Fiber morphology of spider silk: The effects of tensile deformation

The fiber morphology of the dragline silk of Nephila clavipes has been investigated by the detailed analysis of wide-angle X-ray diffraction (WAXD) patterns. WAXD gives the crystal lattice dimensions, the orientation distribution? the crystalline fraction, and an estimate of the crystal size. It is found that the crystals are very small and well oriented. The mean (minimum) crystal dimensions are 2 x 5 x 7 nm, and the angle, phi, between the molecular chains in the crystals and the fiber axis has a full width at half-maximum (fwhm) of 15.7 degrees and an orientation function f=0.981. The X-ray crystallinity is in the range 10-15%, and the amorphous diffraction is divided 60:40 between an isotropic ring and an oriented halo with fwhm 30 degrees. This means one-third of the material is in the oriented amorphous state, with a chain orientation of fwhm 43 degrees and f=0.87. When the fiber is extended up to 10%,, the orientation of the crystals increases as predicted for affine deformation at constant volume. There is no observable change in crystallinity and apparently a small reduction in the lateral crystal size on deformation.