Chemical Modification of Nanostructured Wood for Functional Biocomposites
Sammanfattning: Recently emerged top-down processing concept has provided new insights into the chemical modification of wood at the nanoscale. Nanostructured wood with naturally aligned cellulose microfibrils, cell wall nanoporosity, and precisely tuned chemical composition has opened up numerous possibilities for advanced design of functional materials. In this thesis, novel chemical modification strategies have been developed to obtain nanostructural control and surface functionalization of nanostructured wood. Functional biocomposite materials with superior mechanical and optical performance, high CO2 adsorption capacity, and immobilized protein have been fabricated using the novel chemically modified nanostructured wood. The direct preparation of nanostructured wood from hardwood balsa was achieved by structure retaining delignification using acidic sodium chlorite. Crosslinking of the matrix polysaccharides using a homobifunctional epoxide compound was necessary as a pretreatment step for softwood spruce to maintain its structure integrity after complete delignification. Further chemical modification of delignified balsa wood through 2,2,6,6-tetrametylpiperidin-1-oxyl (TEMPO)-mediated oxidation selectively oxidized surface hydroxyls to carboxyl groups and induced fibrillation of cellulose microfibrils within the cell wall. Therefore, TEMPO-oxidized wood (TO-wood) with high carboxylate content (0.78 mmol g-1), high specific surface area (249 m2 g-1), and large mesopore volume (0.78 cm3 g-1) was successfully produced. Tunable microstructure of TO-wood was subsequently obtained by incorporating different counterions (H+, Cu2+, Al3+, Zn2+) or by employing different drying methods (super critical drying and freeze drying). In addition, surface amination method was also developed on highly mesoporous delignified spruce cellulose scaffold to introduce a reactive handle for immobilization of biomolecules. These chemically modified nanostructured wood have inspired the fabrication of wood-based biocomposites with new functionalities that are not possible with traditional wood materials.Delignified balsa wood showed stronger hydrophilicity and larger porosity, which allowed the formation of composite hydrogels through infiltration of gelatin and crosslinking with genipin. The composite hydrogels showed high mechanical strength under compression and low swelling in physiological condition. The preserved cellular structure and fibrillated cellulose microfibrils in TO-wood enabled facile fabrication of compressible aerogel and exceptionally strong film (tensile strength of 449 MPa and Young’s modulus of 51 GPa) upon different drying conditions. Fibrillation of cellulose microfibrils was also found critical to the inter-penetration between cell wall and poly(N-isopropylacrylamide) (PNIPAM) hydrogel network, producing tough and highly transparent composite hydrogel with a total transmittance of 85.8% at thickness of 2 mm. The TO-wood/PNIPAM hydrogel was able to reversibly switch between transparent and brightly white in response to environmental temperature change between 25 and 40 °C. Surface carboxyl groups of TO-wood also facilitated the surface coordination of cell wall to multivalent metal ions, which subsequently enhanced the in situ synthesis of metal organic frameworks (MOFs). The resulting TO-wood/Cu3(BTC)2 (copper benzene-1,3,5-tricarboxylate) composite aerogel showed high specific surface area of 471 m2 g-1 and high CO2 adsorption capacity of 1.46 mmol g-1 at 25 °C under atmosphere pressure. The highly mesoporous and mechanical robust spruce derived cellulose scaffold laden with reactive amine groups allowed covalent immobilization of functional biomolecules, such as a lectin protein concanavalin A, which demonstrated potential glycoprotein-binding and separation applications.
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