Study on the Mechanism of Viscoelasticity of Nanocellulose Suspensions in Dilute and Semi-Dilute Regions

Abstract

The viscoelastic relaxation mechanisms of cellulose nanocrystals (CNCs) and individualized cellulose nanofibers (iCNFs) in the dilute and semi-dilute regions were investigated. The dynamic viscoelasticity and dynamic birefringence of CNCs and iCNFs were measured and compared with the viscoelastic theory for semiflexible polymers (Shankar–Pasquali–Morse theory). We first developed a high-sensitive apparatus capable of measuring minute dynamic birefringence of dilute CNC and iCNF suspensions. Thanks to the apparatus, we were able to distinguish the respective modes contributing to the viscoelastic relaxation of CNCs and iCNFs, particularly CNCs. The viscoelastic relaxation of CNCs was well explained by Shankar–Pasquali–Morse theory. We first found that CNCs behaved as ideal semiflexible rods in fluids. In contrast, iCNFs showed specific viscoelastic relaxation behaviors. Shankar–Pasquali–Morse theory couldn't fully describe the viscoelastic relaxation of the iCNFs at high angular frequencies, and the existence of additional relaxation modes were indicated. These results were found regardless of average lengths and solvents of iCNFs. Furthermore, the rotational relaxation times of iCNFs showed significantly stronger concentration dependencies than those of CNCs and other semiflexible polymers.