A study of ultrastructure of wood and pulped fibers by small angle x-ray scattering

This research examines the ultrastructure of wood and pulped fibers of loblolly pine (Pinus taeda L.) by the technique of small angle x-ray scattering (SAXS). Pulped samples were prepared by kraft cooks. The experimental setup contains a pin-hole collimation system. The scattered intensity was recorded on x-ray film which was then scanned with a microdensitometer.; SAXS is shown to be a quick and accurate method to measure both the mean and the variation of the microfibrillar winding angle in the S2 layer in the fiber wall. Because a large number of microfibrils can be evaluated at one time, the result is reliable. The cross-sectional dimensions of cellulose microfibrils were estimated using three different methods. The average width of microfibrils decreases from about 53A in ring 3 to about 28A in ring 36. The tangential wall and the radial wall in a fiber were found to scatter x-rays differently. A lamellar arrangement of microfibrils in the tangential wall is proposed to explain the strong scattering streak. However, the diffuse intensity along the streak observed implies that long range order does not exist in the cell wall. Pores in both wood and pulped samples were studied by SAXS. In wood fibers, some slit-shaped pores exist and are oriented approximately parallel to the microfibrils. However, most pores are large and symmetric about the fiber axis. In pulped samples, most pores are slit-shaped and parallel to the microfibrils. Both wood and pulped fibers dried from water were studied by SAXS. The strong scattering observed at very small angles indicates that dried fiber wall contains a substantial amount of voids. These voids are sealed completely in the cell wall and not accessible from the outside. The meridional streaks, not reported in the literature, were observed for both wood and pulped samples, indicating that the meridional scattering is caused by cellulose in the cell wall. A model is proposed assuming that the microfibril consists of alternating crystalline and amorphous regions along the microfibril. The amorphous regions of the microfibrils are arranged in horizontal bands, which produce the necessary variation of electron density along the fiber axis.