Analysis of Cellulose Nanocrystal Aerogels Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/vq27zs27g

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  • Aerogels are interesting material systems with outstanding properties such as low density, high surface area, and low thermal conductivity. They can be used for a variety of applications, such as solar cells, sensors, or catalysts. Additionally they provide a unique model system to study the properties of small particles. Cellulose Nanocrystals (CNCs) are the nanoscale extractions of large cellulose fibers in a variety of plants and animals such as wood, paper, cotton, or tunicates. Their nanoscale size coupled with high mechanical strength, biodegradability, and renewability make them an interesting alternative to oil based materials for composites with applications in various areas such as electronics, packaging, or tissue engineering. In this work aerogels are used to study CNCs by first creating CNC aerogels followed by structure investigations of aerogels, and finally the study of their mechanical properties to find the strength of CNC bonds. With the understanding of the bond strength, CNC nanocomposite material system development can advance to the next level. CNC aerogels are produced from carboxylated nanocrystalline cellulose organogels by acetone exchange followed by supercritical CO₂ drying. Atomic layer deposition is used to coat CNC aerogels with a thin conformal layer of Al₂O₃. The resulting three dimensional nanoporous structure is analyzed via transmission electron microscope tomography to find the number of CNC bonds within the aerogel. Uniaxial compression tests are performed to determine the mechanical properties of the aerogels. The resulting aerogels are highly transparent, extremely light weight and have a large surface area. The deposited layer of alumina increases the resistance of the aerogels to moisture and elevated temperatures, and provides protection for CNCs against the electron beam for imaging. Tomographic data shows the nanoscale structure of the CNCs in the aerogel and enables the qualitative and quantitative characterization of the bonds. The mechanical properties are found to depend mostly on the density of the aerogel including the number of CNCs and the number of bonds. The required work to mechanically break CNC bonds is measured and the resulting CNC bond strength suggests that the CNCs are bonded together by hydrogen bonds located at the surface of the CNCs.
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