3.3.1. Fourier Transformed Infrared Spectroscopy

FTIR spectroscopy was performed to identify the functional groups of the polymers and the specific interactions between poly(ClAETA) and CNF, as well as to understand how this reinforcement interferes with the hydrogel base.

Figure 3a shows the characteristic signal of CNF, where the characteristic band of the -OH stretching (3319 cm−1), band for -CH stretching (2900 cm−1), and the band with the signal of the -C=O (1610 cm−1) indicate the occurrence of -COOH groups in CNF, suggesting oxidation in the glucose ring of the hydroxyl group at the C-6 position [41,42]. Figure 3b,c shows the characteristic signals for two synthesized hydrogels, Hy06 and Hy10, corresponding to NH stretching (3357 cm<sup>−</sup>1) and disappearance of the vinyl group (C=C) (1200 cm−1), respectively. The carbon double bond signal disappeared, and the signals of the functional groups of the 2-(acryloyloxy) ethyl trimethylammonium chloride monomer (ClAETA) remained, corroborating the formation of the polymer [43,44]. The signals indicating the stretching of CH in the polymer (2977 cm−1), the carbonyl bond

(C=O) (1736 cm−<sup>1</sup> and 1726 cm−1), and the quaternary ammonium group (-N+ (CH3)3) (1500 cm<sup>−</sup>1) were also observed [44].

**Figure 3.** FTIR analysis of (**a**) CNF, (**b**) Hy06 (hydrogel without CNF), and (**c**) Hy10 (hydrogel with CNF).

3.3.2. Morphological Analysis by Scanning Electron Microscopy

The microstructural changes in the hydrogels were analyzed using SEM, and the micrographs are shown in Figure 4.

**Figure 4.** SEM images of Hy05 (2 wt% CNF) samples at (**a**) ×1000, (**b**) ×2000, and (**c**) ×5000 magnification. Scale bar: 10 μm.

Figure 4 shows the structure of Hy05, which contained 2 wt% CNF. Separated fibers were observed on the sample surface; it is known that the surface of poly(ClAETA) (without CNF) has a smooth morphology, as reported in previous studies [43]. CNF was partially carboxylated, where the carboxylate groups interacted electrostatically with the quaternary ammonium groups present in the polymer, according to the SEM images. The CNF fibers were confirmed to be homogeneously distributed in the hydrogel network. Hy05 presented uniform roughness across the surface of the material; in addition, this physical mixture of CNF with the polymer chains helped in the stability and compactness of the structure [45]. In prior research studies, it has been reported that if one of the polymeric components has

negatively charged functional groups, the formation of porosities is facilitated because of the repulsion forces between charges [40].
