Hygroscopic swelling and shrinkage of latewood cell wall micropillars reveal ultrastructural anisotropy
Hygroscopic swelling and shrinkage of latewood cell wall micropillars reveal ultrastructural anisotropy
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晚材细胞壁微柱的吸湿膨胀和收缩揭示超微结构的各向异性
DOI：
10.1098/rsif.2014.0126
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发表时间：
2014
影响因子：
3.9
通讯作者：
J. Carmeliet
中科院分区：
文献类型：
--
作者：
A. Rafsanjani;M. Stiefel;K. Jefimovs;R. Mokso;D. Derome;J. Carmeliet
我们记录的吸湿膨胀和收缩的中央和最厚的次生细胞壁层的木材（命名为S2）在响应环境湿度的变化，使用同步辐射相衬X射线断层扫描纳米显微镜。S2层是一种天然纤维增强的纳米复合聚合物，对水有很强的反应性。利用聚焦离子束，从挪威云杉软木晚材细胞壁的S2层中制备出截面为几微米的微柱。去除薄的相邻单元壁层以防止在溶胀或收缩期间阻碍或抑制水分引起的变形。该实验旨在进一步了解木材各向异性湿膨胀的微观起源。结果表明，膨胀/收缩应变在细胞壁的横截面上具有高度的各向异性，在垂直方向上大于在平行于细胞壁厚度方向上。这种超微结构各向异性可能是由于纤维素微纤丝的同心分层，因为纤维素微纤丝角在横向溶胀各向异性中的作用是可以忽略的。细胞壁材料的体积膨胀被发现是基本上大于一个生长轮内的木材组织和木材样品制成的几个生长轮。木材的层次结构最佳地增加了其尺寸稳定性，以应对具有更高复杂性的潮湿环境。
We document the hygroscopic swelling and shrinkage of the central and the thickest secondary cell wall layer of wood (named S2) in response to changes in environmental humidity using synchrotron radiation-based phase contrast X-ray tomographic nanoscopy. The S2 layer is a natural fibre-reinforced nano-composite polymer and is strongly reactive to water. Using focused ion beam, micropillars with a cross section of few micrometres are fabricated from the S2 layer of the latewood cell walls of Norway spruce softwood. The thin neighbouring cell wall layers are removed to prevent hindering or restraining of moisture-induced deformation during swelling or shrinkage. The proposed experiment intended to get further insights into the microscopic origin of the anisotropic hygro-expansion of wood. It is found that the swelling/shrinkage strains are highly anisotropic in the transverse plane of the cell wall, larger in the normal than in the direction parallel to the cell wall's thickness. This ultrastructural anisotropy may be due to the concentric lamellation of the cellulose microfibrils as the role of the cellulose microfibril angle in the transverse swelling anisotropy is negligible. The volumetric swelling of the cell wall material is found to be substantially larger than the one of wood tissues within the growth ring and wood samples made of several growth rings. The hierarchical configuration in wood optimally increases its dimensional stability in response to a humid environment with higher scales of complexity.