The green source, industrial abundance, and rich functional groups of lignin enable lignin’s potential to serve as an attractive and sustainable alternative to fossil resources for fuels, chemicals, and materials. However, applying bulky lignin directly for materials is significantly challenged by the heterogeneity of lignin’s chemical structure, originating from plant sources and isolation processes. Molecular weight (MW) has been shown to significantly influence the physicochemical properties of lignin and its valorization to end-products. Lignin fractionation is an efficient method to improve structural homogeneity by decreasing the molar mass dispersity (ĐM) of lignin. The structure of the lignin fraction determines its behavior of process and the properties of end-products. However, the impacts of fractionation approaches on the detailed physicochemical properties of lignin fractions are still unclear. In this study, we use the three most common fractionation methods, namely, solvent exchange, acid precipitation, and membrane ultrafiltration to fractionate Indulin AT kraft lignin for obtaining lignin fractions with reduced heterogeneity. We have thoroughly studied the physicochemical features of the fractions, including MW, compositional units, functional groups, thermostability, morphology, surface charge, and surface energy. We have found that the fractionation greatly reduced the molecular heterogeneity of the kraft lignin with an MW of 6,586 g/mol and an ĐM of 5.8. Solvent fractionation led to MW of 20,605 g/mol, 4,635 g/mol, 1,739 g/mol, and ĐM of 10.2, 2.8 and 1.9, respectively. Acid precipitation led to MW of 7,162 g/mol, 5,673 g/mol, 4,435 g/mol, and ĐM of 5.9, 4.5 and 3.7. We also found the morphologies of the fractions are very different in that the solvent fractionation resulted in particle sizes of approximately 100 µm, 10 µm, and 200 nm, respectively. The Inverse gas chromatography analysis showed that the fractions have very different surface areas, dispersive surface energy, and acid-base surface energy. Our study provided important guidance to reduce the heterogeneity of industrial lignin and its valorization using the appropriate fractionation method for potential materials.
