Hygromechanical deformation of wood cell walls regulated by the microfibril angle

Abstract

Wood, a ubiquitous biomaterial, exhibits exceptional mechanical and functional properties owing to its hierarchical microstructure. Understanding the mechanisms underlying wood moisture absorption is crucial for advancing the application of wood-based materials, yet the process has been hindered by the intricate interplay between microstructure, moisture dynamics, and deformation. Here, we employed large-scale molecular dynamics simulations to comprehensively investigate the hygromechanical deformation of wood cell walls, accounting for the coupled effects of microfibril angle (MFA) and hygroscopicity difference in the main microstructural components: hemicellulose and cellulose nanofibril. Our molecular model, incorporating the practical microstructure and mass fractions of the two main components, provides quantitative insights previously unaddressed. We identified three key mechanisms by which MFA influences wood cell wall deformation. This influence occurs through the regulation of water molecule diffusion stemming from the competition of hydrogen bonds among cellulose nanofibrils, hemicellulose, and their interfaces. At low moisture levels, void filling promotes intermolecular cross-linking, leading to slight structural contraction. At high moisture levels, the impact of MFA becomes pronounced, affecting water absorption behavior, saturation moisture content, and the degree of deformation. Furthermore, the swelling deformation of wood cell walls can be further enhanced by improving hemicellulose content and optimizing MFA to below 23°. Our study underscores the crucial role of MFA in the moisture absorption of wood and establishes a theoretical foundation for the design and optimization of biomimetic materials and humidity-controlled robots.