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Title: Physical mechanism of silk strength and design of ultra-strong silk
Authors: WU XIANG
Keywords: Silk, Semicrystalline, Mechanical Properties, Crystallite Orientation, Hierarchical Structure, Viscoelasticity
Issue Date: 17-Aug-2010
Source: WU XIANG (2010-08-17). Physical mechanism of silk strength and design of ultra-strong silk. ScholarBank@NUS Repository.
Abstract: Natural silk fiber spinning, as one of the mainstays of the economy centuries ago, continues to exert its power in various fields nowadays. The spider silk, hailed as super fiber, is the toughest silk among natural silk family, marked by the combination of both its great strength and extensibility. Despite of extensive studies of silk, it remain not fully understood, especially concerning the undergoing mechanism of their good performance. Current studies ranging from micro-structure probing to mechanical measurements still have not formed a solid relation between the mechanical properties and the structures of silk materials. Hence the general aim of my doctorial dissertation is to uncover the mystery of the mechanical properties of silk through the way of decoding the conformation of its hierarchical structures. Theoretical modeling, hand in hand with experimental measurements and probing, turns out to be the principal method I adopted in most of the research topics to achieve the ultimate goal. To understand the structure-property relation of silk, the structural origin of the mechanical stress-strain profiles was investigated in the first place. It was found that the existence of the ?-crystallites plays an important role in the occurrence of the yield point of silkworm silk, whereas the work-toughness region of spider dragline silk was attributed to the fully dissolution of small-sized ?-sheets into the amorphous matrix. Besides, the increased reeling or extruding speeds enhanced the mechanical performance of both silks, and elevated the level of the splitting force of crystallites. Such change of the mechanical strength by fast reeling was then interpreted by the properties of ?-crystallites, most important of which was their orientation. It showed that better alignment of the crystallites became more efficient in resisting external stress, thus reinforcing the backbone of silk fibers. This finding also provided the engineering insight that the mechanical properties of silks, generally speaking, those of semicrystalline biomaterials, could be optimized by fine tuning of their nano-structures. The time-resolved mechanical behavior of silks was also taken into account. Abnormal stress relaxation behavior was measured, and a bipartite pattern of stress relaxation by fixing at various strains was found with the separation strain around the yield point. Meanwhile, the linearly increased energy dissipation ratio was interpreted by the mechanism of local strain induced thickening, via the viewpoint of the conformational change of fibrils. The last but not least topic is the extensive investigation of the influence of twisting on the change of longitudinal mechanical properties. An inferior tendency of properties at failure was detected but the twisting manner could still enhance the energy absorbability in the low-strain range. The general aim was finally achieved by synthesizing the aforementioned topics through various dimensions, scales and domains. The method of combining theoretical prediction and experimental verification always turned out to be a powerful strategy for soft biomaterial design. Meanwhile, by revealing the mechanism of the super-strength of silk and implementing it with feasible experimental methods, important insights were provided in the improvement of silk quality, which could further benefit the development of silk engineering and widen the application of silk.
Appears in Collections:Ph.D Theses (Open)

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