Investigating moisture behavior of wood nanostructure using experimental and simulated scattering

Wood is a complex, hierarchical material used in practical applications and as a precursor to bio-based materials. As wood is formerly conductive tissue of a living organism, a tree, it is intrinsically linked with water. In most applications of wood, its interactions with water are important, as almost all of its properties depend on the moisture content (MC). Bulk of the wood structure consists of thick secondary cell walls and thus, many of its properties are fundamentally derived from the nanoscale properties and their moisture interactions. Studying the wood nanostructure in intact cell walls is difficult, thus the exact nanostructure remains elusive. This is especially true for the crystalline cellulose microfibril, despite it being the main structural component of wood. Scattering methods allow for non-destructive characterization of the wood nanostructure and how it responds to moisture in practical, ambient conditions. As scattering methods require prior assumptions about the underlying structure to interpret the results, they need to be supplemented with other methods. One such way are molecular dynamics (MD) simulations which allow recreating cell wall constituents on the atomic scale. By combining these two methods, complementary results on the cell wall nanostructure and how it responds to moisture are created. In this work, X-ray and neutron scattering experiments on Norway spruce were conducted. These were complemented with MD models of aggregated cellulose microfibrils, based on an up-to-date understanding of their structure. Computed scattering was also determined from the models. Neutron scattering of drying wood revealed that the changes in fibril packing depend on the drying region, with much less bundle deswelling during the initial constant-rate drying compared to the later falling-rate drying. MD simulations of fibril bundles revealed two distinct regions of water diffusivity as the system dried. X-ray scattering experiments of wood undergoing a moisture cycle combined with MD models revealed that the models predicted experimentally observed changes well. Both models and experiments showed that fibrillar aggregation below 10-15% MC is a significant factor in changes to cellulose crystallites. These changes remained even after the wood was delignified, with delignification not fundamentally changing the fibril moisture interactions, despite increasing the fibril packing distance. The impact of hemicelluloses and fibril size on computed scattering was investigated with MD models to aid with interpreting experimental data. The results indicated that hemicellulose-coated fibrils produce a notable scattering contribution to the small-angle region and can increase the apparent fibril size seen with scattering methods. Overall, the results of this thesis provide insight into the moisture-related changes inside the wood cell wall and how to interpret experimental scattering patterns from wood.