Spider silk is well-known for its outstanding mechanical properties. However, there is a significant variation of these properties in literature and studies analyzing large numbers of silk samples to explain these variations are still lacking. To fill this gap, the following work examines the mechanical properties of major ampullate silk based on a large ensemble of threads from Nephila clavipes and Nephila senegalensis. In addition, the effect of relative humidity (RH) on the mechanical properties was quantified. The large effect of RH on the mechanical properties makes it plausible that the variation in the literature values can to a large extent be attributed to changes in RH. Spider silk's most remarkable property-its high tenacity-remains unchanged. In addition, this work also includes hysteresis as well as relaxation measurements. It is found that the relaxation process is well described by a stretched exponential decay.

Spider dragline silk is a natural semicrystalline polymer that is well-known for its unusual combination of tensile strength and extensibility. However, the correlation between structure and mechanical properties is still largely unknown. We have investigated the changes of the fiber's structure as probed by single fiber X-ray diffraction using a microfocus synchrotron beam, under controlled simultaneous mechanical load and variation of the humidity. Crystallite orientation, size, and lattice constants have been monitored while pulling the fibers.

We have investigated the mechanical properties of spider dragline fibers of three Nephila species under varied relative humidity. Force maps have been collected by atomic force microscopy. The Young's modulus E was derived from the indentation curves of each pixel by the modified Hertz model. An average decrease in E by an order of magnitude was observed upon immersion of the fiber in water. Single fiber stretching experiments were carried out for comparison, and also showed a strong dependence on relative humidity. However, the absolute values of E are significantly higher than those obtained by indentation. The results of this work thus show that the elastic properties of spider silk are highly anisotropic, and that the silk softens significantly for both tensile and compressional strain (indentation) upon water uptake. In addition, the force maps indicate a surface structure on the sub-micron scale.

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Abstract.We analyze the wide-angle X-ray scattering from oriented spider silk fibers in terms of a quantitative scattering model, including both structural and statistical parameters of the β-sheet crystallites of spider silk in the amorphous matrix. The model is based on kinematic scattering theory and allows for rather general correlations of the positional and orientational degrees of freedom, including the crystallite's size, composition and dimension of the unit cell. The model is evaluated numerically and compared to experimental scattering intensities allowing us to extract the geometric and statistical parameters. We show explicitly that for the experimentally found mosaicity (width of the orientational distribution) intercrystallite effects are negligible and the data can be analyzed in terms of single-crystallite scattering, as is usually assumed in the literature.PACS. 81.07.Bc Nanocrystalline materials -87.15.-v Biomolecules: structure and physical properties -61.05.cc Theories of X-ray diffraction and scattering