The twisted plywood or helicoidal structure is a commonly observed microstructural motif in biological composites that enables superior toughness via extrinsic toughening routes such as crack twisting, deflection, and arrest, as well as by the filtering of shear wave energies. These mechanisms have motivated numerous efforts to mimic helicoidal structures through top-down assembly or 3D printing. However, attempts to emulate strong and tough natural composites using wood cellulose nanocrystals (CNCs) fall short in mechanical performance due to the low shear transferability of CNC interfaces in their helicoidal ensembles. This shortcoming is ascribed to the small CNC-CNC overlap lengths that minimize the cumulative interaction strengths. In this presentation, we will demonstrate a simple green strategy to fabricate superior helicoidal CNC films with properties that rival those of the best natural materials. Assembling the wood CNCs (w-CNCs) with a minority fraction of large aspect ratio tunicate derived CNCs (t-CNCs), we report remarkable synergistic enhancements in modulus, strength, ductility, and toughness. These mechanical properties (E = 20.2 GPa ± 0.9 GPa, σf = 138 MPa ± 4.3 MPa and toughness = 1.09 MPa/m3 ± 0.01 MPa/m3), which are the best reported for photonic CNC films thus far, rival those of gold-standard natural composites such as cortical bone, nacre, ivory and stomatopod clubs. Using scanning electron microscope-based fractography and multiscale simulations, we explain the observed enhancements to be due to increased effective overlap lengths (i.e., enhanced CNC-CNC interfaces) and the activation of additional toughening mechanisms such as bridging by the long CNCs. The effectiveness of this strategy and role of parameters such as long-CNC modulus and aspect ratio are evaluated by preparing a second set of bimodal mixtures using cellulose nanofibrils that are of lower modulus and crystallinity as compared to t-CNCs. All blended films with long-CNC contents greater than 5 % by mass also display UV reflecting behavior. We identify these films as excellent starting materials or scaffolds for biomimetic nanocomposite fabrication. As is, these damage tolerant optically active materials hold potential for application as protective coatings in the automobile, aerospace, electronics and infrastructure industries.

Bimodal cellulose nanocrystal blends for bioinspired damage tolerant photonic films

255th National Meeting (2018)

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