Nanopapers formed by stiff and strong native cellulose nanofibrils are emerging as mechanically robust and sustainable materials to replace high-performance plastics or as flexible, transparent and "green" substrates for organic electronics. The mechanical properties endowed by nanofibrils crucially depend on mastering structure formation processes and on understanding interfibrillar interactions as well as deformation mechanisms in bulk. Herein, we show how different dispersion states of cellulose nanofibrils, that is, unlike tendencies to interfibrillar aggregation, and different relative humidities influence the mechanical properties of nanopapers. The materials undergo a humidity-induced transition from a predominantly linear elastic behavior in dry state to films displaying plastic deformation due to disengagement of the hydrogen-bonded network and lower nanofibrillar friction at high humidity. A concurrent loss of stiffness and tensile strength of 1 order of magnitude is observed, while maximum elongation stays near constant. Scanning electron microscopy imaging in plastic failure demonstrates pull-out of individual nanofibrils and bundles of nanofibrils, as well as larger mesoscopic layers, stemming from structures organized on different length scales. Moreover, multiple yielding phenomena and substantially increased elongation in strongly disengaged networks, swollen in water, show that strain at break in such nanofibril-based materials is coupled to relaxation of structural entities, such as cooperative entanglements and aggregates, which depend on the pathway of material preparation. The results demonstrate the importance of controlling the state of dispersion and aggregation of nanofibrils by mediating their interactions, and highlight the complexity associated with understanding hierarchically structured nanofibrillar networks under deformation.

Renewable nanofibrillated cellulose (NFC) and nanofibrillated chitin (NFCh) are attractive fibrillar bionanoparticles due to their remarkable properties such as outstanding mechanical stiffness and strength, thermostability, barrier properties, and also for their global availability from renewable resources and food waste. One major bottleneck to maximize the mechanical properties of materials based on these bionanoparticles (e.g., nanopapers and macroscale fibers) is to find pathways to control their direction of alignment and understand how preferred alignment correlates with macroscale properties. Herein, we will demonstrate how strain-rate controlled wet-stretching of rehydrated macroscale fibers composed of nanofibrillated chitin and cellulose (NFCh, NFC) induces a high degree of orientation and how the degree of alignment scales with macroscale mechanical stiffness. We find similar degrees of alignment in both types of nanofibril-based macrofibers, yet substantially different macroscale stiffness, with the NFC-based fibers (E(NFC) = 33 GPa) outperforming the NFCh-based ones (E(NFCh) = 12 GPa) considerably. These differences can be correlated to the mechanical properties of the underlying cellulose I and α-chitin crystals and the degree of crystallinity of the nanofibrils, which both govern the stiffness of an individual nanofibril. Our study likely demonstrates the maximum performance in terms of stiffness of materials prepared by NFC and NFCh and reveals a critical difference in the performance of both classes of bionanoparticles.

A sacrificial templating process using lithographically printed minimal surface structures allows complex de novo geo-metries of delicate hydrogel materials. The hydrogel scaffolds based on cellulose and chitin nanofibrils show differences in terms of attachment of human mesenchymal stem cells, and allow their differentiation into osteogenic outcomes. The approach here serves as a first example toward designer hydrogel scaffolds viable for biomimetic tissue engineering.

Natural high-performance materials inspire the pursuit of ordered hard/soft nanocomposite structures at high fractions of reinforcements and with balanced supramolecular interactions. Such biomimetic design principles remain difficult to realize for bulk nanocomposites. Herein, we establish an effective drawing procedure that induces a high orientation of crystalline cellulose nanocrystals (CNCs) in a matrix of carboxymethylcellulose (CMC) at high level of reinforcements (50 vol %). We show such alignment in rather thick bulk films and report synergetic improvement with a simultaneous increase of stiffness, strength, and work-to-fracture as a function of the degree of alignment. Scanning electron microscopy and two-dimensional X-ray diffraction quantify the alignment of the cylindrical nanoparticles and link it to the extent of drawing and improvements in mechanical properties. We further show that the decline in mechanical properties of such waterborne all biobased 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 Cu(2+) and Fe(3+). This contribution demonstrates the importance of aligning one-dimensional 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.