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Coconuts are one of nature’s toughest lignocellulosic materials, possessing a fracture toughness on par with dentin and a compressive strength ten times that of bamboo. The coconut’s hierarchical structure has been characterized before, except prior studies left out one key aspect, the smallest length scales, approaching the molecular level. Here we exfoliate the hard shell of Cocos nucifera, revealing the true cellular organization and the dimensions of the crystalline cellulose nanofibrils found in the cell walls. After chemical pretreatments, we found entanglement between elongated sclereid cells that was not visible in the untreated coconut shell. This may contribute to the mechanical performance of the endocarp; it also utilizes elongated, high-aspect ratio structural elements at the cellular level, in addition to the nanofibrillar level previously known. Compared to other wood-like materials, the cellulose nanofibrils were shorter and represented a smaller weight fraction. This reduced length and the lower filler-to-matrix ratio could be the optimal lignocellulosic nanostructure for tough biomaterials. These newly discovered unique features explain how the endocarp of Cocos nucifera mechanically outperforms materials consisting of the same molecular components.
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Cellulose–Hemicellulose–Lignin Interaction in the Secondary Cell Wall of Coconut Endocarp
The coconut shell consists of three distinct layers: the skin-like outermost exocarp, the thick fibrous mesocarp, and the hard and tough inner endocarp. In this work, we focused on the endocarp because it features a unique combination of superior properties, including low weight, high strength, high hardness, and high toughness. These properties are usually mutually exclusive in synthesized composites. The microstructures of the secondary cell wall of the endocarp at the nanoscale, in which cellulose microfibrils are surrounded by hemicellulose and lignin, were generated. All-atom molecular dynamics simulations with PCFF force field were conducted to investigate the deformation and failure mechanisms under uniaxial shear and tension. Steered molecular dynamics simulations were carried out to study the interaction between different types of polymer chains. The results demonstrated that cellulose–hemicellulose and cellulose–lignin exhibit the strongest and weakest interactions, respectively. This conclusion was further validated against the DFT calculations. Additionally, through shear simulations of sandwiched polymer models, it was found that cellulose–hemicellulose-cellulose exhibits the highest strength and toughness, while cellulose–lignin-cellulose shows the lowest strength and toughness among all tested cases. This conclusion was further confirmed by uniaxial tension simulations of sandwiched polymer models. It was revealed that hydrogen bonds formed between the polymer chains are responsible for the observed strengthening and toughening behaviors. Additionally, it was interesting to note that failure mode under tension varies with the density of amorphous polymers located between cellulose bundles. The failure mode of multilayer polymer models under tension was also investigated. The findings of this work could potentially provide guidelines for the design of coconut-inspired lightweight cellular materials.
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- PAR ID:
- 10412989
- Date Published:
- Journal Name:
- Biomimetics
- Volume:
- 8
- Issue:
- 2
- ISSN:
- 2313-7673
- Page Range / eLocation ID:
- 188
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/biomimetics8020188
