The Fractal Ultrastructure Of Nano-fibrils In Wood Cell Wall And Its Influences On Wood Physical Properties

In order to quantitatively analyze complex ultrastructure in wood cell wall and further investigate the relationship between the ultrastructure and macroscopic properties of wood, fractal theory was first introduced into the description for ultrastructure of wood cell wall in this study. The cellulose chain clusters around nano-scale were viewed as a fractal object that consists of many fibrillar structural units with different scales including microfibrils. On the basis of the morphological data of wood cell wall, fractal dimensions (D) of multi-level fibrillar structural units were calculated by fractal-geometry approach, and then the morphological and structural characteristics of fibers as well as the fractal self-nesting character of the ultrastruture in wood cell wall were investigated according to D. For the experiment of variation of fractal fibers in wood cell wall, D in radial or tangential direction was obtained by the mean microfibril angles in S₂ layer determined using the technique of polarized light microscopy. Besides, the influences of fractal ultrastructure in wood cell wall on macroscopic wood properties were investigated according to the relationship between the D variation of fractal fibers and hygroscopicity, crystallinity of wood.The main results from this research were summarized as follows:1. The D of microfibrils is slightly larger than elementary fibrils and S layers of cell wall. It indicates that microfibrils twist more abruptly and their rigidity coefficient is higher. Elementary fibrils, with D less than microfirils, may endow the fiber with some characteristics more closely relating to flexibility or tenacity. Compared to the natural fiber, fractal self-nesting characters of wood cell wall are more typical. The value of D of twist fibrils changes just in one magnitude.2. In radial direction, D of the heartwood close to the pith is maximuim. It can be divided two stages during D-decreasing course to the sapwood outsides: the first sharp-moving descent and the second slow-moving descent. The minimum of D appears in the sapwood close to the bark, thus D at the position most close to the bark will have a rebound.Fibrillar units in heartwood have a higher fractal degree that shows fractal fibrils also change immediately along with wood turning sapwood into heartwood. Some changes in nature during that process can be explained by fractal theory.3. In tangential direction, the movement of D_max is not obvious. The unstable descents of D are more mild without distinct stage characteristic; At the part close to the bark, D have no obvious rebound.4. D is not related to the relative hygroscopicity necessarily in both radial and tangential directions.