankind although it is currently used, in the form of microfibers, mainly in the paper and pulp industry. However, many efforts are being directed towards retrieving even smaller cellulose constituents such as nanofibers and nanocrystals (i.e., nanocellulose), which can actually be used in high performance materials. In order to do so, a better understanding of the behavior and interactions between these novel nanomaterials are required. Moreover, the combination of nanocellulose with inorganic nanoparticles bears a great potential that can open the door to multifunctional materials based on a renewable component.In this work, the anisotropic behavior, i.e., the formation of a chiral nematic phase, of cellulose nanocrystals (CNC) initially dispersed in aqueous media spanning a wide volume fraction range has been studied by small angle X-ray scattering (SAXS) and laser diffraction. The analysis shows that the twist angle between neighboring CNCs increased from ~1° up to ~4° as the CNC volume fraction increased from 2.5 to 6.5 vol%.Also, the drying of an aqueous CNC droplet immersed in a binary toluene/ethanol mixture was studied and monitored in-situ by polarized video microscopy, where the influence of the water dissolution rate on the morphology of the resulting microbeads was investigated by scanning electron microscopy. The morphology of the microbeads depends not only on the drying speed but also on the initial starting CNC volume fraction. In this regard, the influence of the degrees of liquid crystallinity on the formation of a chiral nematic phase on films has also been studied.Lastly, the fabrication and various properties of hybrids and composites prepared from cellulose nanofibers (CNF) and inorganic constituents are presented. The structure and chemistry of a museum sample of a traditional African textile (Bogolan) is analyzed and the chemical foundation of the dyeing method is outlined. This Bogolan dyeing method was used to pattern CNF films, and to study the details of how the surface-bound iron-tannin complexes are formed on the cellulose surface.Also, the formation of transparent, hard and flexible films based on CNF-titania (anatase) nanoparticle hybrids was studied, where the influence of the composition of the hybrids on the optical and mechanical properties is discussed on the basis of results from electron microscopy, spectrophotometry and nanoindentation.