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

Abstract

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.

Chiral Photonic Cellulose Films Enabling Mechano/Chemo Responsive Selective Reflection of Circularly Polarized Light

“…The chiral photonic cellulose films exhibit high stretchability with the maximum strain‐at‐break of 40.8%, which is the highest among chiral photonic cellulose materials reported so far. [12c,16d,20–22] They enable left‐handed circularly polarized light (L‐CPL) by selective reflection. Of note is that chiral photonic cellulose films having a certain composition exhibit a broad‐wavelength‐range response (from the visible to the near‐infrared region) of the selective reflection of left‐handed circular polarized light to bending, uniaxial tensioning, and water vapor (relative humidity (RH): 10–100%).…”

The Self‐Assembly of Cellulose Nanocrystals: Hierarchical Design of Visual Appearance

“…The co‐assembly of CNCs with weakly interacting additives significantly reduces the brittleness of the produced films while preserving the formation of the helicoidal nanostructure and causing a red‐shift of the optical bandgap (Figure e). This has been demonstrated with zwitterionic surfactants, neutral or anionic polymers, amino resins or sol–gel precursors, and crosslinked latex nanoparticles . The cholesteric arrangement of CNCs can also accommodate small nanoparticles without disruption of the phase, e.g., plasmonic gold nanorods, allowing complex chiroptical effects …”

Binary Cellulose Nanocrystal Blends for Bioinspired Damage Tolerant Photonic Films

“…a) Schematic of model helicoidal structure formed by the assembly of cellulose nanocrystals. b) Comparative Ashby plot of the elastic modulus (GPa) versus the strain to failure (%) from helicoidal CNC films and composites reported in literature . Error bars represent reported uncertainties.…”

“…This gap is attributable to the following observations. All studies on cellulose‐rich helicoidal materials utilize w‐CNCs as building blocks and the mechanical properties of the pristine w‐CNC films (100% w‐CNC) leave much to be desired. The pristine w‐CNC films are excessively brittle (strain to failure <0.5%).…”