Nanomaterials 2022 , 12 , 790
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networks of fibrils start to separate due to the shear forces at higher stirring speeds. In addition, in E-CNF hydrogel in which gelation has occurred after homogenization, clusters begin to break. Then, it is observed that increasing stirring speed, an increase in H s /H o is produced that indicates the E-CNF structure is more open and spongier with the dissolution of clusters. The fact that relative height was in the denominator of Equation (2) makes thatØ g decrease during this first period until a certain velocity gradient value, in which theH s /H o is maximum. Then, at higher speeds, H s /H o decreased because the hydrogen bonds between fibrils would break them and, therefore, the networks would be destroyed. In addition, the mechanical forces during agitation would shorten the length of the fibrils that are individual separated, compacting them in the base of the graduated cylinders and, as a consequence, increasing Ø g . However, the optimal stirring speed to obtain as many dispersed fibers as possible without breaking the network is different for each hydrogel. Analyzing them one by one, C-CMF show the highest Ø g studied (Figure 3), due to cotton that was ground in powder obtaining the lowest aspect ratio during all agitation stages, as Figure 4 shows. The high crystallinity of the sample with a high rigidity makes the sample not produce a network of fibers without an increase in the sponginess with the stirring. The AR in C-CMF is almost invariable with a maximum obtained at very low agitation conditions, with a velocity gradient around 10 s − 1 associated to the initial agglomeration of the sample. This hydrogel without any pretreatment contains mainly microfibrils with scarce branches from the primary structure due to the powder state of the sample, so the crossover network is hardly possible and at low agitation the effect of fiber breakages is already evident. However, moderate agitation produces a strong shortening of the fibers as aspect ratio shows. On the contrary, the R-CMF shows a higher number of branches around the cellulose backbone due to the pulp was refined as pretreatment before the homogenization, this mechanical treatment produce an increase in the AR compared with other pretreatments as TEMPO- mediated oxidation or enzymatic hydrolysis [53]. A greater crosslinking of networks as in R-CMF would produce, at low agitation speeds, the increase in the sponginess stretching the networks as it shakes. A higher velocity gradient is required to obtain the spongier network, above 500 s − 1 . Then, the high hydrodynamic conditions would produce the breakage of refined microfibrils separating them from the main structure and at the same time the shortening of fibrils. As for CNF hydrogels, R-CNF obtained from the same raw material than R-CMF but pretreated with TEMPO-mediated oxidation instead of refining, shows a minimum Ø g around 50–100 s − 1 , less agitation than R-CMF. This is because TEMPO-mediated oxida- tion produces electrostatic repulsion between microfibrils and the breakages of cellulose chains, favoring the fibers separation with the breakage of primary structures. Both effects would reduce the crossover between networks and, therefore, at lower stirring speeds the breakage of the fibers is observable. Finally, the other CNF hydrogel, E-CNF with an almost total nanofibrillation, shows the minimum Ø g around 500 s − 1 . Despite the higher nanofibrillation yield of E-CNF, the number of carboxyl groups is less than R-CNF due to a lower NaClO content in the TEMPO-mediated oxidation, decreasing the repulsion of fibrils. This fact would produce less breakage and separation of fibers and, therefore, E-CNF keep a greater crosslinking of the fibers that are more difficult to separate mechanically requiring more energy. In addition, in E-CNF is not produced a great variation of deposits at very high stirring speeds, probably due to the almost total nanofibrillation of the fibers that prevents a great variation of heights from occurring. 3.2. Validation of Gel Point Methodology to Quantify Nanocellulose Dispersion To verify the shortening of the fibrils and the AR, the two hydrogels from the same raw material were selected in order to see the effect of refining and TEMPO pretreatments on the dispersion degree. TEM images of R-CNF and R-CMF were taken at different stirring speeds and magnifications from 400 × to10,000 × . Diameter distributions in logarithmic
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