Viscoelastic and Birefringence Relaxation of Individualized Cellulose Nanofibers in the Dilute and Semidilute Regions

Viscoelastic relaxation mechanisms of individualized cellulose nanofibers (iCNFs) dispersed in glycerol in the dilute and semidilute regions were investigated by linear viscoelastic and dynamic birefringence measurements. The birefringence relaxation of the iCNFs was described by the orientational and curvature modes of an existing viscoelastic theory for ideal semiflexible polymers (Shankar-Pasquali-Morse theory). However, the Shankar-Pasquali-Morse theory could not fully describe the iCNF viscoelastic relaxation at high frequencies. Considering the results for birefringence relaxation, the experimental tension mode of the iCNFs was evaluated to be higher than the theoretical value. These results show that the viscoelastic relaxations of the iCNFs are different from those of ideal semiflexible polymers, in contrast to cellulose nanocrystals (CNCs). As the iCNF concentration increased, the orientational mode dramatically slowed, which was more drastic than other semiflexible polymers, including CNCs. This anomalous behavior is likely due to the nonideal nature of iCNFs.