Nanomaterials 2023 , 13 , 2536
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from the smallest fines, as shown by the clean background of the optical image in Figure 2f. These CNFs are not visible with an optical microscope. Furthermore, some degradation of the amorphous part of the fibers by side-reactions can contribute to the external fibrillation (Figure 2f), which is in accord with the above. Furthermore, the TEMPO-mediated oxidation process resulted in the production of a significant quantity of fines and microfibers that decreased the values of coarseness and diameter. Some of the microfibers were too small to be clearly observed with an optical microscope and appeared as dots in Figure 2e. The optical images of TEMPO- mediated oxidized pulps reveal the treated fibers retain their structure prior to undergoing mechanical nanofibrillation. However, these images do not show the electrostatic repulsive forces occurring within the fibers, which are responsible for inducing nanofibrillation when shearing forces are applied. 3.2. Effect of the Pretreatment on the Properties of the Cellulose Nanofibers Table 6 shows the results of the quantitative characterization of different CNFs pro- duced using the highest intensity of HPH that corresponds to Sequence 5, as indicated in Table 3. GP was used to determine the aspect ratio of the fibers using crowding number (CN) theory [37,38,47].
Table 6. Properties of cellulose nanofibers produced by different pretreatments and followed by Sequence 5 HPH (3 passes at 300 bar + 3passes at 600 bar + 3 passes at 900 bar).
Yield (%)
Transmittance at 600 nm (%)
CD ( μ eq-g/g)
Aspect Ratio (GP)
Mec
16.2 28.0 30.0 41.4 62.3
15.2 26.3 28.9 38.5 59.8
202 233 228
173
Enz_80
91
Enz_240
109
TEMPO_5 TEMPO_15
1108 1908
47
7.3
The aspect ratio of the nanofibers in the obtained hydrogels depends on the pulp pretreatment. The mechanically pretreated fibers (refined pulp) showed the highest aspect ratio, approximately 173. Figures 3 and 4 provide visual qualitative evidence of the signifi- cantly high aspect ratio of this pretreated pulp compared to the others. They are optical (Figure 3) and TEM micrographs (Figure 4) of the twenty-five CNFs produced. Each CNF produced was analyzed by capturing twenty images, and each image in Figures 3 and 4 was selected as a representative from those twenty images. Figures 3 and 4 display, in the case of CNFs obtained by refining (Mec), the presence of highly fibrillated fibers longer than 500 μ m, with a structure thinner than those obtained through enzymatic degradation followed by HPH. Optical images provide a higher visual field, but with lower resolution than TEM images. For example, the fluff shown in Figure 3 must be interpreted with the information given by TEM images, which show the level of nanofibrillation achieved. Figure 5 shows the D2 of refining pretreated pulp at different HPH sequences compared to the other pretreatments. Under soft HPH, it is possible to observe a high D2 value, near 2.0, due to the fibrils joining to the cellulose backbone, forming bundles. Increasing the intensity of homogenization, a decrease is observed in the D2, associated with fibrils separation from the main structure. Both enzymatic hydrolysis and TEMPO-mediated oxidation pretreatments result in a decrease in fiber length and, consequently, aspect ratio compared to mechanically treated pulp. Enz_80 CNF suspension contains a substantial amount of short and thick fibers as D2 shows in Figure 5, with a lower value than for Mec. Figure 3 shows the length and Figure 4 shows the higher thickness of fibers that remained after the HPH process. A more intense enzymatic pretreatment, Enz_240, reduces the thickness, as evidenced by Figure 4, and the
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