Rafsanjani Ahmad, Stiefel Michael, Jefimovs Konstantins, Mokso Rajmund, Derome Dominique, Carmeliet Jan
Swiss Federal Laboratories for Materials Science and Technology, EMPA, , Duebendorf, Switzerland.
J R Soc Interface. 2014 Mar 26;11(95):20140126. doi: 10.1098/rsif.2014.0126. Print 2014 Jun 6.
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.
我们使用基于同步辐射的相衬X射线断层扫描纳米显微镜，记录了木材中央和最厚的次生细胞壁层（称为S2）在环境湿度变化时的吸湿膨胀和收缩情况。S2层是一种天然纤维增强的纳米复合聚合物，对水具有强烈的反应性。利用聚焦离子束，从挪威云杉软木晚材细胞壁的S2层制备出横截面为几微米的微柱。去除相邻的薄壁细胞壁层，以防止在膨胀或收缩过程中阻碍或限制水分引起的变形。所提出的实验旨在进一步深入了解木材各向异性吸湿膨胀的微观起源。研究发现，细胞壁横截面上的膨胀/收缩应变具有高度的各向异性，垂直方向的应变大于平行于细胞壁厚度方向的应变。这种超微结构的各向异性可能是由于纤维素微纤丝的同心层状排列，因为纤维素微纤丝角度在横向膨胀各向异性中的作用可以忽略不计。研究发现，细胞壁材料的体积膨胀远大于生长轮内木材组织以及由多个生长轮制成的木材样品的体积膨胀。木材中的分层结构以更高的复杂程度，最佳地提高了其在潮湿环境中的尺寸稳定性。
