Structural and Thermal Characterization of Milled Wood Lignin from Bamboo (Phyllostachys pubescens) Grown in Korea

The structural and thermal characterization of milled wood lignin (MWL) prepared from bamboo (Phyllostachys pubescens) grown in Korea was investigated, and the results were compared with bamboo MWLs from other studies. The C9 formula of the bamboo MWL was C9H7.76O3.23N0.02 (OCH3)1.41. The Mw and Mn of MWL were 13,000 and 4400 Da, respectively, which resulted in a polydispersity index (PDI) of 3.0. The PDI of the prepared MWL was higher than other bamboo MWLs (1.3–2.2), suggesting a broader molecular weight distribution. The structural features of MWL were elucidated using FT-IR spectroscopy and NMR techniques (1H, 13C, HSQC, 31P NMR), which indicate that MWL is of the HGS-type lignin. The major lignin linkages (β-O-4, β-β, β-5) were not different from other bamboo MWLs. The syringyl/guaiacyl ratio, determined from 1H NMR, was calculated as 0.89. 31P NMR revealed variations in hydroxyl content, with a higher aliphatic hydroxyl content in MWL compared to other bamboo MWLs. Thermal properties were investigated through TGA, DSC, and pyrolysis-GC/MS spectrometry (Py-GC/MS). The DTGmax of MWL under inert conditions was 287 °C, and the Tg of MWL was 159 °C. Py-GC/MS at 675 °C revealed a syringyl, guaiacyl, p-hydroxyphenyl composition of 17:37:47.