Counterion Size and Nature Control Structural and Mechanical Response in Cellulose Nanofibril Nanopapers.

Nanopapers formed from aqueous dispersions of cellulose nanofibrils (CNFs) combine stiffness, strength, and toughness. Yet, delicate interactions operate between the CNFs during nanopaper formation and mechanical deformation. We unravel in detail how counterions, being either of the organic alkyl ammonium kind (NR4+) or of the earth metal series (Li+, Na+, Cs+), need to be chosen to achieve outstanding combinations of stiffness, strength, and toughness, extending to previously unreached territories. We relate structure formation processes in terms of colloidal stabilization to nanostructural details such as porosity and ability for swelling, as well as to interfibrillar interactions in bulk and macroscale mechanical properties. We demonstrate that our understanding also leads to new levels of ductility in bioinspired CNF/polymer nanocomposites at high levels of reinforcements. These results contribute to future rational design of CNF-based high-performance materials.

Understanding Toughness in Bioinspired Cellulose Nanofibril/Polymer Nanocomposites.

Cellulose nanofibrils (CNFs) are considered next generation, renewable reinforcements for sustainable, high-performance bioinspired nanocomposites uniting high stiffness, strength and toughness. However, the challenges associated with making well-defined CNF/polymer nanopaper hybrid structures with well-controlled polymer properties have so far hampered to deduce a quantitative picture of the mechanical properties space and deformation mechanisms, and limits the ability to tune and control the mechanical properties by rational design criteria. Here, we discuss detailed insights on how the thermo-mechanical properties of tailor-made copolymers govern the tensile properties in bioinspired CNF/polymer settings, hence at high fractions of reinforcements and under nanoconfinement conditions for the polymers. To this end, we synthesize a series of fully water-soluble and nonionic copolymers, whose glass transition temperatures (Tg) are varied from -60 to 130 °C. We demonstrate that well-defined polymer-coated core/shell nanofibrils form at intermediate stages and that well-defined nanopaper structures with tunable nanostructure arise. The systematic correlation between the thermal transitions in the (co)polymers, as well as its fraction, on the mechanical properties and deformation mechanisms of the nanocomposites is underscored by tensile tests, SEM imaging of fracture surfaces and dynamic mechanical analysis. An optimum toughness is obtained for copolymers with a Tg close to the testing temperature, where the soft phase possesses the best combination of high molecular mobility and cohesive strength. New deformation modes are activated for the toughest compositions. Our study establishes quantitative structure/property relationships in CNF/(co)polymer nanopapers and opens the design space for future, rational molecular engineering using reversible supramolecular bonds or covalent cross-linking.

Supramolecular Engineering of Hierarchically Self-Assembled, Bioinspired, Cholesteric Nanocomposites Formed by Cellulose Nanocrystals and Polymers.

Natural composites are hierarchically structured by combination of ordered colloidal and molecular length scales. They inspire future, biomimetic, and lightweight nanocomposites, in which extraordinary mechanical properties are in reach by understanding and mastering hierarchical structure formation as tools to engineer multiscale deformation mechanisms. Here we describe a hierarchically self-assembled, cholesteric nanocomposite with well-defined colloid-based helical structure and supramolecular hydrogen bonds engineered on the molecular level in the polymer matrix. We use reversible addition-fragmentation transfer polymerization to synthesize well-defined hydrophilic, nonionic polymers with a varying functionalization density of 4-fold hydrogen-bonding ureidopyrimidinone (UPy) motifs. We show that these copolymers can be coassembled with cellulose nanocrystals (CNC), a sustainable, stiff, rod-like reinforcement, to give ordered cholesteric phases with characteristic photonic stop bands. The dimensions of the helical pitch are controlled by the ratio of polymer/CNC, confirming a smooth integration into the colloidal structure. With respect to the effect of the supramolecular motifs, we demonstrate that those regulate the swelling when exposing the biomimetic hybrids to water, and they allow engineering the photonic response. Moreover, the amount of hydrogen bonds and the polymer fraction are decisive in defining the mechanical properties. An Ashby plot comparing previous ordered CNC-based nanocomposites with our new hierarchical ones reveals that molecular engineering allows us to span an unprecedented mechanical property range from highest inelastic deformation (strain up to ∼13%) to highest stiffness (E ∼ 15 GPa) and combinations of both. We envisage that further rational design of the molecular interactions will provide efficient tools for enhancing the multifunctional property profiles of such bioinspired nanocomposites.

Bioinspired Mechanical Gradients in Cellulose Nanofibril/Polymer Nanopapers.

Mechanical gradients are important as tough joints, for strain field engineering in printable electronics, for actuators, and for biological studies, yet they are difficult to prepare and quantitatively characterize. We demonstrate the additive fabrication of gradient bioinspired nanocomposites based on stiff, renewable cellulose nanofibrils that are bottom-up toughened via a tailor-made copolymer. Direct filament writing of different nanocomposite hydrogels in patterns, and subsequent healing of the filaments into continuous films while drying leads to a variety of linear, parabolic and striped bulk gradients. In situ digital image correlation under tensile deformation reveals important differences in the strain fields regarding asymmetry and step heights of the patterns. We envisage that merging top-down and bottom-up structuring of nanocellulose hybrids opens avenues for aperiodic and multiscale, bioinspired nanocomposites with optimized combinations of stiffness and toughness.

Humidity and multiscale structure govern mechanical properties and deformation modes in films of native cellulose nanofibrils.

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.