Using an in situ combination of tensile tests and x-ray diffraction, we have determined the mechanical properties of both the crystalline and the disordered phase of the biological nanocomposite silk by adapting a model from linear viscoelastic theory to the semicrystalline morphology of silk. We observe a strong interplay between morphology and mechanical properties. Silk's high extensibility results principally from the disordered phase; however, the crystals are also elastically deformed. DOI: 10.1103/PhysRevLett.100.048104 PACS numbers: 87.85.Jÿ, 61.05.cp, 81.70.Bt, 87.15.La Natural silks exhibit extraordinary mechanical properties, combining high tensile strength with a high elongation at failure. Producing man-made fibers with such properties requires a considerable input of energy compared to natural silk fibers spun from aqueous protein solution at ambient temperature. The toughest silk known is spun by spiders. However, it has recently been shown that, chemically and rheologically closely related, silkworm silk [1] could reach comparable parameters in an optimized spinning process [2].The similarity between spider and silkworm silk fibers extends to their morphology. Both are semicrystalline nanocomposites, with ordered regions (-sheet protein nanocrystals) embedded in a softer matrix of disordered material (spider silk, [3]; silkworm fibroin, [4]). Tensile deformation of silk fibers involves a strong contribution of viscoelastic material, presumably in the disordered regions [5]. However, detailed information on the relaxation mechanism is still missing.The mesoscopic structure of polymer nanocomposites is the key to their mechanical properties [6]. Mimicking nature's spinning process to produce artificial fibers with optimized morphology and, thus, optimal mechanical performance either from silkworm or recombinant spider silk spinning dope [7] would be highly desirable. The models of silk available today do not directly connect structure and macroscopic properties. Termonia's model [8] describes silk as a hydrogen-bonded rubberlike matrix with embedded stiff crystals serving as cross-link sites. At their surface, another ordered phase of protein molecules is required in order to explain the static mechanical properties. The assumed high rigidity of the -sheet crystals has been questioned by x-ray diffraction results [9]. While the recent model by Porter et al. [10] and Vollrath and Porter [11] includes morphological parameters such as ordered/ disordered fractions, which are very difficult to obtain experimentally, deformation processes in crystals and matrix are neglected. A molecular model for spider capture silk is available but restricted to this very specialized silk type [12].The aim of this Letter is to establish a model for silk incorporating the macroscopic mechanical behavior, in particular, viscoelasticity, on the basis of its semicrystalline morphology. To this end, we combined high-resolution cyclic stress-strain measurements of single silkworm silk fibers (such as reported in [13][14...

The molecular structure of Bombyx mori silkworm silk fibers is investigated in situ upon externally applied tensile stress using synchrotron X-ray diffraction, while the molecular vibrational response is investigated using cold neutron time-of-flight spectroscopy. The aligned silk fibers are therefore exposed to a tensile force along the fiber axis generated by stretching machines adapted to X-ray and neutron scattering, respectively, and the stress-strain curves are measured in situ. The applied force in both cases is sufficient to reach the yield point of plastic deformation. In the case of neutron spectroscopy, different regions within the hierarchical silk structure are masked by selective deuteration. The X-ray studies confirm the assumption that most of the deformation upon extension of the fibers is due to the amorphous regions of the silk. The neutron results indicate that the externally applied force is not reflected by any noticeable effect on the molecular vibrational or diffusional/ reorientational level in the range accessible to neutron time-of-flight spectroscopy. This observation of unaffected molecular dynamics is in agreement with a model of entropy elasticity.

Silk fibers are semi-crystalline nanocomposite protein fibers with an extraordinary mechanical toughness which changes with humidity. Diffusive or overdamped motion on a molecular level is absent in dry silkworm silk, but present in humid silk at ambient temperature. This microscopic diffusion distinctly depends on the externally applied macroscopic tensile force. Quasi-elastic and inelastic neutron scattering data as a function of humidity and of tensile strain on humid silk fibers support the model that both the adsorbed water and parts of the amorphous polymers participate in diffusive motion and are affected by the tensile force. Notably, the quasi-elastic linewidth of humid silk at 100% relative humidity increases significantly with the applied force. The effect of the tensile force is discussed in terms of an increasing alignment of the polymer chains in the amorphous fraction with increasing tensile stress which changes the geometrical restrictions of the diffusive motions.

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Wandering spiders climb vertically and walk upside‐down on rough and smooth surfaces using a nanostructured attachment system on their feet. The spiders are assumed to adhere by intermolecular van der Waals forces between the adhesive structures and the substrate. The adhesive elements are arranged highly ordered on the hierarchically structured attachment hair (setae). While walking, it has been suggested that the spiders apply a shear force on their legs to increase friction. However, the detailed mechanical behavior of the hair's structures during attachment and detachment remains unknown. Here, gradients of the mechanical properties of the attachment hair on different length scales that have evolved to support attachment, stabilize adhesion in contact, and withstand high stress at detachment, examined by in situ experiments, are shown. Shearing helps to self‐align the adhesive elements with the substrate. The study is anticipated to contribute to the development of optimized artificial dry adhesives.