The Analyses of Lignin Structure and Molecular Weight Distributions of Small Fractions from Hardwood Dissolved Alkali Lignin

The Soda-AQ cooking of poplar was carried out in the study. Acid-precipitation was used in the process of the classification and separation of lignin from the black liquor, and different lignin samples were obtained. The molecular weight distributions of lignin samples were analysed by GPC(gel permeation chromatography). The results showed that the last stage of the dissolved lignin sample had low molecular weight. The structure of lignin was analysed by 31P-NMR,and the content of the aliphatic hydroxyl group, phenolic hydroxyl group and other functional groups was obtained. According to the molecular weight and the content of functional groups, the structural diagrams of lignin fragments which had small molecular weight was set up.