Surface modification and micro/nano structure control of cellulose nanofibrils film and wet-spinning fibers
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Nano-cellulose, including cellulose nanofibrils (CNFs), nanocrystals (CNC) and tempo-oxidized cellulose nanofibrils (TOCNs), have been of great research interest due to their advantages of abundance, biodegradability, non-toxicity, excellent biocompatibility and recyclability. The aim of this thesis is the surface modification of cellulose nanofibrils, controlling nano/micro structures of the composite films and wet-spinning fiber developments. In the first part, patterned nanocellulose membrane surfaces were fabricated by a simple pressing process. After chemical modification with non-fluorinated and low-cost industrial chemicals, some nanocellulose membranes showed water contact angles >150°. The combination of micro-sized pattern and nanosized cellulose fibers created a hierarchical structure that contributed to the superhydrophobic properties. In the second project, PLA composite films were prepared with the reinforcement of hydrophobically modified nanocellulose. The hydrophobic modification of nanocellulose was conducted in situ in water suspension using an AKD emulsion, which alternates the CNF surface from hydrophilic to hydrophobic directly in water. The mechanical strength measurement verified the reinforcement of CNF in the composites. In the third project, 3D porous and simultaneously high-strong nanocellulose frameworks were prepared and used as reinforcement for the PLA composite fabrication. The re-wetting and freeze-drying resulted in a porous structure and large specific area that improves the interaction with the PLA polymeric matrix. Furthermore, with the chemical modification of isocyanates, e.g., TDI and HDI, PLA molecules were crosslinked and covalently bonded to the nanocellulose frameworks. As a result, the CNF-PLA composite films with high mechanical performances were obtained. In the last part, nanocellulose filaments were prepared by wet-spinning. To improve the mechanical strength, optimization of operation conditions of wet-spinning, chemical modification and the post-treatments including wet- stretching and wet-twisting were investigated. With optimized operations, the nanocellulose filament with the highest tensile strength of 211 MPa and modulus of 18.2 GPa in this study was obtained.