Biomimetics - Architectural Considerations for Functional Nanocomposites

This thesis will focus on synthetic nanomaterials that combine hard nanocellulose reinforcement with soft molecularly engineered synthetic polymeric components. The architectural designs have all been inspired by natural nanocomposites, such as silk, animal bone and plant fibres. Typically, the design of natural structures are built around hard reinforcing nanodomains bound together by energy dissipating sacrificial networks. These natural materials consist of proteins, carbohydrates or brittle minerals. Separately each component is usually mechanically weak; however, when combined in the balanced and hierarchical ways, each component will synergistically contribute towards mechanically excellent networks, by combining strength, toughness and stiffness. Hence, one of the main focuses in this thesis will be the structural design of the constituting components and how they relate to the mechanical properties of the overall composites. Another key feature is the compatibility issues between the reinforcing nanocellulose with the synthetic supramolecular networks. Each material has been designed in a way that yields a homogeneous dispersion of all colloidal components. This allows more efficient reinforcment as well as allows the components to more effectively together. And finally, specific functionalities were engineered into the nanocomposite materials either through the supramolecular binding or by the use of functional polymers. In Publication I, highly dynamic supramolecular nanocomposite hydrogels were developed that uniquely combine: high stiffness, approaching those of solid elastic networks; self-healing within seconds; and temporal stability, allowing for self-healing of the exposed surface areas even after prolonged storage. In Publication II, biomimetic sacrificial bonds were chemically engineered into one-component nanocomposites. This resulted in engineered fracture energy dissipation, which considerably increased the toughness of the glassy nanocomposite. In Publications III and IV, functional nanocomposite hydrogels were formed by linking colloidal nanocellulose by adsorbing functional polysaccharides onto the surface. In the first example, a thermoresponsive nanocomposite with switchable modulus was shown. In the second example, both the modulus and yield-strain were enhanced by adding an interconnected sacrificial network into the nanofibrillar network. These nanocellulose-based nanocomposites demonstrate promising new concepts for material design as well as never before seen combinations of mechanical properties and functionalities.