Enhancement And Toughening Of Artificial Silk Materials

As one of the toughest materials, spider dragline silk has roused widespread attention. Preparation of artificial spider silk has become a comprehensive subject in biochemistry, bionics and materials science. Because of the cannibalistic feature, it is impracticable to rear spiders. Thus the spider silk resource cannot be obtained easily from nature like silkworm cocoon silk. There are three general strategies to prepare artificial spider silk, such as genetic engineering, biomimetic spinning using regenerated silk fibroin (RSF) and modification of natural silkworm silk. The artificial spider silk based on genetic engineering is high-cost and low-yield. The biomimetic artificial RSF silks have mechanical properties exceeding silkworm silk but far from spider silk.As the in situ synthesized nano-titanium dioxide (TiO2) greatly toughened natural spider silk, a hydrophilic nano-TiO2was used to modify the artificial RSF silk. In the first section of this study, the effects of TiO2content and post-treatment parameters on the mechanical properties and micro-structures of hybrid artificial silks were discussed, respectively. It is found that the proper incorporation of TiO2(0.05-0.7wt%) into RSF can greatly increase the mechanical properties of artificial RSF silk. It is of great interest that the hybrid silk with0.1wt%TiO2exhibited an elongation of88.3%and a breaking energy of93.1MJ m-3exceeding that of silkworm silk. Fourier transform infrared spectroscopy (FTIR) demonstrated that as the increase of TiO2in RSF fiber, the (3-sheet content reduced and the random coil/a-helix content increased. The crystalline structure of hybrid artificial silks was characterized by synchrotron radiation hard X-ray micro-focus wide angle X-ray diffraction (SR-WAXD). The SR-WAXD results showed that the crystallinity, crystallite size and orientation degree of hybrid artificial silks were all less than those of neat RSF silk. While the TiO2content raised, the crystallinity declined but the mesophase content increased. The properties and micro-structures of hybrid artificial silks can also be influenced by the post-treatment method. Lager draw ratio conduced to better mechanical properties and the ascending of β-sheet content and orientation degree. The mechanical properties of the hybrid artificial silks drawn and immersed with fixed length were superior to those of silks drawn and immersed in relaxed state. For better understanding of the mechanism of nano-TiO2toughened RSF artificial fiber, in situ X-ray diffraction/scattering were performed on RSF/TiO2solution and fiber, respectively. in situ small angle X-ray scattering (SR-SAXS) combining optical shearing system was applied on the RSF/TiO2solution to demonstrate the stable interface existing between RSF and TiO2. in situ SR-WAXD was done on single fiber during drawing. It was found that the crystallinity and the orientation degree of hybrid fiber raised when the strain was increased in plastic deformation zone. The interface orientation lagged behind that of crystal orientation. A mechanism of confined crystallites toughening artificial RSF silk was suggested. For the interaction of chelation and H-bonds between RSF and TiO2, the crystallization of RSF molecules nearby TiO2was confined in nano-space so that smaller crystallite, lower crystallinity and more mesophase formed. When the plastic deformation happened during drawing, the amorphous and crystalline regions oriented first. Some part of amorphous RSF and mesophase transformed to crystals which resulted in strain-hardening of hybrid silk. The mesophase between RSF and TiO2oriented in higher strain zone which lead to the second yield point in stress-strain curve of hybrid silk. The orientation of crystalline and amorphous region in low strain and the orientation of mesophase in high strain contributed to the large breaking elongation and breaking energy.Although the toughness and initial modulus of artificial RSF silk had been improved significantly by incorporating TiO2, the breaking energy of silks were still far from natural ones. Therefore, functional multiwall cabon nanotubes (MWNTs) were employed to further enhance the artificial RSF silk material. The RSF/MWNTs (0-1.5wt%) electrospun mats were prepared and the influence of MWNTs content on the mechanical properties of material was discussed. The results showed that the MWNTs can enhance RSF electrospun mats obviously and the composite mats with1.0wt%MWNTs exhibited a2.8-fold increase in breaking strength, a4.4-fold increase in Young’s modulus and a2.1-fold increase in breaking energy.