Effects of UV irradiation of TEMPO-oxidized cellulose nanofibril/water dispersions on chemical structure, molar mass, and morphology

A commercial TEMPO-oxidized cellulose (TOC) and three laboratory-made TOC samples prepared under different conditions were converted to aqueous 0.4% dispersions of TEMPO-oxidized cellulose nanofibrils (TEMPO-CNFs) by high-pressure homogenization in water under the same conditions. The obtained TEMPO-CNF/water dispersions were irradiated with ultraviolet (UV) light for 0-48 h using a high-pressure mercury lamp at a main wavelength of 365 nm. The changes in the light transmittance, viscosity, pH, zeta-potential, and average particle size of the dispersions, and mass recovery ratio, molar mass, and carboxylate content of the acid-insoluble and freeze-dried fractions separated and isolated from the UV-irradiated dispersions were investigated. The results were analyzed in terms of the UV-irradiation time. The dispersion pH, and the mass recovery ratio, carboxylate content, and mass-average degree of polymerization (DPw) of the acid-soluble fraction decreased with increasing UV-irradiation time. This shows that some acid-soluble acidic compounds were formed from the TEMPO-CNFs via UV-induced depolymerization, degradation, and removal of carboxylate groups from the TEMPO-CNFs in water, depending on the UV-irradiation time. The average CNF length, which was determined from atomic force microscopy images, decreased with increasing UV-irradiation time. After UV irradiation of dispersions of the laboratory-made TEMPO-CNFs for 12 h, the average CNF lengths were 130-150 nm and their length distributions were narrower. UV irradiation of TEMPO-CNF/water dispersions is therefore a promising method for efficiently decreasing the dispersion viscosity, DPw, and average CNF length, depending on the TEMPO-CNF properties and UV-irradiation conditions.