Alkaline soda lignin (AL) was sequentially fractionated into six fractions of different molecular size by means of solvent extraction and their phenolic hydroxyl groups were chemoselectively methylated to determine their effect on nanoparticle formation of lignin polymers. The effect of the lignin structure on the physical properties of nanoparticles was also clarified in this study. Nanoparticles were obtained from neat alkaline soda lignin (ALNP), solvent-extracted fractions (FALNPs, i.d. 414–1214 nm), and methylated lignins (MALNPs, i.d. 516–721 nm) via the nanoprecipitation method. Specifically, the size properties of MALNPs showed a high negative correlation (R² = 0.95) with the phenolic hydroxyl group amount. This indicates that the phenolic hydroxyl groups in lignin could be influenced on the nucleation or condensation during the nanoprecipitation process. Lignin nanoparticles exhibited high colloidal stability, and most of them also showed good in vitro cell viability. This study presents a possible way to control nanoparticle size by blocking specific functional groups and decreasing the interaction between hydroxyl groups of lignin.

In general, lignin exhibits unpredictable and nonuniform thermal properties due to the structural variations caused by the extraction processes. Therefore, a systematic understanding of the correlation between the extraction conditions, structural characteristics, and properties is indispensable for the commercial utilization of lignin. In this study, the effect of extraction conditions on the structural characteristics of ethanol organosolv lignin (EOL) was investigated by response surface methodology. The structural characteristics of EOL (molecular weight, hydroxyl content, and intramolecular coupling structure) were significantly affected by the extraction conditions (temperature, sulfuric acid concentration, and ethanol concentration). In addition, the correlation between the structural characteristics and thermal properties of the extracted EOLs was estimated. The relevant correlations between the structural characteristics and thermal properties were determined. In particular, EOLs that had a low molecular weight, high phenolic hydroxyl content, and low aryl–ether linkage content exhibited prominent thermal properties in terms of their initial decomposition rate and a high glass transition temperature, T_g. Correspondingly, EOL-PLA blends prepared using three EOL types exhibited improved thermal properties (starting point of thermal decomposition and maximum decomposition temperature) compared to neat PLA and had thermal decomposition behaviors coincident with the thermal properties of the constituent EOLs.