*3.7. Thermal Stability Analysis*

The thermal stability of each of MCC, MD-MCC, AH-NC, HH-NC, and US-NC was investigated by TGA, as shown in Figure 6. The onset of thermal decomposition temperature (Ton) corresponds to the beginning of degradation and the maximum decomposition temperature (Tmax) corresponds to the temperature of the maximum rate of degradation. The Ton, Tmax, mass loss at Tmax, and the char residuals at 500 ◦C are given in Table 2. Generally, the thermal degradation of MCC and NC occurred in two steps, as shown in Figure 6. The initial mass loss, which was caused by the evaporation of absorbed water, was observed below 120 ◦C and was slightly different for the different cellulose samples.

**Figure 6.** (**a**) Thermogravimetric analysis (TGA) and (**b**) derivative thermogravimetry (DTG) curves of MCC, MD-MCC, AH-NC, HH-NC, and US-NC.

As shown in Table 2, the Ton of each NC was less than that of MCC, which indicates that NC has decreased thermal stability. This decreased thermal stability might be attributed to the large number of cellulose chain segments that were damaged during the preparation of NC and formed low molecular chain segments and weak points in the cellulose chain on the surface of NC. At elevated temperatures, these low molecular chain segments and defects absorbed heat and thus began to degrade first, which resulted in the reduced thermal stability of NC. Furthermore, the nanoscale lateral dimensions of NC mean that NC has a higher surface to volume ratio than MCC and thus is heated more efficiently, which also decreases thermal stability [49]. Compared with AH-NC and US-NC, the lower thermal stability of HH-NC could be due to the increased damage to the crystalline region of cellulose during the high-pressure homogenization process as indicated by the XRD results [50].

The main thermal degradation stages of MCC, MD-MCC, AH-NC, HH-NC, and US-NC occurred in the range of 250–430 ◦C, 200–430 ◦C, 210–360 ◦C, 240–405 ◦C, and 250–405 ◦C, respectively; this degradation was mainly due to the thermal decomposition of the crystalline cellulose chains. The mass losses caused by the thermal decomposition of AH-NC, HH-NC, and US-NC at Tmax were 50.8%, 52.9%, and 50.4%, respectively, and the corresponding Tmax were 298 ◦C, 349 ◦C, and 365 ◦C. The mass losses of each type of NC were very similar but the corresponding Tmax varied significantly, which indicates that acid hydrolysis, high-pressure homogenization, and high-intensity ultrasonication had different effects on the thermal stability of NC. The sulfate groups that were added to the cellulose chains during acid hydrolysis could facilitate the thermal degradation of cellulose [51], which would explain why the Tmax of AH-NC was less than those of HH-NC and US-NC. Moreover, this result was consistent with the previous study by Wang et al. [36]. Above 450 ◦C, the thermal decomposition temperature of each cellulose sample leveled off and a slow thermal degradation profile was obtained. This continued degradation could be attributed to the carbonization of polysaccharide chains caused by the cleavage of C-C and C-H bonds.

The mass of char residue in AH-NC, HH-NC, and US-NC at 500 ◦C was 20.6%, 14.2%, and 8.5%, respectively, which are all greater than that of MCC (7.6%). The high char residue of AH-NC has been ascribed to the direct solid-to-gas phase transitions of decarboxylation catalyzed by the sulfate groups on the surface of AH-NC [52]. The high char yield of HH-NC might be due to its relatively high dehydration at low temperatures, which carbonized cellulose as confirmed by its relatively low degradation temperature. The low char residue yield of US-NC might be ascribed to its low crystalline content [2].
