Abstract:

Objective Bombyx mori silk fibers have an interesting and elaborate structure, but its mechanical properties are inferior to spider dragline silk. Therefore, it is of great significance to prepare Bombyx mori silk comparable to spider silk. Traditional silk processing technology, however, is difficult to enhance the performance of cocoon silk, and even damage its morphology and properties. In addition, artificial silk produced by a variety of spinning methods cannot fully replicate the properties of natural silk. Herein, we proposed a simple and effective strategy for preparing high-strength Bombyx mori silk, which not only retains the hierarchical structure of natural silk, but also advances its mechanical properties.

Method Inspired by spider silking, this work designed a conical forced reeling device that can consistently reel silk from the fifth instar silkworm at controlled reeling speed. In this study, the silkworms were forcibly reeled at 1-4 cm/s, resulting in the force-reeled silk fibers (FRSF-1, FRSF-2, FRSF-3, and FRSF-4). During forced reeling, the silk was fully stretched due to a recombination of the reeling force and the gravity of silkworm, which increased the orientation and crystallinity of FRSFs. Meanwhile, the cocoon silk fibers (CSFs) selected from the same batch were set as a control group. In addition, the characteristic of fiber morphology, fineness, and mechanical properties of all silk samples were performed.

Results The FRSFs have a smooth and uniform morphology, and a bow-shaped cross-section, which differs significantly from that of CSFs. Specifically, the transversal surface of FRSF-1 is almost a round triangular cross-section shaped like CSFs, and both the rounded triangular and bow-shaped cross-section appeared in FRSF-2. With the reeling speed further increasing, only bow-shaped transverse section can be found in FRSF-3 and FRSF-4. Furthermore, the mechanical properties including breaking elongation, stress, elastic modulus, and specific work of rupture of FRSFs are superior to that of CSFs. First of all, the stress of FRSFs increases with reeling speed increasing. More specifically, the stress of FRSF-1 is approximately 3.0 cN/dtex, which is similar to CSFs. In addition, the stress of the FRSF-2, FRSF-3, FRSF-4 increases to 3.20 cN/dtex, 3.34 cN/dtex, and 3.39 cN/dtex, respectively, with the increasing reeling speed. As for the elastic modulus, there is a clear phenomenon that the modulus of FRSFs is obviously larger than that of the CSFs, and increases with reeling speed. Accordingly, FRSF-1 has an elastic modulus of 93.10 cN/dtex, while the elastic modulus of CSFs is 76.90 cN/dtex. Besides, the specific work of rupture of FRSF-1, FRSF-2, FRSF-3, and FRSF-4 is as follows: 0.54 cN/dtex, 0.55 cN/dtex, 0.60 cN/dtex, and 0.62 cN/dtex. In contrast, the specific work of rupture of CFSs is 0.47 cN dtex. Moreover, the breaking elongation of FRSFs increases after force-reeling and shows a similar value with increasing reeling speed. As a result, the FRSF-4 exhibits integrated mechanical properties of superior breaking elongation (22.49%), excellent strength (3.39 cN/dtex), outstanding elastic modulus (95.76 cN/dtex), and supreme specific work of fracture (0.62 cN/dtex), which increases by 7%, 13.76%, 24.53%, and 31.91%, as compared to the average values of CSFs.

Conclusion In summary, the FRSFs with smooth morphology and improved mechanical properties was achieved by force-reeling. Notably, stress of the FRSFs increases as the reeling speed increases, and the elastic modulus of all these FRSFs is significantly higher than that of CSFs. This is due to the strong shear effect of the high-speed stretch that facilitates the orientation of the nanofibrils and improves the crystal structure of the nanocrystallites, thereby promoting the mechanical properties of FRSFs. The obtained FRSFs with satisfying elongation, high strength, superior modulus, and specific work of rupture broaden its application prospects in impact-resisting materials, biomedical materials, smart textiles and wearable electronics. Besides, the forced reeling processes and device need to be further modified to reduce the unstability of reeling tension and the fluctuation of fiber fineness.

Key words: Bombyx mori silkworm, silk, forced reeling, force-reeled silk, mechanical property