| Both silkworm and spider can efficiently dry spin outstanding fibers under environmentally friendly conditions using water as solvent at atmospheric pressure and room temperature. Recently, the studies on biomimicking the spinning processes of spider and silkworm have been carried out with great attentions by researchers in many fields, especially in the field of polymer materials. However, the poor protein amount produced by spider has set a restriction to the further research on animal silk. In contrast, silkworm silk has similar amino acid composition and formation mechanism as spider silk. Meanwhile, silkworm silk can be obtained easily and shows excellent mechanical properties as spider silk under certain spinning conditions. Therefore, silkworm silk has been selected as a model to perform the biomimetic spinning of silk by many researchers.When studying the biomimetic spinning of silk, it is necessary to understand the spinning system of silkworm and the structures and properties of the silk fibroin (SF) aqueous solution in vivo, then a suitable spinning dope can be prepared by mimicking the SF state and an environmentally friendly dry spinning technology can be explored to produce high performance regenerated silk fibroin (RSF) fibers. However, the investigations on the rheological properties of the natural SF solutions and the structural change of SF aggregates along the silk gland are rarely reported. Meanwhile, the formation mechanism of silk is still incompletely known. Therefore, before studying the biomimetic spinning of silk, the structures and properties of the natural spinning dopes in different divisions of silk gland were firstly investigated in this thesis. Then the changing process of natural dope was mimicked in vitro and the effects of various factors on the structures and properties of RSF aqueous solution were discussed to better understand the formation mechanism of silk. Based on these works, an environmentally friendly and simple dry spinning technology was further explored to prepare high performance RSF fibers. To understand the formation process of silk in vivo, in this thesis, the structures and properties of the SF aqueous solutions from different divisions of silk gland were investigated using polarized microscope, rotational rheometer, FT-IR spectrometer, Raman spectrometer and dynamic laser light scattering instrument. It was found that only the anterior division and the anterior part of middle silk gland showed birefringence. With flowing from the posterior part to the anterior part in the middle silk gland, the viscosity and elasticity of the SF aqueous solution decreased, and its conformation was also gradually transformed into P-sheet. Meantime, the aggregation of SF gradually occurred and the larger aggregates with more uniform size were eventually formed in the anterior part of middle silk gland. These results indicated that as the SF solution flowed from the posterior division to the anterior division in the silk gland, it was gradually changed from isotropy to anisotropy and the liquid crystal structure might be initially formed in the anterior part of middle silk gland.In order to biomimic the SF dope in silk gland, the condensed RSF aqueous solution was prepared.Then, the effects of solution concentration, pH value and Ca2+content on the structures and properties of the RSF solution under different shear rates were studied using plate shear device. It was found that the RSF aqueous solution was changed to anisotropy after being applied sufficient shear. Meanwhile, it was also found that the increase of concentration, decrease of pH value and the addition of Ca2+promoted the solutions’structural transition from isotropy to anisotropy. Furthermore, the effect of shear time on the structures and properties of condensed RSF aqueous solution system was investigated online by using optical shearing instrument, synchrotron radiation small angle X-ray scattering and rotational rheometer. It was found that the extension of shear time and the increase of shear rate (stress) were equivalent, and both could lead the aggregation of RSF molecules and the conformational transition to β-sheet. As the shear time increased, the viscosity of the RSF aqueous solution gradually increased to a maximum. Meantime, rod like structures were formed due to the aggregation of enough RSF molecules along the shear direction, and the corresponding solution showed birefringence. As the shear time was further extended, the solution viscosity decreased and the length/diameter ratio of RSF aggregates increased. The rod like structures and the birefringence of the solution became more obviously. These results indicated that the RSF molecules might gradually form a rod-like liquid crystalline structure in the solution when the condensed RSF aqueous solution was optically anisotropic after being applied sufficient shear.In addition, both the conventionally prepared RSF aqueous solution (with the adjustment of pH value and Ca2+concentration) and the simply prepared RSF aqueous solution (with solely adjusting the Ca2+concentration) showed similar results. This indicated that the adjustment of pH value had little effect on the formation of liquid crystal structure in the RSF aqueous solution.In view of the shear experiments’ results, a new and more environmentally friendly dry spinning technology with solely adding Ca2+was proposed to prepare RSF dry-spun fiber using capillary spinning equipment. The resultant RSF fiber exhibited poorer structures and mechanical properties compared with the natural degummed silk, which was due to the deficient drawing of the artificial silk in air. In order to improve its mechanical properties, the parameters for the spinning and post-treatment were further adjusted. It was found that a proper increase of the take-up rate, spinning dope concentration and the length/diameter ratio of capillary promoted the formation of β-sheet conformation and hence improved the mechanical properties of the fibers. In this thesis, the suitable take-up rate, spinning dope concentration and the length/diameter ratio of capillary were about3cm/s,53wt%and80, respectively. Moreover, the ethanol aqueous solution was chosen as the post-treatment agent, and then the RSF fiber was drawn and immersed in the solution to improve its properties and structures. It was found that compared with the conventionally prepared RSF post-treated fiber, the simply prepared RSF post-treated (Sim-Post) fiber showed better secondary structure, crystalline structure and orientation, as well as better mechanical properties. After post-treatment, the crystallinity of the Sim-Post fiber was about48%, which was similar to that of the natural degummed silk. Meanwhile, the mechanical properties of the Sim-Post fiber were also greatly enhanced, with an average breaking strength of357.3Mpa, an initial modulus of10.3GPa and a breaking energy of52.7kJ/kg, which were even better than those of the natural degummed silk. In addition, the microfluidic chip was tried to dry spin RSF fibers from the simply prepared RSF aqueous solution. It was found that the structures and properties of the resultant RSF fiber could be improved by using this technology. The above results indicated that high performance RSF fibers could be obtained more simply and environmentally friendly by simplifying the preparation process of RSF spinning dope, which was without the adjustment of pH value and with solely adding Ca2+,using a dry spinning technology and a proper post-treatment.Based on the above work and in order to understand the role of Ca2+on the formation of RSF fibers, the RSF aqueous solutions with different concentrations of Ca2+were prepared for dry spinning and post-treatment. Meanwhile, the effects of Ca2+on the structures and properties of RSF aqueous solutions and the resultant fibers were investigated. Results showed that the concentration of Ca2+greatly affected the rheological properties and spinnability of RSF aqueous solutions. As the concentration of Ca2+increased, the viscosity of the solution decreased. In this thesis, when the concentrations of Ca2+were1.25～2.50mmol/g, the RSF aqueous solutions showed good spinnability. Meanwhile, the addition of Ca2+prompted the formation of β-sheet and the aggregation of RSF, which further caused the increase of β-sheet content and crystallinity in the resultant as-spun and post-treated RSF fibers. Moreover, Ca2+was favorable for the orientation of macromolecular chains and crystalline regions in the fibers, which resulted in better mechanical properties. Studies on the role of Ca2+on RSF fiber formation showed that the Ca2+could form a COO--Ca2+--OOC bond between two negatively charged hydrophilic spacers of RSF chains, which promoted the formation of intermolecular hydrogen bond and led to the β-sheet conformation. Moreover, this packed structure could be assembled orderly after physical shearing and drawing during spinning and post-treatment, which resulted in forming high performance RSF fibers. |
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