Changes in microfibril angle in cyclically deformed dry coir fibers studied by in-situ synchrotron X-ray diffraction

Dry coir fibers are characterized by wide-angle X-ray scattering coupled with tensile tests. The fibers exhibit elastic and plastic behavior with the yield point at a strain of about 2%. In-situ experiments document that the cyclic loading and unloading beyond the yield point does not reduce the stiffness of the fibres, since they recover their initial stiffness by every increase of the strain. The diffraction data show that the microfibril angle (MFA) of cellulose fibrils in the coir fibre cells is inversely proportional to the magnitude of the applied strain. In average, the relatively high MFA of about 45° in the unstrained state decreases linearly upon straining until the fibers break at about 35% strain. When the strain is released during the tensile experiment the MFA tends to recover its original magnitude. No significant differences in the dependence of MFA on strain are detected in elastic and plastic regions, respectively. The results demonstrate that the tissue with helical architecture does not have to be saturated with water in order to exhibit the effect of the recovery of the mechanical function when cyclically loaded. This indicates differences in the architecture of the coir cell wall in comparison with that of compression wood with high MFA whereby similar phenomena were observed in the wet state.