Structural changes during tensile testing of an all-cellulose composite by in situ synchrotron X-ray diffraction

Tensile experiments with a regenerated cellulose film reinforced by microcrystalline cellulose, combined with simultaneous synchrotron wide angle X-ray scattering, show that, when mechanically loaded at constant strain rates, the cellulose crystallites tend to orient with their crystallographic c-axis parallel to the force direction and an irreversible preferred orientation is induced. The magnitude of the orientation factor calculated numerically from the experimental data depends linearly on the applied strain and is independent of the actual stress, also in the case of cyclic loading–unloading experiments. Upon cyclic unloading and loading in the plastic region, the stiffness of the all-cellulose composite equals approximately the original one or, in the case of the initial unloading stiffness, even shows a significant increase. During tensile straining, the observed degree of preferred orientation was significantly higher for the cellulose I part of the composite compared to the cellulose II part, respectively.