Cellulose nanocrystals: insights into the preparation and incorporation into new materials templates

Defence is held on 8.10. 2021 12:00 – 15:00 https://aalto.zoom.us/j/64004385532This thesis is focused on a novel cellulose hydrolysis technique via gaseous HCl and on the utilization of that technique in building up different materials templates, namely, those in hybrid multilayered structures of cellulose nanocrystals (CNCs) and ZnO as well as in micron-sized, porous particles of cross linked CNCs that retain the native cellulose microfibrillar alignment. Cellulose hydrolysis, typically in liquid/solid systems, has been extensively studied due to its significance in the fields of biofuels, chemicals production, and renewable materials. Herein, we focused on a gas/solid system by constructing a reactor for HCl (g) to overcome the limitations in previously introduced HCl vapor hydrolysis systems. Cellulose fibers were hydrolyzed down to the leveling-off degree of polymerization (LODP), indicating degradation of the disordered segments in cellulose microfibrils without visibly affecting the fiber morphology. Yet prolonged exposure was found to induce a partial collapse of the fiber structure. The exposure of hardwood cellulose nanofiber (CNF) thin films deposited on silica to HCl (g) resulted in visualization of the order/disorder transitions in cellulose microfibrils.Bacterial cellulose (BC) aerogels were hydrolyzed under 1 bar of HCl (g) and subse-quently TEMPO-oxidized to isolate the CNCs in a stable dispersion in water. Comprehensive microscopic analysis provided proof of concept on CNC formation. Crosslinking of hydrolyzed fibers by citric acid led to the formation of crosslinked CNC particles that maintained the native cell wall alignment. Thermoporosimetry combined with high resolution microscopy revealed the formation of a well-defined, nanoporous network consisting of end-to-end connected CNCs. The preparation of ZnO/CNC multilayered structures took place by alternation of atomic layer deposition (ALD) and spin coating, resulting in discrete CNC layers between the ZnO laminates. Thermal conductivity measurements revealed the effective thermal insulation by CNCs. Even the incorporation of submonolayer amounts of CNCs was able to significantly reduce the thermal conductivity of the system