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.

中文翻译：

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受微纤丝角调节的木细胞壁的湿机械变形


木材是一种无处不在的生物材料，由于其分层微观结构而表现出卓越的机械和功能特性。了解木材吸湿机制对于推进木质材料的应用至关重要，但微观结构、水分动力学和变形之间错综复杂的相互作用阻碍了这一过程。在这里，我们采用大规模分子动力学模拟全面研究了木细胞壁的湿机械变形，解释了微纤丝角 （MFA） 和主要微观结构成分：半纤维素和纤维素纳米纤丝的吸湿性差异的耦合效应。我们的分子模型结合了两个主要成分的实际微观结构和质量分数，提供了以前未解决的定量见解。我们确定了 MFA 影响木细胞壁变形的三个关键机制。这种影响是通过调节水分子扩散而发生的，水分子扩散源于纤维素纳米原纤维、半纤维素及其界面之间的氢键竞争。在低水分含量下，空隙填充促进分子间交联，导致轻微的结构收缩。在高水分含量下，MFA 的影响变得明显，影响吸水行为、饱和水分含量和变形程度。此外，通过提高半纤维素含量和优化 MFA 至 23° 以下，可以进一步增强木细胞壁的溶胀变形。我们的研究强调了 MFA 在木材吸湿中的关键作用，并为仿生材料和湿度控制机器人的设计和优化奠定了理论基础。