The remarkable mechanical properties of biological materials reside in their complex hierarchical architecture and in specific molecular mechanistic phenomena. The fundamental importance of molecular interactions and bond recovery has been suggested by studies on deformation and fracture of bone and nacre. Like these mineral-based materials, wood also represents a complex nanocomposite with excellent mechanical performance, despite the fact that it is mainly based on polymers. In wood, however, the mechanistic contribution of processes in the cell wall is not fully understood. Here we have combined tensile tests on individual wood cells and on wood foils with simultaneous synchrotron X-ray diffraction analysis in order to separate deformation mechanisms inside the cell wall from those mediated by cell-cell interactions. We show that tensile deformation beyond the yield point does not deteriorate the stiffness of either individual cells or foils. This indicates that there is a dominant recovery mechanism that re-forms the amorphous matrix between the cellulose microfibrils within the cell wall, maintaining its mechanical properties. This stick-slip mechanism, rather like Velcro operating at the nanometre level, provides a 'plastic response' similar to that effected by moving dislocations in metals. We suggest that the molecular recovery mechanism in the cell matrix is a universal phenomenon dominating the tensile deformation of different wood tissue types.

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.

Highly oriented native cellulose fibres (flax) and softwood (pine) have been investigated by means of X-ray diffraction. Local structural information was obtained by using X-ray microbeams. Tensile tests were performed in situ, revealing a change of orientation of cellulose microfibrils in materials with tensile strain. In flax fibres, the microfibrils rotate during the first percent of stretching, into a more parallel orientation with respect to the fibre axis. For wood, a decrease of orientation with the onset of strain hardening is found for the first time.

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