In biological architectures, material properties are optimized by combining multiple components ordered both at the nano- and macro-scales. For instance, the natural armors of certain reptiles and mammals consist of tiled scales of highly ordered protein nanofibers. Such nano- to macro-scale superstructuring enables programmed yield points, which maximize toughness and minimize the otherwise high brittleness imbued by high strength building blocks. In this study, we used cellulose nanocrystals (CNCs) as high strength (E<sub>T </sub>&gt;100 GPa) nano-scaled building blocks and controlled their assembly into chiral-nematically ordered structures from the nano- to the macroscale. Thus, bridging previous developments in biomimetic materials focusing on either of these scales individually. Specifically, the CNC films were tesselated by microtemplating onto a flexible, soft, substrate consisting of a mesh with unit-cells defined by the diameter of the filaments and size of the open areas between the filaments. We evaluated two meshed architectures, with two distinct feature sizes, and compared their opto-mechanical properties with featureless CNC films. In addition to the meshed substrate, incorporated into the final bi-component composite, a separate rigid support was used to control the capillary stresses developed during the assembly. When optimized, microtemplating enabled the formation of chiral nematically ordered CNC films that replicated, with high fidelity, the topography inherent to the chosen meshed substrates. The director of the helicoidal, chiral-nematic, assembly followed the contour of the template, which enabled fine-tuning of the reflection properties of the material as well as its toughness. Finally, we demonstrate that the microtemplated CNC structures promote multi-scaled fracture propagation that can contribute to enhancing the material’s toughness through a highly tortuous fracture propagation in all (x, y, z) directions. We expect that the presented findings pave the way towards bridging the large body of research in cellular metamaterials, e.g., with 3D tessellation of repetitive motives or origami patterns, with the research focusing on the generation of nano-scaled biomimetic materials, e.g., from mismatched layers of clay, cellulose nanocrystals or even chitin.

Opto-mechanical properties of microtemplated chiral-nematic cellulose nanocrystals films

2019 Spring National Meeting

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