Studies On The Structures And Properties Of Silk Fibroin Dry-Spun Fibers By Using Synchrotron Radiation Technology

Recently, considerable interest has arisen in mimicking biological high-performance materials because of their extraordinary mechanical properties, such as silkworm silk and spider silk. Although commercial Bombyx mori silk is considerably weaker than spider dragline silks, silkworm silks with comprehensive mechanical properties approaching those of spider silks can be obtained by forcibly reeling directly from silkworm. This is attributed to the improved secondary structures and less defects in the obtained silks. Thus, the secondary structures and mechanical properties of natural silk could be highly improved by optimizing spinning process. To prepare artificial tough silk, it is crucial to understand the relationship between the structures and the mechanical properties of the fibers. Due to limited sample amount, it is not possible to measure the crystalline structures of regenerated silk fibroin (RSF) fibers using traditional X-ray diffraction instrument. The intermittently dry-spun RSF single fibers and natural cocoon silk were studied by synchrotron radiation micro-focus wide angle X-ray diffraction (SR-WAXD) to obtain the details of the crystalline structure. In addition, the effects of intermittent post-treatment conditions on the crystalline structure were comparatively studied against natural degummed cocoon silk. Though the crystalline structure of the as-spun RSF fiber was poor, it could be improved greatly and even become similar to that of the degummed cocoon silk by post-treatment. As the post-treatment process was performed, the peak of [021](d-Spacing=0.37nm) lattice plane weaken while the peaks of [020](d-Spacing=0.45nm) and [200](d-Spacing=0.43nm) lattice planes grew intensely and became dominated finally. The crystallinity increased with the increase of draw rate and decrease of draw ratio, while the crystalline size of the RSF fibers all decreased with the increase of draw rate and ratio. The investigation of the structure-property relationships of the RSF fibers provides a wealth of possible new insight into the design of high-performance artificial animal silk.Generally, the as-spun fiber obtained by batched dry-spinning technology had to be stored in a sealed container before being post-treated. Thus the single fiber exhibited unstable mechanical properties. Moreover, the small sample amount limited the application of the RSF fiber on a large scale. In this paper, a custom-built continuous dry-spinning equipment was applied to prepare RSF/silk sericin(SS) fibers from RSF/SS aquous solutions. The RSF/SS fiber was firstly extruded from a capillary dry-spun equipment, then introduced into a pre-crystallization bath so that it could be easily drawn subsequently in a post-treatment bath. The post-drawing was performed by adjusting the rolling rates of the first and second rollers. Finally, the post-drawn RSF fiber was wound on the second roller. The continuous dry-spinning equipment produces fibers which can been post-treated once they were extruded. Thus the spinning efficiency was improved and the mechanical properties of the RSF/SS fiber became steady. The effects of continuous dry-spinning parameters on the spinnability of RSF/SS solution, the structures and properties of the RSF/SS fibers were investigated. The concentration of spinning dope, type of post-treatment bath and draw ratio were adjusted accordingly. It was found that increasing draw ratio is more effective than extending immersion time to further improve the structures and mechanical properties of the RSF/SS fibers. After being drawn4times in80vol%ethanol aqueous solutions, and then immersed in the same solution for another8h, the obtained RSF/SS fiber presented a breaking stress of259.2MPa, a breaking strain of13.7%, an initial modulus of9.6GPa and a breaking energy of13.0kJ·kg-1, respectively. These approached those of the RSF fibers prepared by batched dry-spinning process. However, the crystalline orientation factor of the resultant RSF/SS fiber was only0.35, which was still much lower than that of the degummed cocoon silk.