## 3 Results and Discussion
### Characterization of CNCs Properties
#### 3.1.2 FTIR and Raman Spectra of Dried Cellulose Nanocrystals  
Figure 1a reports the FTIR spectra of the three CNCs samples. Spectrum relative to \(\mathrm{CNC_{COOH}}\) sample showed a peak at 1743.76 cm\({}^{-1}\) that can be attributed to the carbonyl stretching in esters. The presence of this peak is a strong proof of the successful functionalization of the cellulose nanocrystals during the process of chemical modification. All three hydroxyl groups of the cellobiose unit are reactive and can participate to the esterification reaction; moreover, it is worth reporting that in situ esterification used in this work can occur on CNCs surface or inside crystals [23; 32]. After studying the kinetic reaction rate of oxidation and hydrolysis of cellulosic raw materials (hybrid poplar) during CNCs production by APS, [PERSON] et al. (2017) reported that, compared to hydrolysis, the oxidation takes much longer (\(>\)10 h) to occur [33]. Therefore, in this work, where raw cotton linters and \(\mathrm{CNC_{SO3H}}\) were put together, it was assumed that the CNCs esterification occurred within 10 h (i.e., a color shift considered to be an indicator of esterification occurred after 2 h) following three steps in this order: (1) rapid oxidation of more reactive -OH groups of already-present crystals \(\mathrm{CNC_{SO3H}}\) to introduce carboxylates groups on their surface forming \(\mathrm{CNC_{COOH}}\); (2) cotton linters hydrolysis which makes reactive -OH groups more available for the esterification; (3) competition between esterification and oxidation. Esterification takes place between \(\mathrm{CNC_{COOH}}\) (1) and a part of reactive -OH groups (2), while the residues of reactive -OH are oxidized to form -COOH, which may not participate in the esterification, as they cannot find free reactive -OH groups. We are testing the same process using CNCs prepared in different ways and, combined with other raw materials, controlling the time of the esterification. In the future, it would also be very important to investigate the ideal proportions between the raw materials and the CNCs, and to evaluate the degree of substitution during the esterification. Although the esterification involves hydroxyl groups (-OH), the signal at around 3350 cm\({}^{-1}\) does not change significantly, because it can be hypothesized that many -OH groups do not contribute to the esterification. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**CNCs Properties** & \(\mathrm{CNC_{SO3H}}\) & \(\mathrm{CNC_{COOH}}\) & \(\mathrm{CNC_{COOH}}\) \\ \hline Hydrodynamic diameter (nm) & 130.55 \(\pm\) 2.35 & 176.78 \(\pm\) 3.52 & 175.02 \(\pm\) 0.53 \\ \hline \(Z\) potential at pH (mV) & \(-\)29.60 \(\pm\) 0.71 & \(-\)35.65 \(\pm\) 0.21 & \(-\)36.30 \(\pm\) 0.42 \\ \hline Conductivity (mS cm\({}^{-1}\)) & 1.31 \(\pm\) 0.05 & 0.82 \(\pm\) 0.04 & 1.37 \(\pm\) 0.07 \\ \hline Polydispersityindex (\%) & 27.53 \(\pm\) 0.51 & 25.24 \(\pm\) 0.48 & 25.3 \(\pm\) 0.53 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Properties of dispersed cellulose nanocrystals. From the Raman spectra (Figure 1b), the band at 900 cm\({}^{-1}\) in the spectra of cellulose is known to be associated with increased disorder [31], and therefore it was hypothesized that CNC\({}_{\text{COOH}}\) were less ordered, due to their negative charges. Moreover, the regions (1200-1400 cm\({}^{-1}\)) are attributed to -CH\({}_{3}\) bending and -COH in-plane bending motions. The bands in that region were sharper and less intense in CNC\({}_{\text{SO3H}}\), with respect to CNC\({}_{\text{COOH}}\) and CNC\({}_{\text{COOH}}\). More importantly, calculated X785-Ram values, which represent the degree of crystallinity, were 63.7%; 62.64% and 60.14% for CNC\({}_{\text{COOH}}\) and CNC\({}_{\text{SO3H}}\), respectively. Such similar values showed that the esterification did not affect the degree of crystallinity of the CNCs significantly.