Bioinspired Bouligand cellulose nanocrystal composites: a review of mechanical properties

Abstract

The twisted plywood, or Bouligand, structure is the most commonly observed microstructural motif in natural materials that possess high mechanical strength and toughness, such as that found in bone and the mantis shrimp dactyl club. These materials are isotropically toughened by a low volume fraction of soft, energy-dissipating polymer and by the Bouligand structure itself, through shear wave filtering and crack twisting, deflection and arrest. Cellulose nanocrystals (CNCs) are excellent candidates for the bottom-up fabrication of these structures, as they naturally self-assemble into 'chiral nematic' films when cast from solutions and possess outstanding mechanical properties. In this article, we present a review of the fabrication techniques and the corresponding mechanical properties of Bouligand biomimetic CNC nanocomposites, while drawing comparison to the performance standards set by tough natural composite materials.This article is part of a discussion meeting issue 'New horizons for cellulose nanotechnology'.

Keywords: Bouligand; cellulose nanocrystals; chiral nematic; extrinsic toughening; nanocomposites; self-assembly.

Figure 1.

(a) Comparison of a 3D model Bouligand structure with SEM image of chitin fibril arrangement in the mantis shrimp dactyl club. (b) Nanoindentation map of hardness and a line profile of elastic modulus from the impact region to the periodic region. Reproduced with permission [7]. Copyright © 2012, AAAS. (c) 3D X-ray computed microtomograph of crack paths (purple) in human cortical bone, showing crack twisting and deflection around Haversian canal network (yellow). Reproduced with permission [8]. Copyright © 2012, Elsevier. (d,e) In situ TEM micrographs of picoindentation performed on focused ion beam milled dactyl club impact region, showing 90° crack deflection at interface between perpendicular fibre bundles (green and blue). Reproduced with permission [9]. Copyright © 2016, Wiley.

Figure 2.

Photographs showing damage in (a) quasi-isotropic and (b) small-angle Bouligand carbon fibre-reinforced epoxy composites. Ultrasound images showing damage fields (indicated by dotted lines) in (c) quasi-isotropic and (d) small-angle Bouligand carbon fibre-reinforced epoxy composites. A visualization of impact damage in finite-element models of (e) quasi-isotropic and (f) small-angle Bouligand CFRPs, with nodal points coloured based on dominant failure mode. The top layer represents the impact surface. The wider distribution of damage in the Bouligand composites is clear in both the experimental (d) and simulation (f) results. Reproduced with permission [50]. Copyright © 2014, Elsevier.

Figure 3.

(a) An SEM image of a photonic CNC film cross-section showing curves arising from CNC Bouligand arrangement. Reproduced with permission [57]. Copyright © 2017 ACS, Elsevier. (b) CD spectra of cellulose films produced from 1% by mass CNC suspensions with 0 (black solid), 0.1 (black dashed), 0.2 (dark grey solid), 0.3 (dark grey dashed), 0.4 (grey solid) and 0.5 mol cm⁻³ (grey dashed) NaCl, showing blue-shifting with increasing ionic strength. Reproduced with permission [58]. Copyright © 2010 ACS, Elsevier. Polarized optical microscopy and SEM images of (c,g) fast-dried, sodium-neutralized, (d,h) slow-dried, sodium-neutralized, (e,i) fast-dried, methyltriphenylphosphonium-neutralized, and (f,j) slow-dried, methyltriphenylphosphonium-neutralized CNC films. The measured pitch and standard deviations are observed to shift to lower values with slower drying (1000 ± 350 to 606 ± 151 nm) and greater surface hydrophobicity [1000 ± 350 to 619 ± 190 nm (fast) and 606 ± 151 to 504 ± 180 nm (slow)]. Reproduced with permission [57]. Copyright © 2017 ACS, Elsevier.

Figure 4.

(a) Comparative Ashby plot of elastic modulus (GPa) versus the strain at break (%) from Bouligand CNC composites reported in the literature [,–73]. The arrow represents the general trend in property change with increased polymer addition. (b) A comparative 3D plot of elastic modulus (GPa) versus the strain at break (%) versus the CNC concentration (%) from Bouligand CNC composites reported in the literature. (c) An Ashby plot of elastic modulus (GPa) versus the tensile strength (MPa) comparing synthetic Bouligand CNC composites with natural Bouligand materials. (d) Ashby plot of elastic modulus (GPa) versus the tensile strength (MPa) comparing synthetic Bouligand CNC composites with commercial materials.