Microstructural characterization of Bombyx mori silk fibers

The microstructure of Bombyx mori silk fibers before and after degumming was examined by transmission electron microscopy (TEM), selected area electron diffraction (SAED), wide-angle X-ray scattering (WAXS), and low-voltage high-resolution scanning electron microscopy (LVHRSEM). LVHRSEM micrographs of the neat cocoon revealed a network of pairs of twisting filaments. After degumming, there were only individual filaments showing a surface texture consistent with an oriented fibrillar structure in the fiber interior. WAXS patterns confirmed the oriented beta-sheet crystal structure common to silkworm and spider (dragline) silks. Low-dose SAED results were consistent with the WAXS data and revealed that the crystallographic texture did not vary significantly across the fiber diameter. TEM observations of individual microtomed fibers indicated a nominally triangular cross section and a 0.5-2 pm sericin coating. After degumming to remove the sericin, a banded feature was revealed with a characteristic spacing of nominally 600 nm along the fiber axis. These bands were oriented in a roughly parabolic or V-shape pointing along one axis within a given fiber. We hypothesize that this orientation was induced by the extrusion and drawing during the spinning process. Equatorial dark field (DF) images revealed that axial and lateral sizes of the beta-sheet crystallites in silk fibroin ranged from 20 to 170 nm and from 1 to 24 nm, respectively. Crazes developed in the degummed silk fiber parallel to the fiber direction. The formation of these crazes suggests that there are significant lateral interactions between microfibrils in silk fibers.