Relevance of Molecular Structure to the Morphological Properties of Lignin Extracted by Various Methods: X-ray Scattering Analysis

Different extraction methods of separating lignin from biomass have resulted in considerable differences in the structure, purity, and properties of the material, which may affect its utilization and valorization. Differences in the lignin molecular structure also affect morphology, including size and structural rearrangement. Furthermore, knowledge of lignin morphology can help us understand its interfacial interactions with other materials. This study investigates the molecular structure, physicochemical characteristics, and morphological features of lignin samples obtained using various extraction processes, including milled wood lignin (MWL), organosolv lignin (OSL), softwood kraft lignin (SKL), and hardwood kraft lignin (HKL). The results indicate notable variations in structural units, total hydroxyl groups, and physicochemical properties among the various lignin samples. In particular, the larger amounts of oxygenated aliphatic linkages and aliphatic OH groups of MWL, along with its lower numbers of C–H aromatic bonds and higher molecular weight (Mw) relative to the other types of lignin, result in a higher glass transition temperature (Tg), a smaller radius of gyration (Rg), and a higher fraction of T-shaped π–π stacking. Conversely, the fewer oxygenated aliphatic linkages, larger amounts of phenolic OH groups, and lower Mw of OSL result in a relatively lower Tg, higher Rg, and greater fraction of sandwich-like π–π stacking. These properties of OSL, along with its good purity, make it more accessible than the other lignins for interactions or reactions with other materials.