# The Impact of Thermal Treatment on Structural Changes of Teak and Iroko Wood Lignins
## 3 Results and Discussion
### Molecular Weight Determination  
Lignin isolated from the untreated teak wood has a molecular weight of about 7100 g\(\cdot\)mol\({}^{-1}\); in the available literature, no adequate data were found for comparison. In the thermal treatment of teak wood, its molecular weight (MW) and polydispersity (PD) decreased at a temperature of 160 \({}^{\circ}\)C (Table 4). Elution profiles confirmed a cleavage of the intermediate molecular portions of the lignin macromolecule (Figure 1). At higher temperatures (180 \({}^{\circ}\)C and 210 \({}^{\circ}\)C), both degradation and condensation reactions occur simultaneously, resulting in increased polydispersity and expanded peaks in the chromatogram (Figure 1). Changes in MW and PD are significant, except the difference in PD between temperatures 180 \({}^{\circ}\)C and 210 \({}^{\circ}\)C. Condensation reactions are predominant at 210 \({}^{\circ}\)C, leading to a lignin molecular weight increase and to a decrease in the yield of NBO products. The reduction in \(M_{\text{n}}\) and \(M_{\text{w}}\) indicates that the lignin molecules were fragmented into smaller units. Oxidized lignin can undergo re-polymerization as can be observed in our GPC data and as reported by other authors [40, 41]. Simultaneous degradation and condensation reactions were also confirmed by the heating of MWL lignin isolated from poplar wood [39]. The lignin thermal degradation leads to the arising of phenoxy radicals through a hydrogen atom abstraction process. They could undergo coupling reactions with the corresponding C5-centered mesomeric radical derivatives to generate new 4-O-5 and 5-5' linkages, respectively [42, 43]. Figure 1: Chromatograms of lignins isolated from untreated and thermally modified teak wood. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Wood** & **20 \({}^{\circ}\)C** & **160 \({}^{\circ}\)C** & **180 \({}^{\circ}\)C** & **210 \({}^{\circ}\)C** \\ \hline \(p\)-Hydroxybenzoic acid & 0.16 \(\pm\) 0.00 & 0.19 \(\pm\) 0.01 & 0.19 \(\pm\) 0.02 & 0.16 \(\pm\) 0.00 \\ \(p\)-Hydroxybenzaldehyde & 1.42 \(\pm\) 0.03 & 1.60 \(\pm\) 0.03 & 1.74 \(\pm\) 0.06 & 1.46 \(\pm\) 0.03 \\ Vanillic acid & 0.23 \(\pm\) 0.02 & 0.35 \(\pm\) 0.04 & 0.52 \(\pm\) 0.08 & 0.30 \(\pm\) 0.04 \\ Vanilline & 20.69 \(\pm\) 0.68 & 20.55 \(\pm\) 0.37 & 20.77 \(\pm\) 0.68 & 19.03 \(\pm\) 0.95 \\ Syringic acid & 1.56 \(\pm\) 0.06 & 2.45 \(\pm\) 0.11 & 2.89 \(\pm\) 0.11 & 1.22 \(\pm\) 0.06 \\ Syringaldehyde & 15.95 \(\pm\) 0.16 & 18.04 \(\pm\) 0.31 & 19.14 \(\pm\) 0.93 & 16.50 \(\pm\) 0.53 \\ Total yield on DL & 40.01 \(\pm\) 0.78 & 43.18 \(\pm\) 0.44 & 45.25 \(\pm\) 1.74 & 38.67 \(\pm\) 1.81 \\ S/G ratio & 0.84 \(\pm\) 0.02 & 0.98 \(\pm\) 0.04 & 1.03 \(\pm\) 0.04 & 0.92 \(\pm\) 0.01 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Nitrobenzene oxidation products of iroko dioxane lignins (mean \(\pm\) SD, %). Lignin isolated from iroko wood has a different molecular weight (about 38% lower) and polydispersity (about 44% lower) when compared to teak lignin. These values (\(M_{\rm w}\) = 4500 g\(\cdot\)mol\({}^{-1}\), \(M_{\rm n}\) = 2100 g\(\cdot\)mol\({}^{-1}\), PD = 2.1) are lower than those published by [PERSON] et al. (2019) [37]--(\(M_{\rm w}\) = 13,900 g\(\cdot\)mol\({}^{-1}\), \(M_{\rm n}\) = 2300 g\(\cdot\)mol\({}^{-1}\), PD = 5.7). The differences may be due to different growth locality, lignin isolation procedure, and the chromatographic method itself. During the thermal treatment, iroko lignin changes in a different way than teak lignin. At 160 \({}^{\circ}\)C, low molecular weight lignin fragments are degraded (Figure 2), resulting in a slight increase in molecular weight and a negligible decrease in polydispersity. Degradation reactions are predominant at 180 \({}^{\circ}\)C and the molecular weight decreases by almost 10% when compared to the original sample. Changes in MW are significant, some changes in PD are significant, while others are not.