Nanomaterials 2023 , 13 , 2536
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Figure 6. Photographs of obtained CNF hydrogels. Thus, the aspect ratio of TEMPO CNFs is notably lower than the others and is de- creased with the NaClO dose. Figure 3 shows that only a few microfibrils in TEMPO are detected by optical microscopy after HPH (shown as small dots) and these become fewer and fewer as the intensity of homogenization increases, as D2 indicates with its decrease due to fiber individualization. However, Figure 4 shows a CNF network being responsible for forming a gel, as evidenced by the photograph in Figure 6 for TEMPO-15. The fact that none of the obtained nanocelluloses were transparent indicates that all of them have some microfibrils or nanocellulose aggregates that dispersed visible light. However, Figure 6 shows, the opacity of the gel suspension was lower in the case of TEMPO-15, as expected from the lower number of microfibers and higher amount of nanofibrillated cellulose and nanocrystals. The thin and short nanofibers observed in Figure 4 for TEMPO-15-CNF-5 account for the low aspect ratio values of these CNFs (Table 6). The sensitivity of cellulose to mechanical forces during the HPH process was increased by the cellulose oxidation and the removal of lignin due to the effect of NaClO during TEMPO-mediated oxidation. The soft yellow color of TEMPO-pretreated CNFs indicates lignin removal during the treatment. In fact, at the first stage of the reaction, NaClO is mainly consumed in lignin oxidation [45]. As a result, an intense HPH treatment caused the highest cutting effect, which accounts for the lowest aspect ratio (around 7) obtained for TEMPO_15-CNF-5, suggesting that the sufficiently intense TEMPO-mediated oxidation followed by an intense HPH process produces cellulose nanocrystals (CNCs), which have lower aspect ratios than CNFs [42]. According to Serra-Parareda et al. 2021, CNCs have D2 values under 1.4–1.5, whereas CNFs have higher D2 [42]. With this criterion, as Figure 5 shows, TEMPO_15-CNF-5 is in the CNC scale while TEMPO_15-CNF-3 and TEMPO_15-CNF-4 range from CNCs to CNFs. 3.3. Effect of Homogenization Sequence on the Properties and Morphology of Cellulose Nanofibers The homogenization pressure and cycles were varied in this study for the isolation of different CNF qualities. The final goal is to optimize the production of CNFs based on the desired morphology and properties for specific applications in the future. Figures 3 and 4 show the significant influence of pretreatment on the nanofibrillation process of the pulps under different HPH intensities. The Mec pulp suffered minimal changes during HPH process, except for some internal fibrillation as shown in Figure 4. Although the fiber wall structure after enzymatic treatments exhibited limited signs of external fibrillation, this fibrillation noticeably increased with the intensity of subsequent mechanical treatment using HPH (Figure 3). Furthermore, Figure 4 shows some internal
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