This work aimed to discriminate among different wood samples on the base of the NMR relaxation of their cell wall water in the hygroscopic range. To develop a non-invasive NMR protocol for in situ investigation useful in the wood industry and/or cultural heritage applications, a portable NMR instrument was used. In the following, to better deal with and discuss the various observations and results obtained, the discussion section was divided into several paragraphs.
4.1 NMR relaxation times components assignment and their dependence on RH
Different behavior of the two components of T1 relaxation and the two components of T2 relaxation is highlighted in Figs. 2 and 3. Considering the longitudinal relaxation time, some authors (Fantazzini et al. 2006; Bonnet et al. 2017; Rostom et al. 2020) pointed out the existence of two different components below the FSP. A fast T1 component of a few ms or hundreds of µs, which is due to water bound to the cell walls, and a slow T1 of tens of ms, which is associated with the protons of wood polymers. In this regard, the component T1,1 displayed in Fig. 2a, which is of the order of few ms, can be attributed to bound water in the cell walls while the second component T1,2 (Fig. 2b), much greater and around tens of ms, can be associated with relatively immobile water within the wood polymers.
Regarding the transversal relaxation time, previous works (Araujo et al. 1992; Hartley et al. 1992; Labbé et al. 2002; Thygesen and Elder 2009) identified a T2 component around 0.2-3 ms increasing with RH as bound water. Therefore, we can associate the fast component T2,1 (Fig. 3a) with water protons tightly bound to macromolecules and the slow component T2,2 (Fig. 3b) with protons of cell wall-bound water (Casieri et al. 2004; Thygesen and Elder 2009). On the other hand, as the investigated wood samples were below the fiber saturation point, we did not detect the lumen water that is characterized by T2 around tens of milliseconds (Labbé et al. 2002). Moreover, because we used a TE = 0.04 ms in the CPMG experiments, we did not measure the faster T2 component belonging to solid wood (Casieri et al. 2004), estimated to be around 0.01 ms by Labbè et al. (Labbé et al. 2002).
4.1.2 Cell walls reservoir. In Fig. 2, an exponential dependence on the relative humidity of the T1 associated with the cell walls (T1,1) is visible for all four wood samples. Particularly, this component (Fig. 2a), ranging from a minimum of 0.8 ms to a maximum of 2 ms, shows a quite different behavior among softwoods and hardwoods. For the two hardwoods, white poplar and akatio walnut, the T1,1 appears to increase with the RH increment. This is due to the growing hydration of the wood cell walls which determines a slower longitudinal relaxation time. In particular, the T1,1 increment is higher when RH increases from RHA1 = 46% to RHA2 = 78% while it is lower from RHA2 = 78% to RHA3 = 94%. In the two softwoods, Russian silver fir and European Virginia pine, the T1,1 seems to be constant when RH changes from RHA1 = 46% to RHA2 = 78% and it starts to increase rapidly when RH grows from RHA2 = 78% to RHA3 = 94% reaching a lower maximum value compared to that of the hardwoods. This result can be explained by the fact that the hardwood samples catch more water molecules thanks to their higher hemicellulose content compared to the softwood samples. Also in the softwoods, the T1,1 increment is associated with the growing hydration of the cell walls, but this seems to be gained drastically for RH > 78%. This behavior may be a consequence of the hemicellulose softening that at room temperature (T = 20°C) occurs around RH = 75% (Olsson and Salmén 2004; Engelund et al. 2013).
The T2,2 component of wood cell walls (Fig. 3b), grows from 0.53 to 1.38 ms describing progressive hydration of the wood cell walls mainly ascribable to hemicelluloses softening. Its growth is faster above RHA2 = 78% due to the glass transition of hemicelluloses as described by Engelund et al. (Engelund et al. 2013). This T2 component also shows a distinct behavior among softwoods and hardwoods reflecting their different hygroscopicity. On this subject,, hardwoods have cell walls made by a higher amount of hemicellulose (Holtzapple 2003) that is characterized by a lot of polar groups (OH groups) able to retain water increasing wood hygroscopicity (García Esteban et al. 2005).
4.1.3 Polymers water reservoir. The T1,2 component in Fig. 2b shows an opposite behavior as a function of RH compared to that of the T1,1 (Fig. 2a). Starting from a value of 50–60 ms at RHA1 = 46%, the T1,2 decreases for both softwoods and hardwoods with the increase of RH, reaching a minimum value around 30–40 ms. This is explainable considering that water tightly bound to polymers gains mobility with the RH increment. In the solid-like range, this increase of mobility is associated with a speeding up of the spin-lattice relaxation time due to the faster exchange of energy between spins and lattice (Brown and Koenig 1992).
In this regard, the T2,1 component (Fig. 3a) spanning from 0.15 to 0.24 ms, appears to slightly grow during the RH increment except for akatio walnut that shows a rapid increase (from 0.18 to 0.25 ms) when RH changes from 78 to 94%. This observation can indicate greater mobility of water protons in akatio walnut macromolecules and that it is approaching the FSP. In fact, as described in the literature (Jankowska and Kozakiewicz 2016), the FSP is negatively correlated with the wood density so denser woods have a lower FSP. The akatio walnut sample investigated in this paper has the highest density (around 560 kg/m3, see Table 1) among the other three samples and for this reason, a low FSP is expected. In general, the T2,1 component is quite similar for softwoods and hardwoods and describes the slow variation of mobility of the water protons tightly bound to the macromolecules.
4.2 Relaxation times vs. density correlation
As expected, the plot in Fig. 4 suggests that T1 moderately correlates with the wood dry density (kg/m3), as previously shown by Stagno et al. . The dependence of T1,1 and T1,2 on the dry density of woods indicates that T1 is also affected by intrinsic features of the wood. Particularly, English walnut is characterized by the longest T1 and the highest dry density associated with its compact structure due to a diffuse-porous ring with infrequent pores and frequent tyloses . Conversely, European silver fir has the lowest density and the shortest T1 because of its homogeneous structure constituted by more than 95% of open elements, i.e. tracheids [3, 38].
4.3.1 Two clusters hypothesis. The main purpose of clustering by hypothesizing two clusters was to detect a possible different behavior among softwoods and hardwoods based on the measured relaxation times. The plot in Fig. 5a shows how the different species of wood are distributed according to their T1,1 and T2,2 relaxation times. A clear differentiation of cell wall reservoir between softwoods and hardwoods is visible. Softwoods cluster centroid is [1.22, 0.69], which indicates that softwood samples are distributed around the median value of T1,1 = 1.22 ms and of T2,2 = 0.69 ms. Indeed, while hardwoods seem to be spread among different values of T1, softwoods occupy a quite narrow region of the plot that roughly ranges from 1 to 1.5 ms. Moreover, the hardwoods cluster centroid is [1.67, 1.25]. Indeed, hardwoods show higher values of T2,2 if compared to softwoods. Softwoods, indeed, have T2,2 always shorter than 0.806 ± 0.004 ms, and hardwoods always longer than 0.90 ± 0.02 ms. Basically, at RHB = 94% the cell wall reservoir of softwoods is characterized by lower values of T1 and T2 than the cell wall reservoir of hardwoods.
In Fig. 5b, a similar result is shown but considering the T1,2 and T2,1 components. Two different clusters can be observed: cluster 1 with a centroid of [74.39, 0.23] and cluster 2 with a centroid of [30.33, 0.20]. As for the plot in Fig. 5a, the T1,2 component of hardwoods is spread to different values. On contrary, softwoods show shorter T1,2. The T2,1 component is quite similar for all the woods. This result suggests that on the base of the polymers water reservoir it is not possible to distinguish among softwood and hardwood samples because of their quite similar T1 and T2 relaxation times. Anyway, two clusters were detected with cluster 1 which contains four hardwoods (sessile oak, sapele mahogany, English walnut, and Australian red cedar) characterized by long T1,2 and T2,1 likely indicating greater hydration of their polymers.
4.3.2 Three clusters hypothesis. To evaluate the existence of other possible clusters by using the T1,1 and T2,2 components, which are the relaxation times that provided a good differentiation among softwoods and hardwoods, a three clusters analysis was performed and shown in Fig. 6. This plot suggests two sub-clusters of the hardwoods cluster. The cluster called hardwoods 2, with centroid [2.05, 1.38], contains the samples with longer T1,1 (English walnut, white poplar, and African walnut), whereas the cluster called hardwoods 1, with centroid [1.66, 1.10], the samples with shorter T1,1 (sapele mahogany, akatio walnut, bahia walnut, sessile oak, tanganyika walnut, and Australian red cedar).
4.4 Final discussion
In Fig. 7a schematic representation of the hygroscopic behavior of the wood cell wall polymers exploited in this work to discriminate between softwood and hardwood is displayed. Specifically, in parallel with the increase of RH, the hemicellulose hydrates more. The hydroxyl groups of the hemicellulose capture water molecules through hydrogen bonds that affect the NMR relaxation times of the cell wall reservoir (T1,1 and T2,2), which allow discriminating between softwood and hardwood. Hardwoods have a higher hemicellulose content compared to softwoods, therefore their cell walls can reach greater hydration with more water molecules that are bound to the hemicellulose hydroxyls.