the ultrasonic output power conducted at 800 W, and more than
75% of the nanofibers had a diameter range of 5–20 nm. Although
bundles of the nanofibers still existed to a certain extent in the suspension,
small gaps among the unindividualized nanofibers can be
observed in the nanofiber aggregates (insert image of Fig. 4b). This
indicates that the hydrogen bonding between the nanofibers was
lower after ultrasonic treatment at 800 W. Sequentially, chemicalpurified
cellulose fibers were subjected to 1000W and 1200W
ultrasonic treatments, and cellulose nanofibers with a uniform
width ranging from 5 to 20 nm and a web-like network structure
were obtained. The average diameter of the obtained nanofibers
was found to be 13.0 and 12.8 nm, respectively. The TEM observation
also revealed that the length of most obtained cellulose
nanofibers was a few microns. When cellulose nanofibers prepared
with 1000 and 1200W ultrasonic treatment were observed using
FE-SEM (Fig. 5) after gold sputtering, fibers in the nano-order scale
and entangled structure were also observed, which satisfactorily
agreed with TEM measurements. It is probable that the nanofibers
were covered with relatively thick gold layers by the sputtering
process, which is necessary to observe such nanoscale cellulose
fibers by FE-SEM. It is thus illustrated that the degree of nanofibrillation
depends on the output power of the ultrasonic treatment,
i.e., the obtained nanofibersbecomemore uniform and slender with
increased intensity. The result indicated that ultrasonic treatments
with an output power equal or greater than 1000Wcan efficiently
individualize cellulose nanofibers and disperse them in an aqueous
suspension. This approach to individualize nanofibers from wood
chemical-purified cellulose fibers may be explained by the effect of
acoustic cavitation of high frequency (20–25 kHz) ultrasound in the
formation, expansion, and implosion of microbubbles in aqueous
solution. The violent collapse induces microjets and shock waves
on the surfaces of the chemical-purified cellulose fibers, causing
erosion of the surface of the fibers to split along the axial direction.
The sonification impact can break the relatively weak interfaces
among the nanofibers, which are bonded to each other mainly by