## 3 Results and Discussion  
### Structural Features of Lignin Fractions  
The yield, molecular weight, phenolic OH, and methoxyl contents of each fraction from SL are shown in Table 1. SL was fractionated with five different organic solvents. Solvation properties of the solvents used in this experiment are as follows: ethyl acetate (Hildebrand solubility parameter, \(\delta\) = 9.1, wavelength shift, \(\Delta\mu\) = 0.12), 2-butanone (\(\delta\) = 9.3), methanol (\(\delta\) = 14.3, \(\Delta\mu\) = 0.28), acetone (\(\delta\) = 9.8, \(\Delta\mu\) = 0.14), and 95% dioxane (\(\delta\) = 10.0, \(\Delta\mu\) = 0.14)/water (\(\delta\) = 23.4, \(\Delta\mu\) is very high). The yields of F1 to F5 and INS were 30.1%, 25.5%, 24.7%, 2.0%, 11.2%, and 6.5%, respectively. The molecular weights of F1 to F5 were 1190, 2550, 3680, 5310, and 14,650, respectively. INS showed low solubility in the gel permeation chromatography solvent, therefore, we could not determine its molecular weight. As the fractionation step proceeded, the molecular weight of the lignin fraction increased. Several studies showed that a tacticity or increase in the solvation power contributed to an increase in the molecular weight [17; 18]. Due to the similarity of solvation properties between methanol and acetone, the yield of  
Figure 1: Schematics of the lignin nanoparticle formation. F4 was relatively lower than that of F3. However, there was a remarkable molecular weight difference between F3 and F4 (3680 and 5310, respectively). F1 to F4 showed improve PDI values (less than 2.6) compared to that of SL (3.0), indicating that the fractions were homogenized by the sequential solvent extraction. The amount of phenolic OH groups in SL was 1.8 mmol/g. Relatively low molecular weight fractions had more phenolic OH groups than SL (2.6 mmol/g and 2.2 mmol/g for F1 and F2). On the contrary, relatively high molecular weight fractions had fewer but similar phenolic OH groups (1.7 mmol/g and 1.6 mmol/g for F3 and F4). F5 showed the lowest phenolic OH group content of all fractions, less than 1.0 mmol/g. This is indirect evidence suggesting that many of phenolic hydroxyl groups in low molecular weight fractions remain free, and those high in molecular weight participate in lignin internal linkages (e.g., \(\beta\)-O-4 and \(\beta\)-5). The methoxyl group content showed a slight decrease from 3.9 mmol/g for F1 to 3.1 mmol/g for F5. Molecular weights of the lignin fractions dissolved in THF at different concentrations and molecular weights are shown in Table 2 since lignin should be dissolved in THF to make nanoparticles by nanoprecipitation. THF soluble fractions showed relatively low PDIs compared to those of raw SL under all concentrations (2.2, 2.5, and 2.5 for 1 mg/mL, 2 mg/mL, and 4 mg/mL, respectively). Molecular weights of the THF soluble fractions were much lower than those of SL and increased with increasing concentration from 1810 to 2490. It is suggested that accession of polymer concentration caused an increased interaction between lignin and other molecules, and thus the average molecular weight and PDI increased. On the other hand, the molecular weight of F1 to F4 did not change much when dissolved in THF. The molecular weight of F5 dissolved in THF, which showed low solubility, greatly declined because low molecular weight lignin fractions specifically dissolved in the solvent. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & **Yield** & \multicolumn{3}{c}{**Molecular Weight**} & \multicolumn{2}{c}{**Phenolic OH**} & **Methoxyl** \\  & **(wt \%)** & **Mw** & **Mn** & **PDI** & **(mmol/g)** & **(mmol/g)** \\ \hline SL & - & 3130 & 1030 & 3.0 & 1.8 & 3.4 \\ F1 & 30.1 & 1190 & 660 & 1.8 & 2.6 & 3.9 \\ F2 & 25.5 & 2550 & 1200 & 2.1 & 2.2 & 3.9 \\ F3 & 24.7 & 3680 & 1630 & 2.3 & 1.7 & 3.5 \\ F4 & 2.0 & 5310 & 2110 & 2.5 & 1.6 & 3.4 \\ F5 & 11.2 & 14,650 & 4700 & 3.1 & 0.8 & 3.1 \\ INS & 6.5 & - & - & - & - & 1.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Yields and molecular weight distribution.