Sequential fractionation of lignin macromolecules with organic solvents and investigation of their potentials for lignin-PLA composites

Abstract

Milled wood lignin (MWL), organosolv lignin (OL), and soda lignin (SL) were sequentially fractionated with ethyl acetate (F1), 2-butanone (F2), methanol (F3), acetone (F4), and dioxane/water (95:5 v/v, F5). Yields of the five MWL fractions were 11.7%, 11.7%, 15.3%, 11.8%, and 49.6%, and yields of OL fractions were 26.2%, 26.1%, 18.7%, 3.7%, and 25.4% where yields of SL fractions were 30.1%, 25.5%, 24.7%, 2.0%, 11.2%, and 6.5% of insoluble fraction (INS) was remained. GPC analysis showed that the molecular weights of lignin fractions increased from F1 to F5. The average molecular weight of F1 ranged from 1000 to 2400 Da. whereas that of F5 was above 10000 Da revealing that molecular weight of fractions increased from F1 to F5. According to functional group analysis, the contents of phenolic hydroxyl groups and methoxyl groups decreased gradually with increasing molecular weight. DFRC analysis revealed that the higher molecular weight fractions yielded larger amounts of DFRC monomers indicating that later fractions contain more aryl ether linkages than earlier fractions. According to Py-GC/MS analysis, main pyrolysis products of each fraction were analysed. S/G ratios obtained by DFRC and Py-GC/MS analysis showed difference since condensed structure of lignin might bias the result of DFRC. TG/DTG analysis suggested that the low molecular weight fractions generally have lower thermal stability than other fractions due to their high content of functional groups. Native SL and SL fraction were individually grafted with L-lactide via ring-opening polymerization to produce lignin-grafted-poly(L-lactide) (lignin-g-PLLA). Conversion ratio of each fraction was calculated by 1H NMR and revealed that SL F1 which contains the largest amount of hydroxyl groups had the highest conversion ratio of 91.2% while SL F5 showed the lowest value of 88.3%. However, SLF1-g-PLLA showed the lowest molecular weight (Mn) where that of SLF5-g-PLLA was the highest indicating that SLF5-g-PLLA had the longest PLLA chain length. According to DSC analysis, it was revealed that SL-g-PLLA had the highest glass transition temperature due to its structural complexity. Surface characterization using scanning electron microscope suggested that surface of lignin grafted with PLLA had is smoother than that of native lignin, while SL F1, F3, and F5 had more smoother surface than SL and SLINS grafted with PLLA. Each copolymer was mixed with PLA 2002D to manufacture a lignin-PLA composite. Tensile strengths of composites were varied by chain lengths and surface properties of copolymers. In case of tensile modulus, the chain length of copolymer affected mainly.