Determination of nanocellulose fibril length by shear viscosity measurement
The lengths of ten types of cellulose nanofibrils were evaluated by shear viscosity measurement of their dilute dispersions. Aqueous dispersions of surface-carboxylated cellulose nanofibrils with a uniform width of ~3 nm were prepared from wood cellulose by 2,2,6,6-tetramethylpiperidine-1-oxyl-mediated oxidation and successive mechanical treatment. Cellulose nanofibril samples with different average lengths were prepared by controlling the conditions of the oxidation or mechanical treatment. The viscosity-average lengths, Lvisc, of the nanofibrils were calculated by applying the shear viscosities of the dilute dispersions to an equation for the dilute region flow behavior of rod-like polymer molecules. The obtained Lvisc values ranged from 1,100 to 2,500 nm and showed a linear relationship to the length-weighted average length, Lw, measured by microscopic observation; the relation was described as Lvisc = 1.764 × Lw + 764. The influences of the electric double-layer of the nanofibrils and surface-carboxylate content on the value of Lvisc were also investigated.