Nanocellulose/polymer nanocomposites with ordered structures

Natural high performance materials inspire to pursue ordered hard/soft nanocomposite structures at high fractions of reinforcements and with balanced supramolecular interactions. Owing to their high stiffness and strength, nanocellulose colloids, i.e. short cellulose nanocrystals and long entangling cellulose nanofibrils, are promising candidates for high performance materials. In my theses, I introduced and investigated three new pathways to utilize nanocellulose as reinforcing particles incorporating with energy-dissipating polymer matrices in bulk nanocomposites by biomimetic design principles. A specific focus is set on achieving ordered structures, on the one hand on the nanoscale via bottom-up self-assembly, and on the other hand via top-down direct writing procedures to make macroscopic gradients.First, I established an effective drawing procedure that induces high orientation of crystalline cellulose nanocrystals (CNCs) in a matrix of carboxymethyl cellulose (CMC) at high level of reinforcements (50 vol%). Alignment in rather thick bulk films and synergetic improvement with a simultaneous increase of stiffness, strength and work-to-fracture as a function of the degree of alignment was reported. Scanning electron microscopy and 2D X-ray diffraction quantify the alignment of the cylindrical nanoparticles and link it to the extent of drawing and improvements in mechanical properties. I could further demonstrate that the decline in mechanical properties of such water-borne, bio-based nanocomposites at high relative humidity can be balanced using supramolecular modulation of the ionic interactions by exchanging the monovalent Na+ counterion, present in CMC and CNC with di- or trivalent Cu2+ and Fe3+. This contribution demonstrates the importance of aligning 1D reinforcements to achieve synergetic improvement in mechanical properties in sustainable bioinspired nanocomposites and suggests pathways to prepare water-stable materials based on a waterborne processing route. Second, I developed a facile, waterborne self-assembly pathway to mimic the multiscale cuticle structure of the crustacean armor by combining hard reinforcing cellulose nanocrystals (CNC) with soft poly(vinyl alcohol) (PVA). Iridescent CNC nanocomposites with cholesteric liquid crystal structure with different helical pitches and photonic bandgaps can be realized by varying the CNC/PVA ratio. Multilayered crustacean-mimetic materials with tailored periodicity and layered cuticular structure can be obtained by sequential preparation pathways. The transition from a cholesteric to a disordered structure occurs for a critical polymer concentration. Crack propagation studies using scanning electron microscopy visualize the different crack growth and toughening mechanisms inside cholesteric nanocomposites as a function of the interstitial polymer content for the first time. Different extents of crack deflection, layered delamination, ligament bridging and constrained microcracking can be observed. Drawing of highly plasticized films allowed to shed light on the mechanistic details of the transition from a cholesteric/chiral nematic to a nematic structure. The study demonstrates how self-assembly of biobased CNCs in combination with suitable polymers can be used to replicate a hierarchical biological structure, and how future design of these ordered multifunctional nanocomposites can be optimized by understanding mechanistic details of deformation and fracture. In the last part, I investigated the fabrication of gradient bioinspired nanocomposites based on cellulose nanofibrils that are bottom-up toughened via a tailor-made synthetic copolymer. Direct filament writing of different nanocomposite hydrogels in patterned arrays, and subsequent healing of those filaments into continuous films while drying allowed the preparation of a variety of linear gradients, parabolic and striped bulk patterns. Detailed in-situ digital image correlation under tensile deformation reveals important differences in the strain fields with respect to asymmetry and step heights of the mechanical gradient. The concept of merging top-down and bottom-up structuring of nanocellulose hybrids opens new avenues for aperiodic and multiscale, bioinspired nanocomposites with optimized combinations of stiffness and toughness