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.

Nanopapers containing cellulose nanofibrils (CNFs) are an emerging and sustainable class of high performance materials. The diversification and improvement of the mechanical and functional property space critically depend on integration of CNFs with rationally designed, tailor-made polymers following bioinspired nanocomposite designs. Here we combine for the first time CNFs with colloidal dispersions of vitrimer nanoparticles (VP) into mechanically coherent nanopaper materials. Vitrimers are permanently cross-linked polymer networks that undergo temperature-induced bond shuffling through an associative mechanism and which allow welding and reshaping on the macroscale. The choice of low glass transition, hydrophobic vitrimers derived from fatty acids and polydimethylsiloxane (PDMS), and achieving dynamic reshuffling of cross-links through transesterification reactions enables excellent compatibility and covalent attachment onto the CNF surfaces. Moreover, the resulting films are ductile, stretchable and offer high water resistance. The success of imparting the vitrimeric polymeric behavior into the nanocomposite, as well as the curing mechanism of the vitrimer, is highlighted through thorough analysis of structural and mechanical properties. The dynamic exchange chemistry of the vitrimers enables efficient welding of two nanocomposite parts as characterized by good bonding strength during single lap shear tests. In the future, we expect that the dynamic character of vitrimers becomes a promising option for the design of mechanically adaptive bioinspired nanocomposites and for shaping and reshaping such materials.

A generic, facile, and waterborne strategy is introduced to fabricate flexible, low‐cost nanocomposite films with room‐temperature phosphorescence (RTP) by incorporating waterborne RTP polymers into self‐assembled bioinspired polymer/nanoclay nanocomposites. The excellent oxygen barrier of the lamellar nanoclay structure suppresses the quenching effect from ambient oxygen (kq) and broadens the choice of polymer matrices towards lower glass transition temperature (Tg), while providing better mechanical properties and processability. Moreover, the oxygen permeation and diffusion inside the films can be fine‐tuned by varying the polymer/nanoclay ratio, enabling programmable retention times of the RTP signals, which is exploited for transient information storage and anti‐counterfeiting materials. Additionally, anti‐interception materials are showcased by tracing the interception‐induced oxygen history that interferes with the preset self‐erasing time. Merging bioinspired nanocomposite design with RTP materials contributes to overcoming the inherent limitations of molecular design of organic RTP compounds, and allows programmable temporal features to be added into RTP materials by controlled mesostructures. This will assist in paving the way for practical applications of RTP materials as novel anti‐counterfeiting materials.

Cellulose nanofibrils (CNFs) are attractive, renewable building blocks for high-performance and lightweight nanocomposites of high sustainability. Following bioinspired design principles, meaning the organization of large fractions of reinforcing CNFs in a minority matrix of suitably designed polymers, promises the best mechanical performance. However, thus far, truly synergetic mechanical behavior in such nanocompositesas often found in biological load-bearing tissueswith a simultaneous increase of stiffness, toughness, and strength have remained elusive. Here we describe such a system realizing outstanding synergies in the relevant mechanical performance indicators by combining anionic TEMPO-oxidized CNFs with a self-cross-linkable PU resin. Strikingly, appropriate counterion selection, that is, an exchange of the commonly used sodium to the large tetrabutyl ammonium, turns out to be of key importance to tailor the interaction between the components in a suitable fashion. Ultimately, at only 10 wt % of PU, the cured nanocomposites achieve twice as high stiffness, yield strength, toughness, and strength than a pure CNF nanopaper, allowing the nanocomposites to reach close to 20 GPa in stiffness, 450 MPa in tensile strength at ca. 14% strain. The resulting materials are located in previously completely unoccupied territories in mechanical properties for waterborne CNF/polymer nanocomposites. The study shows that subtle engineering of interactions and attractive PUs containing various noncovalent interaction motifs provide unforeseen opportunities in reaching remarkable mechanical property areas in bioinspired nanocomposites which are promising in applications for future lightweight high-performance sustainable materials.

We demonstrate waterborne, unimolecularly dissolved vitrimer prepolymer systems that can be transferred into a vitrimer material using catalytic transesterification. The one-component prepolymer system can be processed via film casting and subsequent heat-induced cross-linking. A variation of the density of side chain hydroxy groups over ester and amide groups in the methacrylate/methacrylamide backbone, as well as of the Lewis acid catalyst loading, allow control of the extent of cross-linking and exchange rates. The increase of the amount of both catalyst and hydroxy groups leads to an acceleration of the relaxation times and a decrease of the activation energy of the transesterification reactions. The system features elastomeric properties, and the tensile properties are maintained after two recycling steps. Thus far, vitrimers have been limited largely to hydrophobic polymers; this system is a step forward toward waterborne, one-component materials, and we demonstrate its use in waterborne bioinspired nanocomposites.