Fabrication, Structures And Properties Of Nanocellulose And Nanocellulose Aerogels

With the increasing attention on sustainable development, utilization of bio-based materials became more and more important. The bio-based materials are supported with nanocellulose. Because nanocellulose has prospective properties such as high Young’s modulus and very low coefficient of thermal expansion, they are expected to be utilized as a filler of next generation nanocomposites. The present dissertation will focus on the preparation, characterization, and application of nanocellulose and their aerogels. First, a series of nanocellulose were produced from various bio-based materials with different nanofibrillation methods. Second, the structures and properties of the nanocellulose were characterized and compared. Third, aerogels were fabricated by freeze-drying the nanocellulose suspensions. The self-aggregation mechanism of nanocellulose and the structures and properties of their aerogels were further presented. The main results can be summarized as follows:(1) Nanocellulose was successfully fabricated from poplar wood using high-intensity ultrasonication combined with chemical pretreatments. In this work, the structural and property evolution during the preparation of nanocellulose were investigated. The effect of ultrasonication treatment on the structures and properties of nanocellulose were also studied. With the removing of lignin and most of hemicelluloses, the size of the cellulose fibers decreased apparently. The diameter distributions of the nanocellulose were dependent on the treating time and output power of the ultrasonication. Uniform nanocellulose can be produced when the treating time reached30min, and the output power was greater than1000W. The ultrasonication treatment has little effect on the chemical composition, crystal structure and thermal stability of the nanocellulose.(2) The structures and properties of the nanocellulose isolated from various bio-based materials with different nanofibrillation methods were investigated. We can isolate long nanocellulose with10～40nm in width from the resources with "low" cellulose content such as wood, bamboo, and wheat straw. However, it is very hard for us to extract long nanocellulose from the resources with "high" cellulose content such as cotton and flax. Nanocellulose can be individualized by nanofibrillation the cellulose fibers using a high-speed blender, an ultrasonicator, and a high-pressure homogenizer. The suspensions after treated with a high-pressure homogenizer exhibited the best stable behavior. Large amounts of bundles were existed in the suspensions. The bundles were>13μm long and30～250nm wide. The bundles were organized with parallel aligned nanocellulose with3～5nm in width.(3) Ultralong and highly uniform cellulose nanofibers were successfully prepared, by using chemical pretreatment combined with high intensity ultrasonication to exact nanocellulose, and then self-aggregation of them into ultralong nanofibers by freeze-drying. The as-prepared bamboo cellulose nanofibers displayed fine structures with lengths>1mm, diameters of30-80nm, and aspect ratios>10,000. Similar findings were also observed from the micrographs of nanofibers fabricated from softwood, hardwood, wheat straw and cane bagasse. With the removal of the matrix materials, the cellulose Ⅰ crystal structure was maintained, whereas the crystallinity and thermal stability of the fibers increased. The crystallinity and thermal degradation temperature of the bamboo cellulose nanofibers reached61.3%and over309℃, respectively.(4) Ultralight and highly flexible nanocellulose aerogels were fabricated by freeze-drying the nanocellulose suspensions which were produced by nanofibrillation the cellulose fibers using an ultrasonicator. The structure, density, and water uptake capability of the aerogels were characterized. The microstructures of the aerogels can be transformed from open3D porous nanofibrillar network to2D sheet-like skeletons by adjusting the concentration of the nanocellulose suspension. When the transparent supernatant fraction, which has-0.018wt%solid content obtained via centrifugation of the nanocellulose suspensions, was subjected to freeze-drying, ultra-low density aerogels (0.2×10-3g/cm3) were successfully produced due to the self-aggregated of the nanocellulose and their bundles along the longitudinal direction.(5) By carefully modulating the nanofibrillation process, four types of nanocellulose with different morphologies and surface properties were readily fabricated. Then, free-standing lightweight aerogels were obtained from the corresponding aqueous nanocellulose suspensions via freeze-drying. The structures of the aerogels could be controlled by manipulating the type of nanocellulose as well as the concentrations of their suspensions. A possible mechanism for the self-aggregation of nanocellulose into aerogel nanostructures was further proposed. Owing to web-like structure, high porosity and high surface reactivity, the nanocellulose aerogels exhibited high mechanical flexibility and ductility, and excellent properties for water uptake, removal of dye pollutants, and the use as thermal insulation materials. The aerogels also displayed high sound adsorption capability at high frequencies.