self-aggregated into long fibers through hydrogen binding in both
lateral and longitudinal directions. The diameter of the fiber of the
TCNF ranged from 200 nm to 1.5 m and that of the PCNF ranged
from 90 to 800 nm. The lengths ofthe nanofibers were about several
micrometers. During the freezing process, the nanofibers would
gather around the edge of ice crystals and massive hydrogen bonds
formed among the adjacent surfaces of nanocellulose in the following
drying process. Consequently, the nanofibers interacted with
each other and eventually self-organized into larger-sized cellulose
fibers or 2D sheet-like structures (Chen et al., 2014).
Because freeze-drying process could cause nanocellulose to
self-assemble and aggregate, preventing formation of hydrogen
bonding among CNCs/CNFs would help retain the nanoscale dimension
of individual nanofiber. tert-butanol could limit nanocellulose
assembly and aggregation via providing steric hindrance to neighboring
nanofiber surface hydroxyls from hydrogen bonding with
another nanofiber (Jiang & Hsieh, 2015). tert-butanol was added
to aqueous nanocellulose to obtain 0.05 wt% tert-butanol/water
(1:1 v/v) suspension, which was rapidly frozen by liquid nitrogen
and freeze-dried. The SCNC (Fig. 2e) and FCNC (Fig. 2f) selfassembling
from freeze-drying in tert-butanol/water formed into
long fiber in the longitudinal direction, instead of 2D sheet-like
structures aggregated from water. The lengths of some of the FCNC
fibers were more than 1 micrometer. The diameter of the fiber of
the SCNC ranged from about 60–330 nm and that of FCNC ranged
from about 35–290 nm. The ultra-fine nanofibers interconnected
with each other and self-organized into web-like entangled structures.
The TCNF/PCNF was similar to that drying fromwater, buthad
thinner diameter (Fig. 2g and h). Thus, the addition of tert-butanol
showed apparent inhibition of aggregation during the freeze drying
process. The entire length of the bundles was not easy to be
determined with the SEM images due to the entanglement of long
cellulosic chains and twisted morphologies of the nanofibers.
SEM has been widely utilized for morphology characterization
of nanocellulose. Coating the surface of sample with gold or platinum
is needed before imaging. But the coating would broaden
the nano-sized structure. Moreover, the cellulose nanofibers might
self-aggregate into thick nanofibers or be dried into thin sheets
during the drying process. Thus, the real structures of nanocellulose
can not be clearly observed by SEM. Compared with traditional
SEM, the resolutions of AFM and TEM are higher, and coating the
sample surfaces before observation is not needed. The TEM and
AFM analyses can more clearly evaluate the size of single nanofiber
and web-like twisted structure (
self-aggregated into long fibers through hydrogen binding in bothlateral and longitudinal directions. The diameter of the fiber of theTCNF ranged from 200 nm to 1.5 m and that of the PCNF rangedfrom 90 to 800 nm. The lengths ofthe nanofibers were about severalmicrometers. During the freezing process, the nanofibers wouldgather around the edge of ice crystals and massive hydrogen bondsformed among the adjacent surfaces of nanocellulose in the followingdrying process. Consequently, the nanofibers interacted witheach other and eventually self-organized into larger-sized cellulosefibers or 2D sheet-like structures (Chen et al., 2014).Because freeze-drying process could cause nanocellulose toself-assemble and aggregate, preventing formation of hydrogenbonding among CNCs/CNFs would help retain the nanoscale dimensionof individual nanofiber. tert-butanol could limit nanocelluloseassembly and aggregation via providing steric hindrance to neighboringnanofiber surface hydroxyls from hydrogen bonding withanother nanofiber (Jiang & Hsieh, 2015). tert-butanol was addedto aqueous nanocellulose to obtain 0.05 wt% tert-butanol/water(1:1 v/v) suspension, which was rapidly frozen by liquid nitrogenand freeze-dried. The SCNC (Fig. 2e) and FCNC (Fig. 2f) selfassemblingfrom freeze-drying in tert-butanol/water formed intolong fiber in the longitudinal direction, instead of 2D sheet-likestructures aggregated from water. The lengths of some of the FCNCfibers were more than 1 micrometer. The diameter of the fiber ofthe SCNC ranged from about 60–330 nm and that of FCNC rangedfrom about 35–290 nm. The ultra-fine nanofibers interconnectedwith each other and self-organized into web-like entangled structures.The TCNF/PCNF was similar to that drying fromwater, buthadthinner diameter (Fig. 2g and h). Thus, the addition of tert-butanolshowed apparent inhibition of aggregation during the freeze dryingprocess. The entire length of the bundles was not easy to bedetermined with the SEM images due to the entanglement of longcellulosic chains and twisted morphologies of the nanofibers.SEM has been widely utilized for morphology characterizationof nanocellulose. Coating the surface of sample with gold or platinumis needed before imaging. But the coating would broadenthe nano-sized structure. Moreover, the cellulose nanofibers mightself-aggregate into thick nanofibers or be dried into thin sheetsduring the drying process. Thus, the real structures of nanocellulosecan not be clearly observed by SEM. Compared with traditionalSEM, the resolutions of AFM and TEM are higher, and coating thesample surfaces before observation is not needed. The TEM andAFM analyses can more clearly evaluate the size of single nanofiberand web-like twisted structure (
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