Post-Treatment Of The Dry-Spun Fibers From Regenerated Silk Fibroin Aqueous Solution

Spiders and silkworms produce natural animal silks with remarkable mechanical performance from aqueous protein solutions by using dry-spinning processes in air. Many researchers tried to biomimetically obtain artificial animal silks using wet-spinning and electrospinning processes, which are very simple and are significant different from the natural spinning processes of spider and silkworm.Our group initially prepared regenerated silk fibroin (RSF) fibers from RSF aqueous solutions by using dry-spinning technology, while the mechanical properties of the fibers are far from the Bombyx mori silk. In order to obtain high performance fibers as natural silks, post-treatment on the RSF as-spun fibers was studied in this thesis. Ethanol aqueous solution (80 vol%) was adopted as a post-treatment agent due to its environmentally friendly chatractistic and the ability to promote the conformational transition of RSF. A post-drawing device specially designed for ultra-fine monofilament was used for the single-step and multi-step post drawing. By using polarized optical microscope, Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy, the storage conditions of as-spun fiber and the conformational transition of RSF during the post-treatment were investigated. This study provides a reference for the fabrication of artificial animal silks with high-performance.To obtain stable RSF fibers, RSF as-spun fibers are recommended to be stored in a sealed container with 30-35% RH at least 3 days. Despite the ethanol atmosphere post-treatment induced the formation of ordered structures containningβ-sheet conformation, the poor orientation resulted in the mechanical property deterioration of the RSF fiber. It is suggest that orientedβ-sheet structure is very important for high performance silk fibers.In order to optimize the post-treatment process for the RSF fiber, the orientation and conformation development of the RSF molecules in two different post-treatment processes were studied quantitatively. The two processes are named as method A (the RSF fiber was immersed in 80 vol% ethanol solution for 1 h then drawn 2 times in steam) and method B (the RSF fiber was drawn 2 times in 80 vol% ethanol solution then immersed in the same solution for 1 h). Furthermore, the relationship between the conformation transition and the improvement of mechanical properties of the RSF fibers was discussed. Comparing to immersing the fiber in ethanol aqueous solution, it is more efficient to transit the RSF conformation and improve the RSF orientation by post-drawing the fiber in ethanol aqueous solution or in steam. Thus, method B instead of method A was adopted for an optimal process due to the resultant highβ-sheet content, orientation and excellent mechanical properties of the RSF fibers.Because of the unstable manual stretching in the previous post-treatment, mechanical drawing was achieved via an improved post-drawing device with many advantages, such as accurate measurement and convenient operation. To further optimize the post-treatment process with single-step drawning, the effects of drawing rate, drawing ratio and fiber diameter on the structures and properties of the RSF fibers were investigated. Resuls show that the mechanical properties of the post-treated fibers mainly depend on the supramolecular structures, such as orientation, crystalline structure and morphology. Drawing ratio is more effective to improve the structure and and the mechanical properties of the fibers than drawing rate.The mechanical properties of RSF as-spun fiber could be significantly improved by post-treatment processes with single-step or multi-step drawing procedures. The post-treatment with single-step post-drawing is more effective than the multi-step one. After being stretched 4 times in 80 vol% ethanol aqueous solution at a rate of 0.9 mm/s, and then being immersed in the solution for 1 h,the as-spun fiber achived a breaking stress of (326.7±97.8) MPa and a breaking energy of (39.7±10.1) kJ·kg-1. The stress and the breaking energy are up to 10 times and 150 times of those of the as-spun fiber, respectively. The post-treated fiber is even superior to the degummed natural silk. When the as-spun fiber was post-treated with multi-step drawing procedures, the stress and the breaking energy are up to 6 times and 90 times of those of the as-spun fiber, respectively.