Changes in the Higher-Order Structure of Silk Fibers Obtained by Forcibly Spinning as Compared to Those Obtained by the Naturally Spinning System of the Bombyx mori Silkworm

Forcibly spinning was applied to fully grown Bombyx mori silkworm larvae, and the higher-order structure of the as-spun filaments was analyzed by Fourier-transform infrared spectroscopy (FT-IR) and wideangle X-ray diffraction (WAXD). The FT-IR spectra of the as-spun filaments were similar to those of naturally spun cocoon filaments. This indicated that the forcibly spun filaments were comprised of β-sheets. However, FT-IR spectra were obtained under ATR mode. Thus, the spectra basically reflected the structure of sericin layer as well as inside fibroin. In the WAXD diagram of forcibly spun filaments, 002 and 101 reflections of the silk-1 crystal were overlapped on the silk-2 crystal reflections, while only silk-2 crystal modification occurred in naturally spun ones. The preferential silk-2 crystal modification was disordered although the shear force loaded by the forcibly spinning on the liquid silk in the duct of a silkworm was thought to be high enough to allow modification for the preferred silk-2 crystal. In addition, the sericin layer of the forcibly spun filaments was so brittle that numerous cracks were generated perpendicular to the fiber axis when the filaments were slightly stretched, suggesting that molecular orientation in the sericin layer of the forcibly spun filaments was extremely low. These structural changes observed in the forcibly spun filaments are due to the interference of forcibly spinning with the naturally spinning system of a Bombyx mori silkworm. For further understanding the inherent spinning system of a Bombyx mori silkworm, the schematic representation was proposed based on the previous studies, and the structural changes observed in the forcibly spun filaments were discussed in relation to the fiber formation process.