Nanomaterials 2023 , 13 , 1931
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The suspensions obtained after the different pretreatments and the mechanical ho- mogenization at several intensities were characterized. The samples pretreated by refining have an proportion of solids of around 1–1.1% in the suspensions and the carboxyl group content was 0.46 ± 0.03 mmol/g pulp. In the case of pulps pretreated by TEMPO-mediated oxidation, the solid content was 1.1–1.2% and the carboxyl group content increased up to 1.16 ± 0.04 mmol/g pulp which suggested good fiber oxidation, with a remarkable increase similar to other studies [63,65,76]. The pulp had a higher content of ashes due to the production of salts as a parallel reaction during the oxidation [89]. In addition, the action of the oxidant removes part of the insoluble lignin and dissolves the cellulose at a higher proportion than the hemicellulose [64,90]. On the other hand, in neither of the pretreatments was a variation in the carboxyl groups observed with the intensity of the homogenization. Table 2 shows the transmittance and aspect ratio of the obtained CMNF suspensions. Transmittance gives an idea of the homogeneity of the CMNFs fibers. When a sample has been highly fibrillated, the minimum possible diameter is reached, and the suspension becomes optically transparent at a test concentration (0.1 wt.%), with a transmittance near to 100% [91,92]. Aspect ratio refers to the relationship between the length and the width of the fibers. It is a fundamental parameter used to describe the morphology and physical properties of CMNFs, providing valuable information that influence their mechanical, thermal, and optical properties [72,93]. Samples obtained by a refining pretreatment have a low transmittance associated with a low light pass and the presence of more microfibrils than nanofibrils. In the case of chemically pretreated samples, the proportion of nanofibrils is higher and the value increases with the intensity of homogenization. It is interesting to point out that the pretreated sample with 5T-oxidation without homogenization (PS0), shows too high a transmittance value due to the sedimentation of the fibers, since the cellulose has not been subjected to any type of defibrillation. As expected, results of the aspect ratio show an increase with the intensity of homogenization. Comparing both pretreatments, it is observed that in the case of refining, the fibers are separated from the primary structure producing more ramified fibers which increase the aspect ratio values. In the case of 5T-Oxidation, besides cellulose oxidation, the oxidant also produces the breakage of the cellulose chains into smaller units as reflected in lower aspect ratios [72].
Table2. Transmittance and aspect ratio of CMNF suspensions.
Transmittance
Aspect Ratio
Homogenization Intensities
Refining 4.5 ± 0.3 4.6 ± 0.4 5.5 ± 0.3 9.5 ± 0.5
5T-Oxidation
Refining 86 ± 5 127 ± 5 127 ± 6 128 ± 5
5T-Oxidation
PS0 PS1 PS2 PS3
39.1 ± 2.2 22.9 ± 0.1 27.6 ± 0.6 40.7 ± 0.2
43 ± 2 76 ± 3 101 ± 7
103 ± 8 Micrographs of the suspensions have been carried out using OM (Figure 4) and TEM (Figure 5) at 5 × and1000 × magnification, respectively. At least five images of each microscopic technique were taken. In addition, the variety of magnifications allows us to also evaluate the degree of homogeneity of the fiber size. The microfibrils after the refining pretreatment (PS0) showed a diameter range from 3 to 40 μ m as observed in the OM micrographs. However, TEM images show some thinner fibers of 100 nm but in a very small proportion. Only after six homogenization passes (PS2) was is possible to observe a representative number of nanofibers in the samples. After the most intensive mechanical treatment (PS3), the diameter range of the sample was reduced to a range between 20 nm and 6 μ m. In the case of the fibers treated with TEMPO-mediated oxidation, the diameter range without homogenization was more heterogeneous with micro and nanofibrils from 30 nm to 60 μ m. Only after the sample was subjected to the mechanical treatment (PS1) did cellulose start to fibrillate in a notable way, being mostly nanofibers.
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