Defence of doctoral thesis: Mathias Nero – Nanoscale Orientation Mapping of Biopolymer Hierarchies Using Scanning Electron Diffraction

The properties of fiber-based composite materials are highly anisotropic and depend on the orientation and organization of their fibers. In natural systems, such as wood or arthropod exoskeletons, nanofibrils like cellulose and chitin are arranged in complex hierarchical structures that enhance strength, toughness, and sometimes iridescence. These native nanofibrils are crystalline, consisting of tightly packed chains of sugar-based molecules (polysaccharides) organized in nanostructures with anisotropic mechanical behavior. This thesis introduces a methodology using scanning electron diffraction (SED) to analyze the nanoscale orientation of structural polysaccharides in both natural and synthetic composite materials. By scanning a nearly parallel electron beam of just a few nanometers across the sample and capturing a diffraction pattern at each probe position, the local crystallographic orientation of nanofibrils can be determined with significantly improved spatial resolution compared to existing methods. Tilt-series acquisition further enables the reconstruction of the complete three-dimensional fibril orientation, revealing the chirality of the fibrillar organization. Applying this method to electron-beam-sensitive materials, such as cellulose and chitin, presents significant challenges. Their crystalline structures are easily damaged by beam radiation, and they generate weak diffraction signals due to their composition of light elements. To address these issues, low-dose conditions and a specialized data analysis pipeline were developed and integrated into a dedicated workflow. While this work focuses on cellulose and chitin, which are increasingly important in creating advanced hybrid materials, the developed method shows potential for broader applications with other fibrous assemblies, such as those found in bone and spider silk.