常态下出土木材干缩湿胀经时演变及其影响机制

The mechanism in the hygroexpansion of dehydrated archaeological wood could provide the scientific basis for the protection and preservation of archaeological wood. While, the present wood hygroexpansion models are not suitable for the archaeological wood. In this project, waterlogged archaeological wood treated by dehydration treatment though supercritical CO2 drying technique will be used to investigate the hygroexpansion exposed to sinusoidally varying humidity. Confocal Raman Imaging technique and Attenuated Total Refraction- Fourier Transform Infrared Spectroscopy Imaging technique will be used to investigate the variations of the content, distribution and chemical structure of lignin, hemicellulose, cellulose and deposition in wood cell wall of the dehydrated archaeological wood. Small-Angle X-ray Scattering/ Wide-Angle X-ray Scattering will be utilized to study the cellulose crystallites structure including crystallinity, crystallites dimension and the content of surface chains in cellulose crystallites and porous structure contained in wood. Besides, the interaction between water and wood during adsorption and desorption processes will be studied though Dynamic Vapour Sorption. Therefore, the correlation between wood cell wall structure of dehydrated archaeological wood and dehydrated archaeological wood-water interaction will be built. Then, the moisture absorption of dehydrated archaeological wood will be ascertained. The mechanism of hygroexpansion of dehydrated archaeological wood exposed to sinusoidally varying humidity will be clarified. Results can provide the scientific and theoretical knowledge for the protection and preservation of waterlogged archaeological wood.

揭示常态下出土木材干缩湿胀经时演变规律是保证出土木质文物长期妥善保存的科学基础，而现有木材干缩湿胀理论无法适用于出土木材。本项目拟针对经脱水处理后处于常态下的出土木材，研究湿度变化环境中木材干缩湿胀性能随时间的变化规律。通过采用共聚焦拉曼成像（Raman Imaging）、傅里叶变换衰减全反射红外成像（ATR FTIR Imaging）和小角X射线散射（SAXS）/广角X射线散射（WAXS）等细胞壁微区结构与化学检测技术，分析出土木材细胞壁纤维素（结晶区和无定形区）、基质成分（半纤维素和木质素）和沉积物的含量、微区分布、分子结构及分子间键接形式，并结合动态蒸汽吸附手段确定细胞壁中水分吸着和解吸变化方式，建立细胞壁结构与细胞壁中水分吸着和解吸变化间相互关系，阐明常态下出土木材干缩湿胀经时演变规律及其影响机制，为保护保存我国出土木质文物提供理论基础和科学依据。

饱水木质文物是我国文物资源的重要组成部分，是传承中华文化的重要载体。出土木材干缩湿胀经时演变规律是保证出土木质文物长期妥善保存的科学基础。项目以不同腐朽程度出土木材为研究对象，提出了出土木材细胞壁结构与干缩湿胀性能的研究方法，探明了腐朽程度对出土木材细胞壁结构和细胞壁中水分吸着和解吸变化的影响规律，建立了细胞壁结构与细胞壁中水分吸着和解吸变化间相互关系，阐明了常态下出土木材干缩湿胀经时演变规律及其影响机制。具体地，腐朽程度决定了出土木材细胞壁结构与细胞壁中水分吸着和解吸行为的变化，严重腐朽出土木材细胞壁呈现半纤维素木聚糖甲基葡萄糖醛酸侧链几乎完全水解，木聚糖骨架降解严重；纤维素无定形区与结晶区均发生降解，纤维素链间氢键网络部分断键；部分木质素发生氧化、脱甲基和脱甲氧基反应，紫丁香基较愈创木基优先降解；纤维素分子排列无序化、结晶区纤维素层间距增大、晶体尺寸减小以及结晶度降低；孔容显著升高的结构特征。因而出土木材细胞壁水分吸着位点数量显著升高，造成了出土木材具备更高的吸湿性，较快的吸湿动力学，以及较强的捕获及保持水分子能力。在湿度变化环境中出土木材干缩湿胀行为经时演变规律显示出土木材的含水率滞后性低于正常木材，对水分变化的响应更快。但出土木材细胞壁多糖含量的降低，微纤丝排列的无序化，以及木质素交联结构的改变造成了出土木材具有大的尺寸变化率，且尺寸变化率滞后于相对湿度变化，径向尺寸响应快于弦向尺寸变化，并且随时湿度循环次数增加，径向尺寸出现了漂移。项目揭示了出土木材细胞壁结构的显著改变是木质文物呈现高饱水特征的根本原因，提出了调控出土木材细胞壁介孔内自由水移动是科学控制饱水木质文物变形、开发脱水保护技术的关键；展望了出土木材细胞壁精细结构的立体化探索将成为突破饱水木质文物精准保护瓶颈的科学基础。本项目研究成果将为保护保存我国出土木质文物提供理论基础和科学依据。