**3. Results**

## *3.1. Chemical Structure*

FT-IR spectroscopy was used to investigate changes in the chemical structure of cellulose sample before and after the treatments. The spectra displayed the intensity of absorption of the functional groups between 4000 and 400 cm<sup>−</sup>1, which can identify the chemical bond in the cellulose molecule. The absorption bands for characteristic chemical groups of the raw material (native BC) and the treated

celluloses (DP-BC and BC-NC) can be observed in Figure 1, and the typical vibration bands are listed in Table 1. The FT-IR spectra were divided into two parts: (1) H-bonding region from 4000 to 2600 cm<sup>−</sup>1, and (2) fingerprint region from 1800 to 400 cm<sup>−</sup><sup>1</sup> [46]. The broad peak in the 3650–3000 cm<sup>−</sup><sup>1</sup> bands was assigned to O–H stretching vibrations, which are characteristic of the hydroxyl groups generally present in cellulose, water, and lignin. In this region, intramolecular hydrogen bonds appeared at 3342 cm<sup>−</sup><sup>1</sup> and 3432 cm<sup>−</sup>1, and were attributed respectively to the two crystalline cellulose allomorphs, cellulose Iα and cellulose Iβ [47].

**Figure 1.** Fourier Transform Infrared (FT-IR) spectra of: 1. native Bacterial Cellulos (BC); 2. depolymerized cellulose, depolymerization of Bacterial Cellulose (DP-BC), (optimum conditions of ultrasonic irradiation step); 3. extracted crystalline regions, bacterial cellulose nanocrystals (BC-NC) (the best results of the catalyzed hydrolysis treatment).

**Table 1.** Typical vibration bands for the Fourier Transform Infrared (FTIR) spectra of the cellulose samples Bacterial Cellulos (BC), depolymerization of Bacterial Cellulose (DP-BC) and bacterial cellulose nanocrystals (BC-NC).


According to Börjesson and Westman [48], these hydroxyl groups were responsible for the stiffness in the polymer chain, and for allowing the linear polymers to form sheet structures. The strong vibration band around 2895 cm<sup>−</sup><sup>1</sup> corresponded to C–H stretching vibrations [49]. This band may be associated with a hydrocarbonate linear chain. Higher values in this specific band are correlated to a decrease in the calculated total crystallinity value [50]. An intense band at 1429 cm<sup>−</sup><sup>1</sup> band can be assigned to the bending of asymmetric angular deformation of C–H bonds. The band found between 1420 to 1430 cm<sup>−</sup><sup>1</sup> was associated with the amount of the cellulose ordered form, while the band appearing at 898 cm<sup>−</sup><sup>1</sup> was assigned to the disordered region [51]. The 1163 cm<sup>−</sup><sup>1</sup> band was assigned to asymmetrical stretching of C–O–C glycoside bonds.
