Combined analysis of microstructures within an annual ring of Douglas fir (Pseudotsuga menziesii) by dynamic mechanical analysis and small angle X-ray scattering

Abstract

Dynamic mechanical analysis (DMA) and small angle X-ray scattering (SAXS) measurements of water-saturated wood of Douglas fir (Pseudotsuga menziesii) in the temperature range of 0 ℃ to 100 ℃ were focused to clarify microstructural changes within an annual ring. The following results were obtained. Thermal softening behavior caused by micro-Brownian motion of lignin was observed in both earlywood and latewood. The peaks of tanδ were found at around 95 ℃ for earlywood and at around 90 ℃ for latewood. These results suggested that the structures of lignin in the cell wall were different between earlywood and latewood. SAXS measurements of water-saturated earlywood and latewood in water were performed with precise temperature control. The scattering intensity increased with increasing temperature, indicating that the density of the matrix was reduced at higher temperature. One-dimensional SAXS intensity at the equator, which approximately represents cellulose microfibrils arrangement in the matrix, was intensively analyzed using the WoodSAS model. The result of this model fitting showed that the cellulose microfibril diameter of latewood was higher than that of earlywood. In addition, the value of interfibrillar distance decreased monotonically in the earlywood, while it decreased rapidly in the latewood from 60 ℃ to 90 ℃. The changes in the cellulose microfibril (CMF) diameter and the interfibrillar distance with increasing temperature between earlywood and latewood by SAXS measurement were different. The differences in CMF diameter and inter-fibril distance between earlywood and latewood measured by SAXS also support the hypothesis that lignin structure differs between earlywood and latewood based on the results of DMA measurements.