Microstructural And Mechanical Properties Of The Spines From Echinocactus Grusonii Cactus

After billions of years of evolution, creatures in nature possess almost perfect structures and functional properties. There are a number of interrelated features that define biological materials and distinguish them from their synthetic counterparts: self-assembly, multi-functionality, hierarchy, hydration, mild synthesis conditions, evolution and environmental constraints and self-healing capability. The seven characteristics are present in a vast number of structures. Multiscale structures ranging from nano, micro to macro are characteristic for biological materials, which play an important role in achieving structural and functional integrity. Multiscale structures of biological materials exhibit inherent multifunctional integration. This special biological solution provides some inspiration for scientists and engineers to design multifunctional artificial materials with multiscale structures.Some plants have sharp defensive plant organs, for which three terms are created to describe: spines when they are made of leaves, thorns when they are made of branches and prickles when they are made of cortical tissues. A very important role for the sharp plant organs is to against vertebrate herbivores, which require a high puncture strength to withstand bending, compression and torsion. The sharp plant organs usually have the advantages of simple structure and convenient reconstruction. Our research would be another source of inspiration for composite materials. Based on physical mechanics methods that combine parallel computation, theoretical modeling with experimental characterizing and synthesis, this project extensively studied the structural and mechanical properties of sharp plant organs. The studies aim to unveil novel structure function corelations of sharp plant organs, reveal their mechanism for high puncture strength properties. The studies will be helpful to develop nanocomposites.In this work the spine from Echinocactus grusonii cactus was studied with chemical analyzing techniques, tensile test and nanoindentation, coupled with advanced micro/nanostructure characterization methods. The hierarchical structure, its composition and mechanical behavior of the spine were systematically investigated.First, the correlation between the microstructures and mechanical properties of spines from Echinocactus grusonii cactus are comprehensively investigated using light microscope, scanning electron microscope and x-ray microanalysis(XRD), as well as nanoindentation and tensile tests. It is found that the cactus spine shares similar appearance with bamboo and exhibits no mineralization phenomenon, but with a different fibrous structure made of only libriform fibres and sclerified epidermis. The indentation modulus of the spine cell wall is slightly lower than that of woods and crops, but similar to that of bamboo, within the same order of magnitude. Although the transverse hardness of the spine is nearly identical to those of woods, crops and bamboo, it has much higher longitude hardness. In addition, the tensile properties of the spine do not show apparent differences with other non-wood plant fibre bundles. Furthermore, the spine has certain toughness when it is fresh, but it becomes brittle if it was dried. The spine fibres have high crystallinity and very small multifibrillar angle. These results suggest that the morphology and structure of spines are responsible for its special mechanical properties.Further, the microstructures and mechanical properties as well as their correlation of single spine fiber cells(SFCs) from the cactus Echinocactus grusonii are systematically investigated. It is found that the SFCs are 0.32–0.57 mm in length and 4.6–6.0 μm in width, yielding an aspect ratio of 53–124. X-ray diffraction and Fourier transform infrared spectrophotometry show that the spine fiber is mainly made up of cellulose I with a crystallinity index up to ~76%. Nanoindentation tests show that a natural spine presents a high modulus of ~17 GPa. Removing hemicellulose and lignin from the SFC significantly reduces its modulus to ~0.487 GPa, demonstrating the critical role of adhesives hemicellulose and lignin in affecting the mechanical properties of the SFCs. Last, molecular dynamic simulations were performed to investigate the mechanical properties of Iβ cellulose with the use of the reactive force field Reax FF-mattsson. The Young’s modulus obtained by simulations with Reax FF-mattsson(103.38-128.33 GPa)is close to the experimental results(120-138 GPa). Furhter, studies were conducted to observe the influence water molecules and temperature have on the mechanical properties of Iβ cellulose in the axial direction. Water molecules increase the toughness and decrease the number of intermolecular hydrogen bonds of the cellulose. The intermolecular hydrogen plays a significant role in the mechanical properties of the cellulose. There is a linear relationship between them. Before reaching phase transition temperature of(475-500 K), the temperature has little effect on the Young’s modulus of the cellulose. When the temperature is 500 K, a large number of intermolecular strong hydrogen reformed and meshed with bonds and prime covalent bonds and the Young’s modulus(87.36 GPa) is decreased by a third from that of 300 K(128.33 GPa). The Reax FF contains the breaking and forming of bonds, van der Waals force and the Coulomb force. So the Reax FF is more suitable to characterize the interaction forces between atoms in cellulose. This study makes preparations for further researches on influence of the mechanical properties of the cell wall with lignin and hemicellulose by molecular dynamic simulations.These results suggest that the cross-linking way of hemicellulose and lignin, neat alignment of the SFCs and the sandwich roll structureaccount for their extraordinary strength and hardness. This finding sheds light on designing novel bio-inspired high-performance composite nanomaterials with aligned nanofibers, such as using hemicellulose and lignin as adhesive in making carbon nanotube fibers.