| Natural cellulose (NC) is the most abundant renewable resources in nature. Due to its excellent biodegradability and biocompatibility, abundant content, various derivatives and so on, NC has been attracted a lot of research in recent decades, and has achieved rapid development. Because of the low solubility and the high potential of crystallization, the understanding of NC characteristics at single-molecule level is not in-depth and comprehensive. In order to utilize NC more effectively and better, it is necessary to thoroughly understand the structure of NC, especially the conformation, structural change and performance of NC at the single molecule level.In this thesis, the mechanical properties of natural cellulose have been studied by single molecule force spectroscopy(SMFS). First, the molecular structure, the crystal structure and the solubility of NC and the application of SMFS have been reviewed. And then we investigated the inherent single-chain stretching elasticity of NC, which is as flexible as those polymers with carbon-carbon (C-C) backbone. The traditional view that NC is rigid or semi-rigid is not correct. Then the inherent stretching elasticities of typical α-1,4-D-linked and β-1,4-D-linked polysaccharides have been studied by SMFS. The inherent elasticities of these polysaccharides are not influenced by the side chain and the different linkages. The single-chain mechanics of NC has been studied in different liquid environments, which reveals that NC is rather hydrophobic even if it is molecularly dispersed. Based on the above systemic investigation, the main conclusions can be summarized as follows:(1) We prepare single chain samples of NC by adsorption from an NC/AMIMC1 solution, then attempt to study the inherent stretching elasticity of a single NC chain by SMFS. The normalized force curves obtained in octane can be superposed well, indicating that the elastic force signals present the inherent elasticity of sing NC chain. We use QM-FJC model to describe the single chain behavior of NC upon elastic elongation, in which the QM single chain elasticity is integrated into the FJC model. When the Kuhn length is 0.514 nm, the fitting curve and the experimental curve can be superposed well. The value of lk is virtually equal to the length of a pyranose unit of NC. This two findings indicate that the QM calculations reflect the inherent elasticity of single NC chain and the QM-FJC model is appropriate for NC. The normalized fitting curves of NC and polymer with C-C backbone have little difference. This result indicates that although NC is much more rigid than common synthetic polymers at the material scale, it is as flexible as those polymers at single molecule level. This interesting finding implies that the high rigidity of cellulose crystal is a consequence of the macromolecular self-assembly, which perfectly demonstrates that the whole is more than the sum of its parts. These numerous weak interactions (H-bonding network) will work synergistically, which effectively reinforce the material. More importantly, our finding casts new light on the design of novel nano-materials.(2) We have studied the inherent elasticity of three kinds of β-1,4-D-linked polysaccharides, i.e., NC, methylcellulose (MC) and carboxymethylcellulose (CMC) by SMFS. The force curves of NC, MC and CMC obtained in octane can be superposed well, indicating that the inherent elasticity (not influenced by the size of side chain) of β-1,4-D-linked polysaccharides is obtained. The QM-FJC model can provide the best fitting curve when lk=0.514 nm, which can be superposed well with the experimental force curves. This finding demonstrates that we have obtained the inherent elasticity of β-1,4-D-linked polysaccharides. Then, we studied the inherent elasticity of amylose and carboxymethyl amylose (CMA), both are α-1,4-D-linked polysaccharides. The force curves of amylose and CMA obtained in octane can be superposed well, indicating that the inherent elasticity of α-1,4-D-linked polysaccharides is obtained. The force curves of α-1,4-D-linked and β-1,4-D-linked polysaccharides can be superposed well, implying that the different linkages have no detectable influence on the inherent elasticity. Thus, for the first time, we find that the side chain and the different linkages have no detectable influence on the inherent elasticity of 1,4-D-linked polysaccharides.(3) The single-chain mechanics of NC has been studied in different liquid environments, which reveals that NC is rather hydrophobic even if it is molecularly dispersed. In a common nonpolar solvent, octane, NC shows the pure elastic elongation, which can be described well by the QM-FJC model. However, NC shows different single chain behavior in water, where a long plateau with a height of～85 pN is observed in the force curve. The force curve after the plateau (i.e., F> 200 pN) can be attributed to an elastic elongation, which can be superposed with those obtained in octane. This result indicates that the force curve obtained in water is also a single chain result, but the plateau should be originated from a single chain behavior which is different from elastic elongation. Further study shows that the height of the plateau is temperature dependent. However, the plateau disappears when an 8M urea solution is used as the liquid environment. AFM images obtained in water show that single NC chains exist in a compact globule conformation on the sample surface. According to the molecular structure, methylcellulose (MC) should be more hydrophobic than NC. However, no plateau can be observed from the MC samples in water. On the basis of all the results above, we can draw a conclusion that both of the hydrophobic effect and the crystallization of NC contribute to the plateau in the F-E curve obtained in water. Our experimental finding validates, for the first time, the hydrophobic nature of NC at the single chain level. With three hydroxyl groups in each sugar ring, NC was conventionally thought to be highly hydrophilic. However, two of the hydroxyl groups are directly linked to the rigid sugar ring, which greatly limits the freedom of the hydroxyl groups. Thus, the sugar ring is actually amphiphilic:The equatorial sides are more hydrophilic, but the axial sides are more hydrophobic. This special hydrophobicity is crucial for NC, which imparts the two contradictory properties, water insolubility and water permeability, to this important nature occurring macromolecule. More importantly, our finding casts new light on the design of novel nano-materials and biomimetic materials. |
