The Preparations Of Cellulose Nanocrystals By Oxidation And The Corresponding Cellulose Nanocrystal Aerogels

In this study, cellulose nanocrystals(CNCs)were prepared from hybrid poplar residue of little hemicellulose by oxidation using ammonium persulfate(APS). The yield and the physical properties of the resulting CNCs were investigated at different oxidation times, and the CNCs were characterized by some different techniques. The mechanism behind the oxidation of cellulose by APS was established. Additionally, CNC aerogels were prepared using supercritical carbon dioxide extraction(SCE) under different conditions, and optimal conditions for SCE were investigated based on the percentages of the shrinkage and the residual solvent mass. And the resulting CNC aerogels were also characterized using some different techiniques.The crystal structure, size, morphology and thermal property of CNC were characterized and analyzed with by fourier transformation infrared spectroscopy(FT-IR), nuclear magnetic resonance(NMR), X-ray diffraction(XRD), transmission electron microscopy(TEM),atomic force microscopy(AFM)and thermogravimetric(TG). The structure, appearance and thermal property were characterized by specific surface area analyzers,scanning electron microscopy(SEM)and thermogravimetric(TG), respectively. The main results are as following:Hybrid poplar residues after extraction of hemicellulose were oxidized for the release of CNCs at 70 ? with 1.5 mol/L APS solution, and the optimal oxidation time was 16 h. At this time, the oxidation degree and the zeta potential of the CNCs were 0.156 and-42.15 m V, respectively.The FT-IR spectra showed that APS could remove the residual hemicelluloses and lignin in the starting material during the oxidation, and some hydroxyl groups were oxidized into carboxyl groups.The CP/Mas 13C-NMR spectra confirmed that the C6 primary OH groups in cellulose preferentially were oxidized. It was showed that APS oxidation of CNCs was regioselective to OH groups.XRD indicated that the CNCs were cellulose type I and cellulose I? dominated. The crystallinity index of CNCs increased with the increase in oxidation time.The TEM and the AFM images of CNCs showed the CNCs were rodlike and their diameters were between 5 and 45 nm.Supercritical carbon dioxide extraction was optimized for the preparation of CNC aerogels and the optimal conditions were as fellows, 120 min,11.04 MPa and 40 ?.The specific surface area results showed that the internal pore of the CNC aerogels were mesoporous and macroporous. As the CNC mass in hydrogels was increased, the BET surface area increased and were in the range of 203~370 m2/g.The SEM images showed that CNC aerogels were in the form of three-dimension network.Therefore,the CNCs could be efficiently extracted from hybrid poplar residue of high lignin content by the APS oxidation, and the supercritical carbon dioxide extraction was a promising candidate for the preparation of CNC aerogel.