Cellulose Nanofibril Nanopapers and Bioinspired Nanocomposites

Cellulose nanofibrils (CNFs) are considered next generation, renewable reinforcements for sustainable, high-performance materials uniting high stiffness, strength and toughness. They allow the formation of pure nanopapers or can be integrated into bioinspired nanocomposites leading to excellent multifunctional properties. The mechanical properties endowed by nanofibrils crucially depend on mastering structure formation processes and on understanding interfibrillar interactions as well as deformation mechanisms in the bulk. In this context, this thesis focus on an in-depth understanding of the mechanical performance of CNF nanopapers and nanocomposites. Chapter II shows how different dispersion states of CNFs, i.e. unlike tendencies to interfibrillar aggregation, and different relative humidities influence the mechanical properties of the corresponding nanopapers. The results demonstrate the importance of controlling the state of dispersion and aggregation of the CNFs by mediating their interactions, and highlight the complexity associated with understanding hierarchically structured nanofibrillar networks under deformation. Chapter III investigates the challenges associated with making defined CNF/polymer nanopaper hybrid structures influenced by polymer properties in order to deduce a quantitative picture of the deformation mechanisms. The study discusses detailed insights on how thermo-mechanical properties of tailor-made (co)polymers govern the tensile properties in bioinspired CNF/polymer settings. The derived understanding expands the ability to tune and control the mechanical properties by rational design criteria. Then, Chapter IV unravels in detail how counterions, being either of the organic alkyl ammonium series (NR4+) or of the earth metal series (Li+, Na+, Cs+), need to be chosen to achieve outstanding combinations of mechanical properties, extending to previously unexplored areas. This understanding also leads to new levels of ductility in bioinspired CNF/polymer nanocomposites at high levels of reinforcements. Finally, the review in Chapter V reflects my results and discusses the current state of the art in the field of CNF nanocomposites, understanding of mechanical performance, and derives general perspectives for developing future CNF-based nanopapers, as well as nanocomposites with high fractions of reinforcements featuring rationally designed and improved property profiles. The influence of various intercorrelated parameters is discussed: fibril chemistry, crystallinity, aspect ratio and degree of polymerization, colloidal stability and film formation, as well as integration with different counterions, polymers and nanoclays. Here, the previous Chapters II-IV are placed as key research to connect and dissect some of these factors by comparing with the most comprehensive studies