Milled wood lignin (MWL), organosolv lignin (OL) and soda lignin (SL) were sequentially fractionated with in order of ethyl acetate (F1), 2-butanone (F2), methanol (F3), acetone (F4), and dioxane/water (F5). Yields of the five MWL fractions F1 to F5 were 11.7%, 11.7%, 15.3%, 11.8%, and 49.6%, and yields of the OL fractions were 26.2%, 26.1%, 18.7%, 3.7%, and 25.4%. In case of the SL, yields of the fractions were 30.1%, 25.5%, 24.7%, 2.0%, 11.2%, and 6.5% of the insoluble fraction (INS). In all lignin, F1s showed the lowest molecular weights which ranged from 1000 to 2400 Da, while F5 had the highest molecular weight above 10000 Da. In addition, polydispersity index (PDI) values of all fractions were low compared with that of original lignins. This indicates that similar size of lignin fragments were rearranged during sequential solvent fractionation of lignin. According to functional group analysis, the contents of phenolic hydroxyl groups and methoxyl groups decreased gradually with increasing molecular weight. DFRC analysis, a parameter for frequency of β-O-4 linkages, revealed that the higher molecular weight fractions yielded larger amounts of DFRC monomers, indicating that those fractions more frequently contain aryl ether linkages. TG/DTG analysis suggested that the low molecular weight fractions generally have lower thermal stability due to their high content of functional groups. The Tg of the fractions ranged from 126 °C to 156 °C, increasing as the molecular weight of the lignin fraction increased. Among lignin fractions, SL fractions were used to investigate the possibility to utilize them as lignin-PLA copolymers. Each fraction was grafted with L-lactide to form lignin-grafted-poly(L-lactide) (lignin-g-PLLA) via ring opening polymerization (ROP) catalyzed by triazabicyclodecene (TBD). Conversion ratios of L-lactide to lignin-g-PLLA were analyzed by 1H NMR and showed that F1 had the highest conversion ratio while F5 had the lowest. Hydroxyl groups of lignin play a role as initiation site of L-lactide polymerization, thus F1 which contains relatively high amount of hydroxyl groups gave L-lactide more chances to be polymerized. Further investigation of mechanical and thermal properties of lignin-PLA copolymer will be performed.

Sequential fractionation of lignin macromolecules with organic solvents and investigation of their potentials for utilization as lignin-PLA copolymers

253rd National Meeting (2017)

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