Determination of length distribution of TEMPO-oxidized cellulose nanofibrils by field-flow fractionation/multi-angle laser-light scattering analysis

Aqueous nanocellulose dispersions were prepared from wood cellulose by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation. The obtained TEMPO-oxidized cellulose was converted into TEMPO-oxidized cellulose nanofibrils (TOCNs) of different lengths by controlling the nanofibrillation conditions in water or using dilute acid hydrolysis. The average lengths and length distributions of TOCNs have been measured from transmission electron microscopy (TEM) and atomic force microscopy (AFM) images. However, because the number of nanocelluloses observable in TEM and AFM images is limited, a more reliable method is needed to obtain the lengths/length distributions of TOCNs. In this study, the aqueous TOCN dispersions were subjected to a combination of field-flow fractionation (FFF) and multi-angle laser-light scattering (MALLS). The optimum FFF operation conditions for the acid-hydrolyzed TOCN were first established to obtain reasonable data. For TOCNs with average lengths > 400 nm, suitable separation could not be achieved using the FFF/MALLS system. In contrast, the TOCNs with average lengths of 170 and 270 nm were adequately separated according to their sizes by the FFF system. The TOCN length distribution patterns corresponded well to those obtained from TEM images. However, the amounts of TOCNs with lengths > 250 nm were underestimated compared with those determined from TEM images. For TOCNs with average lengths of 170 and 270 nm, the molar mass at each TOCN length was determined using the FFF/MALLS system combined with a refractive index detector, where a specific refractive index increment of 0.165 mL/g was used for TOCN.