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.

Metrics & MoreArticle Recommendations CONSPECTUS: Bioinspired materials engineering impacts the design of advanced functional materials across many domains of sciences from wetting behavior to optical and mechanical materials. In all cases, the advances in understanding how biology uses hierarchical design to create failure and defect-tolerant materials with emergent properties lays the groundwork for engaging into these topics. Biological mechanical materials are particularly inspiring for their unique combinations of stiffness, strength, and toughness together with lightweightness, as assembled and grown in water from a limited set of building blocks at room temperature. Wood, nacre, crustacean cuticles, and spider silk serve as some examples, where the correct arrangement of constituents and balanced molecular energy dissipation mechanisms allows overcoming the shortcomings of the individual components and leads to synergistic materials performance beyond additive behavior. They constitute a paradigm for future structural materials engineering in the formation process, the use of sustainable building blocks and energy-efficient pathways, as well as in the property profiles that will in the long term allow for new classes of high-performance and lightweight structural materials needed to promote energy efficiency in mobile technologies. This Account summarizes our efforts of the past decade with respect to designing self-assembling bioinspired materials aiming for both mechanical high-performance structures and new types of multifunctional property profiles. The Account is set out to first give a definition of bioinspired nanocomposite materials and self-assembly therein, followed by an in-depth discussion on the understanding of mechanical performance and rational design to increase the mechanical performance. We place a particular emphasis on materials formed at high fractions of reinforcements and with tailor-made functional polymers using self-assembly to create highly ordered structures and elucidate in detail how the soft polymer phase needs to be designed in terms of thermomechanical properties and sacrificial supramolecular bonds. We focus on nanoscale reinforcements such as nanoclay and nanocellulose that lead to high contents of internal interfaces and intercalated polymer layers that experience nanoconfinement. Both aspects add fundamental challenges for macromolecular design of soft phases using precision polymer synthesis. We build upon those design criteria and further develop the concepts of adaptive bioinspired nanocomposites, whose properties are switchable from the outside using molecularly defined triggers with light. In a last section, we discuss how new types of functional properties, in particular flexible and transparent gas barrier materials or fire barrier materials, can be reached on the basis of the bioinspired nanocomposite design strategies. Additionally, we show new types of self-assembled photonic materials that can even be evolved into self-assembling lasers, hence moving the c...

The fusion of tetrazole/maleimide-based photochemical ligation (self-reporting, selective) with cellulose nanofibrils (good mechanical properties, biocompatible, sustainable) establishes a versatile platform for bio-based advanced materials.

Supramolecular engineering of multibody colloidal systems provides flexible ways of manipulating superstructures and material properties. We investigate a coassembling microgel (MG) system, in which host- and guest-modified MG partners coassemble by molecular recognition, and show in detail how electrostatic repulsion needs to be balanced for the supramolecular recognition to take place. We observe a gradual change from repellent MGs to stable clusters and ordered flocculates upon decreasing electrostatic repulsion. The adaptive nature of the multivalent interactions embedded in the soft MG shell leads to kinetically trapped scenarios and fibril formation from spherical building blocks.