Acid-Free Preparation of Cellulose Nanocrystals by TEMPO Oxidation and Subsequent Cavitation

Softwood bleached kraft pulp (SBKP) and microcrystalline cellulose (MCC) were oxidized using a 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated system. The TEMPO-oxidized SBKP prepared with 10 mmol/g NaClO (SBKP-10) had a higher mass recovery ratio and higher carboxylate content than the other prepared celluloses including the TEMPO-oxidized MCCs. The SBKP-10 was then exposed to cavitation-induced forces through sonication in water for 10-120 min to prepare aqueous dispersions of needle-like TEMPO-oxidized cellulose nanocrystals (TEMPO-CNCs) with homogeneous width of 3.5 to 3.6 nm and average lengths of ∼200 nm. The average chain lengths of the cellulose molecules that make up the TEMPO-CNCs were less than half the average lengths of the TEMPO-CNCs. Compared with conventional CNCs prepared by acid hydrolysis, the TEMPO-CNCs prepared by the acid-free and dialysis-free process exhibited higher mass recovery ratios, significantly higher amounts of surface anionic groups, and smaller and more homogeneous widths.