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 orien tation distribution? the crystalline fraction, and an estimate of the crystal size. It is found that the crystals are very small and well or iented. The mean (minimum) crystal dimensions are 2 x 5 x 7 nm, and th e angle, phi, between the molecular chains in the crystals and the fib er 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 iso tropic 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 fo r affine deformation at constant volume. There is no observable change in crystallinity and apparently a small reduction in the lateral crys tal size on deformation.