3.2.2. FTIR

Figure 6 shows the FTIR spectra of the CNF, ChNF and ChNF–CNF composites with different ChNF concentration. The peak in the range of 3200–3500 cm−<sup>1</sup> of the ChNF–CNF composites was assigned to the hydrogen-bonded O–H stretching in both CNF and ChNF. Chitosan showed broader O–H bonded as compared to cellulose with higher intensity. Due to the combination of these two materials, with 20% chitosan, the intensity of the peak at 3200–3500 cm−<sup>1</sup> appeared similar to pure chitosan. The CNF showed stronger and higher peak of hydrogen-bonding than the ChNF. When ChNF and CNF were blended, peaks at 3000 and 3500 cm−<sup>1</sup> were reduced, which might be due to the intra-bonding between two materials. The peak at 1596 cm−<sup>1</sup> assigned for –C=O stretching is clear for CNF. The peak at 1736 cm−<sup>1</sup> was assigned for the C=O stretch of the –COOH group. This was only observed in the CNF at first, and the intensity of this peak decreased due to the addition of ChNF. At last, this peak became weak and disappeared when the ChNF concentration increased. This phenomenon indicates the interaction between ChNF and CNF. The peak at 1596 cm−<sup>1</sup> only appeared for cellulose and composites but not clearly shown for chitosan. This peak might be associated with the aromatic ring stretching of lignin or hemicellulose remained small amount in the bleached kraft pulp. From the FT-IR analysis, the structure of CNF and ChNFs were confirmed in the composites.

**Figure 6.** FTIR of CNF, ChNF and ChNF–CNF composites.

## 3.2.3. Crystallinity Index

The crystallinity index (CrI) of the original chitosan and ChNF were obtained by XRD and calculated according to Equation 1 as shown in Figure 7a. The crystalline peaks of chitosan appeared at 9.7◦ and 19.7◦. The peak at 19.7◦ decreased by the ACC treatment, while the strong peak at 9.7◦ increased, which might be associated with the incorporation of bound water molecules of α-chitin [38]. It means that a higher ACC treatment leads to more water molecules bound on hydrophilic surface of ChNF. The CrI values of the original chitosan and ChNFs with 10, 15 and 30 ACC passes were shown to be 65.4%, 44.0%, 44.0% and 36.2%, respectively. The CrI of chitosan highly degraded after ACC treatment. Note that 30 ACC passed ChNF was used for the ChNF–CNF composite preparation.

**Figure 7.** Crystallinity index of: (**a**) ChNF with number of ACC passes; and (**b**) CNF and ChNF–CNF composites.

The XRD patterns of the ChNF–CNF composites are shown in Figure 7b with different ACC passes. As one can see, cellulose peaks at around 15.8◦ and 22.7◦ were clearly shown and as the ChNF content increased, chitosan peaks were slightly appeared by interfering with cellulose peaks. Thus, CrI of the composites was calculated mainly by the dominant cellulose peaks at 22.7◦. The CrI values for low ChNF content composites slight improved at low chitosan content (ChNF3 and ChNF5) from 63% to 65%, and then it slightly decreased to 58% with addition of ChNF content. This might be due to good miscibility between ChNF and CNF.
