Multi-scale instrumental analyses for structural changes in steam-treated bamboo using a combination of several solid-state NMR methods

Abstract

Steam-treated bamboo was analyzed for structural changes at multiple scales using several solid-state NMR methods, including relaxation time analysis. Subtle changes due to different processing conditions were analyzed by 13C magic angle spinning (MAS) NMR using cross polarization (CP), single pulse with dipolar decoupling (DD), and pulse saturation transfer (PST). The analyses also used 1H spin-lattice relaxation time (T1H) integrated with attenuated total reflection infrared spectroscopy (ATR-IR), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The first change due to the steam treatment appeared in the hemicellulose methyl groups and the aromatic carboxylic acid; with further processing the carbohydrate ring of hemicellulose decreased and the pyrolysis products having quaternary olefinic and aromatic groups increased. During steam treatment, pyrolysis products appear to have restrained molecular motions of the steam-heated bamboo, while cleavage of lignin increased molecular motion of the OCH3 group. Bamboo treated with pressurized steam retained its hemicellulose and produced a smaller amount of pyrolysis products, while both hemicellulose and lignin were reduced by treatment with NaOH solution. The physical fracture of the sample observed at higher processing temperatures and longer times occurred not because of chemical changes in specific constituents but through physical destruction of vascular bundle units.