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.

Thermal softening properties of various wood species within an annual ring

“…For softwoods, the peak temperature of tanδ of EW at 0.5 Hz appeared at higher temperatures than those of LW as in the previous report [8]. In addition, the peak temperature of tanδ of R specimens was similar to those of EW except for Pseudotsuga menziesii and Abies sachalinensis, for which the peak tanδ of R specimens was not observed.…”

“…However, the relationship between lignin structure and thermal softening properties within an annual ring has not been investigated. Our previous studies have shown that EW and LW of Douglas fir (Pseudotsuga menziesii) have different thermal softening properties [8]. The peak of tanδ was found at around 95 °C for EW and around 90 °C for LW [8].…”

“…The peak of tanδ was found at around 95 °C for EW and around 90 °C for LW [8]. It was found that the difference in thermal softening properties between EW and LW could not be explained only by the difference in specific density [8]. In other words, the microstructure may be different within an annual ring.…”

“…Our previous studies have shown that EW and LW of Douglas fir (Pseudotsuga menziesii) have different thermal softening properties [8]. The peak of tanδ was found at around 95 °C for EW and around 90 °C for LW [8]. It was found that the difference in thermal softening properties between EW and LW could not be explained only by the difference in specific density [8].…”

“…Understanding the thermal softening properties of wood is considered important for understanding the molecular state of wood. Dynamic mechanical analysis (DMA) of water-saturated wood in the temperature range from 0 to 100 °C are often used as a method to determine the thermal softening properties of wood and bamboo [3][4][5][6][7][8]. Furuta et al reported that the peak of tanδ obtained by dynamic viscoelasticity measurements from 0 to 100 °C is due to micro-Brownian motion of lignin [3].…”