*3.1. E*ff*ect of Preparation Method on the Yield of NC Prepared from MCC and Modified Microcrystalline Cellulose (MD-MCC)*

Low NC yield is a limiting factor in industrial production and commercial applications. The yields of NC prepared from MCC and MD-MCC acid hydrolysis, high-pressure homogenization, and high-intensity ultrasonication are shown in Table 1. The yield of AH-NC prepared from MD-MCC was 30.63% which increased by 48.1%, compared with that prepared from MCC. Moreover, the yields of HH-NC and US-NC prepared from MCC were only 2.04% and 3.57%, respectively, whereas the yields from MD-MCC were 33.08% and 14.18%; these correspond to increases of approximately 16.2 and 3.97 times, respectively. These results demonstrate that cationic etherification of MCC could significantly increase the NC yield of different preparation methods, especially high-pressure homogenization and high-intensity ultrasonication. The NC yields of AH-NC and HH-NC prepared from MD-MCC are comparable to other reports of NC obtained under the similar conditions [30,31], and it can be increased with the optimization of the treatment conditions; however, there is minimal information on the effect of separate ultrasound processing on NC yield as ultrasonication has primarily been used as an auxiliary processing method.


**Table 1.** Effect of preparation method on the yield, zeta potential, and average particle size of NC prepared from MCC and MD-MCC.

Natural cellulose is insoluble and tends to agglomerate in water as it forms an extensive network of intermolecular and intramolecular hydrogen bonds, which can clog valves in homogenizers. Thus, the HH yield of NC prepared from MCC is typically low [32]. During the preparation of US-NC, ultrasonication affected the surface to the inner amorphous regions of MCC and caused MCC to break into submicron fragments instead of directly forming NC. However, the small size of the MCC fragments impeded the ultrasonication process and resulted in the low NC yield [12,18]. Conversely, acid molecules can rapidly penetrate into the inner amorphous regions of the cellulose fibrils where they disintegrate amorphous regions, reduce the size of the cellulose fibers, and ultimately release cellulose nanofibrils [33]; thus, AH-NC had a higher yield than HH-NC or US-NC. Considering these processes, cationic modification of MCC can substantially affect NC preparation by acid hydrolysis, high-pressure homogenization, and high-intensity ultrasonication methods.
