## 3 Results and Discussion
### Preparation and Structures of CCNF  
In this study, a colloid mill was used to split CC of 2% in a suspension into cellulose nanofibers under mechanical action, instead of the traditional high-pressure homogenizer [35]. In the processing parts of the colloid mill, there is a high-speed rotating rotor and a stationary stator matched. The processing material is ground in a spiral motion from the inlet to the outlet of the grinding area and is driven by its own weight or the pressure of the external pump through its clearance between the rotor and the stator. Different from the pressure-type homogenizer, the colloid mill is a kind of centrifugal equipment, in which a powerful vortex shear force of friction is generated. In this way, it can effectively crush particle material to achieve the effect of ultra-fine grinding. Therefore, it has the advantage of simple structure, simple equipment maintenance, and low energy consumption, which is suitable for processing high viscosity materials and large particles materials. As the amino group of CC can capture protons to become cations in an acidic medium [36], the electrostatic repulsion between cationic CC molecules will contribute to the mechanical splitting of cellulose during colloid milling. The pH of 2% CC (DS = 0.123) suspensions was adjusted with or without dilute sulfuric acid to 1, 3, and 7, respectively. After colloid milling, the pH value was basically unchanged due to the low content of the CC suspension. As expected, the zeta potential of the supernatant increased as the pH decreased, as shown in Table 2. Also, Figure 4 clearly shows SEM photos of CCNF from the supernatant after centrifugation of CC suspensions, which had been colloid-milled at different pH values. CCNF diameter decreased with the decrease of pH of the CC suspension ground (Table 2). It is because that the amino -NH\({}_{2}\) of -OCONH\({}_{2}\) on CC is transformed into a cation of -NH\({}_{3}\)\({}^{+}\) under acidic conditions. Due to the repulsive interaction of the same cation charges, cellulose chains were more likely to be cleaved under the action of high shear force. The results also confirmed the conclusion that the ionized modified cellulose pulp is easier to be split up, such as anionic modification [19] and cationic modification with N-(2-3-epoxypropyl) trimethylammonium chloride [37]. Figure 3: Solid-state \({}^{13}\)C-NMR spectra of cellulose and cellulose carbamate (DS = 0.123). The crystalline structures of a polymer can be obtained by X-ray diffraction (XRD). The XRD patterns of cellulose and modified cellulose are shown in Figure 5. The diffraction peaks of eucalyptus lignocellulose are 15.74\({}^{\circ}\), 22.53\({}^{\circ}\), and 34.41\({}^{\circ}\) in 20 angles, corresponding well to the crystalline form of cellulose I [38; 39]. The CC with DS 0.123 and CCNFs ground at different pH values had a similar diffraction pattern to the original cellulose, indicating that the carbamation occurred mainly at the surface and in amorphous regions, without greatly affecting the inner crystalline structure of cellulose. However, the crystallinity decreased with the processing steps (carbamation and colloid milling), and also with the decrease in pH in the colloid milling, as presented in Table 2. This can be ascribed to two reasons: First, it can be due to that the crystal of the original cellulose disintegrated during the processes, but no new recrystallization occurred. Second, with decreasing pH in the colloid milling, more hydroxyl groups of cellulose were substituted, which affected the inner crystalline structure of cellulose.