How to regulate moisture-induced stresses in composites: The answer from nanostructure of S2 layer in Wood cell wall

Abstract

S2 layer of the Wood cell wall is fundamentally a biocomposite with strong hygroscopicity and undergoes significant changes in mechanical properties upon water adsorption. How the internal stress develops during the adsorption process is still unclear. By employing a multi-scale modeling scheme, we successfully resolve the stress distributions within the S2 layer at different moisture levels. We find that the shear stress developed on the fibril-matrix interface is much smaller than that of ordinary fiber-reinforced composites with the same geometrical dimensions due to the heterogeneous nature of the microfibrils. Besides, we demonstrate that the stress developed during dehydration is larger than that during hydration, which is attributed to the fact that the amorphous polymers comprising the matrix suffer from softening upon water adsorption. These unraveled mechanisms can deepen people’s understanding on the origin of the moisture-induced stress in the S2 layer.