In softwood, the peaks of tanδ are found 80 ℃ to 90 ℃, whereas in hardwood, that is found 60 ℃ to 70 ℃ at 0.05 Hz [15]. Softwood lignin has guaiacyl-propane units, while hardwood lignin has syringyl-propane units in addition to it, and it is generally believed that the degree of condensation of softwood lignin is higher than those of hardwood lignin [23]. These results suggested those the difference in the peaks temperature of tanδ between hardwood and softwood is due to the difference in the cross-linking density derived from the structure of lignin [15]. Since softwood was used in this experiment, the lignin of specimens is thought to have mainly guaiacyl-propane units for both earlywood and latewood. However, it is suggested that the lignin structure, such as cross-linking density, may be different between earlywood and latewood even if the same guaiacyl-propane units are used.

Saka and Thomas reported the distribution of lignin in the cell walls of earlywood and latewood in loblolly pine by bromination technique with the SEM–EDXA system [24]. The lignin concentration of earlywood was 0.20 g/g in the S2 layer and 0.49 g/g in the compound middle lamella (CML), whereas of latewood, those were 0.18 g/g in the S2 layer and 0.51 g/g in the CML [24]. It is generally known that the thickness of the S2 layer is different between earlywood and latewood. Moreover, the fractional volume of the S2 layer for earlywood and latewood in loblolly pine was 60% and 80%, respectively [24]. In the DMA in the tangential direction, the microfibrils and the matrix of the S2 layer are arranged in series concerning the tensile direction, so the deformation is considered to be less constrained by the microfibrils [25]. As a result, it is more affected by the quantitatively larger S2 layer than the quantitatively smaller CML [25]. Therefore, the results of the DMA in the tangential direction strongly reflect the influence of the S2 layer. As shown in Fig. 5, the density of earlywood and latewood differed greatly, which also suggested those the absolute amount of lignin was different. However, the behavior of E′/ρ and E″/ρ did not match between earlywood and latewood, indicating that the density and the amount of lignin alone cannot explain the difference in thermal softening behavior as shown in Fig. 4. These results suggested that the structure of lignin in the cell wall is different between earlywood and latewood, because the thermal softening behavior, especially tanδ, is different between earlywood and latewood. Since the lignin in both earlywood and latewood is mainly composed of the guaiacyl-propane units, a possible difference in the structure of lignin could be the different number of βO-4 linkages.

Small angle scattering is one of the structural analyses methods gaining popularity in the field of wood science. For example, Penttilä et al. have studied the relationship between moisture content changes and microstructures changes in wood using X-ray and neutron scattering [13, 14, 26]. In these experiments, in situ measurement plays an important role, which is one of the advantages of small-angle scattering. In this study also, we tracked the structural change of the wood cell wall in the water-saturated state that is occurring in increasing temperature by in situ SAXS measurement. Furthermore, the SAXS data in this study, which were obtained by in situ measurements, are comparable with DMA as shown in Fig. 3. This means that the insightful interpretation of DMA about the structural change with increasing temperature [15] is now possible to test by the native structural data of SAXS.

The increase in SAXS intensity with increasing temperature as shown in Fig. 6 is considered to be due to the increases in the density difference between CMFs and matrix components. Simply there are two possibilities for this observation: CMF becomes denser, or matrix part becomes sparser with increasing temperature. Given that the thermal expansion of cellulose crystals in wood is very little in the range of 0 °C to 100 °C [27], the latter possibility is a favorable interpretation. The sparser matrix at a high temperature fits the thermal softening of lignin, which has been a well-accepted model [15].

We further tried quantitative interpretation of SAXS data with the WoodSAS model [13], which describes the lateral CMF arrangement in the matrix. Two structural parameters obtained by WoodSAS model seemed to vary in correlation to the DMA result: the increase in 2(overline{R }) (the diameter of CMFs) and the decrease in a (interfibrillar distance) were observed for the increased temperature or the progress of thermal softening. We believe that these parameter changes are owing to the structural change of the matrix as indicated by DMA and important information to know the change of the matrix in wood cell wall during the thermal softening. However, we in this study do not directly discuss the change of the matrix, as the CMF in the matrix is the direct target of the analysis of the SAXS data in this study.

A simple interpretation for an increase in the CMF diameter (2(overline{R })) at higher temperatures is the thermal expansion of cellulose microfibril. However, this is clearly ruled out given the previous report showing that the cellulose crystal in wood hardly expands in the range of 0 °C to 100 °C [27]. We then made an interpretation that this is due to the change of the matrix in the thermal softening, as in the results of the DMA measurements, which then result in an apparent change of CMFs. As discussed above, the matrix of water-saturated wood will become sparser at higher temperatures, which will reduce the electron density of the background or the matrix part. Assuming that the electron density reduction of the matrix is striking only apart from the surface of CMFs but not close to the CMFs, at the temperatures where the thermal softening occurs, the matrix in contact with the CMFs will be more apparent in the matrix background, and accordingly the SAXS signal will represent a larger diameter of the CMFs. This hypothesis could explain the higher value of 2(overline{R }) at higher temperatures despite no change in the diameter of CMFs themselves. In the future, we would like to perform the similar measurements for the wood samples whose chemical component(s) was/were specifically extracted to demonstrate the thermal softening mechanism with regard to the microstructure of the wood cell wall.

Another parameter, interfibrillar distance (a), showed a monotonous decrease when increasing the temperature from 25 °C to 90 °C. Penttilä et al. reported that in European fir (Abies alba) and Norway spruce (Picea abies) specimens, the interfibrillar distance decreases by 1–2 nm when drying from near the fiber saturation point to a few percent [14], demonstrating that the distance between CMFs becomes smaller due to the removal of water molecules in drying. We suppose that the similar situation will be found in the wood cell wall at higher temperatures given that the equilibrium moisture content (EMC) of wood cell walls decreases with increasing sample temperature in moisture-saturated wood [28]: the decrease in EMC with a 1 ℃ increase in temperature was estimated to be 0.1% [29]. Thus, one of the apparent changes at higher temperatures is the decrease of the bound water, which consequently results in the decrease of the interfibrillar distance as well as drying. In contrast, it is generally known that the occupied volume of polymers increases with increasing temperature due to the activation of molecular motion [30]. As shown by DMA (Fig. 3), the micro-Brownian motion of lignin is more active at higher temperatures, and then the occupied volume of lignin will be increased, which simply results in a larger distance between the CMFs. In total, in the water-saturated wood at higher temperatures, the increasing and the decreasing trend of the interfibrillar distance might cancel each other, or the decreasing trend (the desorption of bound water) would be slightly more apparent than the increasing trend (the thermal softening of lignin). Although the changes of the interfibrillar distance were not as significant as those of the CMF diameter, it is noticeable that their variation seems to correlate well with tan δ and/or E” (Figs. 3 and 8).

Earlywood showed a relatively continuous drop of a while latewood showed a small decrease from 25 °C to 60 °C and a rapid decrease from 60 °C to 90 °C. Given that tan δ and E” reflect the micro-Brownian motion in the amorphous region, this correlation suggests that the temperature-dependent variation of the interfibrillar distance is, in part, owing to the variation of the micro-Brownian motion in the matrix part in the wood cell wall. The difference in the change in the CMF diameter and the interfibrillar distance between earlywood and latewood by SAXS measurement also supported the hypothesis that the lignin structure differs between earlywood and latewood, which was obtained from DMA measurements. Further correlative studies with DMA and SAXS at finer temperature steps will be required for sophisticating this hypothesis. We believe that the direct comparison between the physical properties clarified by DMA and the structural information by SAXS should be valuable for precisely understanding the structure–function relationship of wood about mechanical and physical properties.

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