Sustainable wood nanotechnologies that combine optical transmittance and mechanical performance are interesting for new functionalities utilizing transparency. Wood is a sophisticated bio-based material with a natural hierarchical, anisotropic and porous structure. The wood cellular structure can be functionalized at the micro and nanostructural level for the design of advanced functional materials. In recent years, the development of transparent wood biocomposites derived from delignified wood substrates have gained interest because they combine attractive structural properties with optical functionality. Nanostructural tailoring of transparent wood biocomposites is required to improve optical transmittance, mechanical performance, and to add new functionalities. In this thesis, environmentally friendly material components and green chemical processes have been developed for the fabrication of nanostructurally tailored transparent wood biocomposites.
Mesoporous delignified wood substrates with preserved microstructure and cellulose microfibril alignment in the cell wall are used as reinforcement in transparent wood biocomposites. Chemical functionalization strategies using renewable maleic, itaconic and succinic anhydrides have been explored for molecular and nanostructural tailoring of delignified cell walls. Cyclic anhydride functionalization results in high degree of esterification, reduces moisture content in the wood substrate, improves monomer diffusion within the cell wall, and further enables interface tailoring at the molecular scale with possibility for covalent attachment with polymer matrix. Transparent wood biocomposites were prepared by methyl methacrylate monomer impregnation followed by in situ polymerization within the chemically modified wood substrates. The anhydride-functionalized transparent wood biocomposites have improved wood-polymer interfacial interactions, resulting in improved optical and mechanical properties. Moreover, a bio-based polymer matrix was designed from renewable limonene oxide and acrylic acid for the fabrication of fully bio-based transparent wood biocomposites. The bio-based monomer can diffuse into the cell wall, and the polymer phase is both refractive index-matched and covalently linked to the wood substrate. The bio-based transparent wood biocomposites are nanostructured and demonstrate superior optical transmittance, low haze, and excellent mechanical performance.
Nanostructural functionalization using phase-change materials is also demonstrated for the design of transparent wood biocomposites that combine thermal energy storage, tunable optical properties, and load bearing functions. Molecular and nanoscale interactions in transparent wood biocomposites are critical as they contribute to the favorable distribution of the phase-change material across the wood structure, which is a key component in optimizing thermal energy storage capacity. Bio-based design of transparent wood is also explored for thermal energy storage applications. Low environmental impact is achieved by combining the use of bio-based resources with green processing routes. Environmentally friendly transparent wood nanotechnologies can compete with petroleum-based plastics in applications such as load-bearing transparent panel and energy saving.
Professor Lars Berglund, KTH / WWSC, Dr Peter Olsén, KTH/WWSC