Hygroscopic swelling and shrinkage of latewood cell wall micropillars reveal ultrastructural anisotropy

Abstract

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 nanocomposite 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.

Wood Moisture-Induced Swelling at the Cellular Scale—Ab Intra

“…The third micropillar in [88], which contained some portion of the S 1 layer, revealed smaller strains in the direction parallel to the cell wall thickness. The obtained values for the third micropillar were 0.6% and 0.8% in the perpendicular and parallel directions, respectively [88]. The behavior of this third micropillar reveals some understanding of the reduction in the moisture-induced swelling at larger length scales of wood.…”

“…Similar swelling strains were measured in each of the primary planes, indicating a transverse isotropic swelling behavior in the microfibrils. Taking this into account and assuming microfibril rectangular cross-section, the swelling at the elementary fibril level was compared with the transverse swelling of the S 2 [88]. Based on this comparison, it was found that the increase in the microfibril cross-sectional area accounts for 55% of the increase in the S 2 transverse area due to moisture-uptake between 25% and 85% RH.…”

“…This is because of the nature of the cell wall structure, where the deformation is restrained by the microfibrils along its longitudinal direction. Furthermore, in [88], three micropillars of the S 2 cell wall layer of Norway spruce latewood were created using a focused ion beam, and the three-dimensional moisture-induced swelling was quantified using high-resolution X-ray nanotomography. They found that the volumetric strains at the cell wall level are significantly larger than those observed at the cellular level.…”

Anisotropy of transversal swelling of Serbian spruce wood (Picea omorika (Pancic) Purkyně)

“…Неки аутори наводе да је варијација ћелијске структуре у самом прстену прираста, где се смењују рано и касно дрво, одговорна за анизотропију бубрења (B o u te l j e , 1962; Watanabe et al, 2000). Према другима је то угао и структура микрофибрила (F r e y -- Wyssling , 1940;Skaar, 1988;Rafsanjani et al, 2014), док неки аутори наводе да велики утицај на анизотропију бубрења имају траке дрвета односно радијални паренхим (M cInto s h , 1954;Usta i G u ray, 2000).…”

“…Some authors state that the variation of the cellular structure in the growth ring with early and late wood is responsible for the anisotropy of swelling (B o u t e l j e , 1962; Watanabe et al, 2000). According to others, it is also the angle of the microfibril structure (F rey-Wyssling , 1940;Skaar, 1988;Rafsanjani et al, 2014), while some authors state that wood rays have a major impact on swelling anisotropy (M cInto s h , 1954; Usta and G u ray, 2000).…”