## 3 Results and Discussion
### X-Ray Diffraction Spectrum  
The X-ray diffraction patterns of the cellulose films are shown in Figure 2. The diffraction peaks at 20 = 22.6\({}^{\circ}\), 16.4\({}^{\circ}\), and 14.9\({}^{\circ}\) indicate that the crystal phase of the original cellulose (MCC) was cellulose type I. The crystal structure of cellulose was not significantly changed using water and acetic acid as the dissolution solvent, but cellulose type I was not observed in the CTA (cellulose dissolved by TA), CQA (cellulose dissolved by QA), and CNA (cellulose dissolved by NA) samples. The regenerated cellulose films, CTA, CQA, and CNA, exhibited the typical diffraction patterns of cellulose type II at 20 = 21.2\({}^{\circ}\) and 20.3\({}^{\circ}\). The results indicate the phase transformation from cellulose I to cellulose II after the dissolution and regeneration in QA RTILs. This phenomenon was also reported in another solvent system for cellulose. The crystallinity index (CrI) defined by Segal is applied to assess the degree of crystallization of the cellulose, as follows [20; 21]:  
\[\text{Cr}\left(\%\right)=\left(1-\text{I}_{\text{AM}}/\text{I}_{200}\right) \times 100 \tag{1}\]  
where I\({}_{200}\) is the peak intensity of the major crystallization peak (002) in the diffraction patterns, containing both the diffraction intensity of the crystalline and the amorphous regions and I\({}_{\text{AM}}\) is the intensity of the amorphous region. The diffraction peaks of I\({}_{200}\) and I\({}_{\text{AM}}\) for cellulose type I are at 22.6\({}^{\circ}\) and 18\({}^{\circ}\), respectively. The peaks at 20 = 20.3\({}^{\circ}\) and 16.2\({}^{\circ}\) represent the crystalline and amorphous regions of the cellulose type II, respectively. According to the above equation, the degree of crystallization of the regenerated cellulose films was estimated, as shown in Table 2. The hydrothermal or acid treatments only slightly reduced the crystallinity of cellulose without changing its crystal phase. This result indicates that QA RTILs can rapidly break intermolecular and intramolecular hydrogen bonds and destroy the original crystalline structure. The crystallinity index of cellulose II is in the order of CNA-5 \(<\) CTA-5 \(<\) CQA-5, which is consistent with the viscosity of ILs. The kinetic energy of  
Figure 1: Polarized light microscopy images of cellulose dissolution in N\({}_{6222}\)OAc at 50 \({}^{\circ}\)C at different times: (**a**) 0; (**b**) 1; (**c**) 5; (**d**) 10; (**e**) 16, and (**f**) 20 min.  
the dissolved cellulose molecules is lower in viscous ILs, so the recrystallization rate for cellulose II in QA is much higher than that in NA. The diffraction pattern of the CNA-5 film showed an amorphous structure. The effect of the cellulose content in NA on crystallinity was also investigated in this work. The type II crystallinity index increases with an increase in cellulose content. In the range of 2 to 10 wt % cellulose content, the degree of crystallinity for cellulose II increased slightly with the increase in cellulose content in ILs. As the cellulose content reached 12 wt %, the crystallinity index rapidly increased to 49%. The NA solution with 12 wt % cellulose became very viscous, and it reached a saturated or supersaturated state, leading to a rapid recrystallization reaction rate and hence could not be used as a proper coating solution. Therefore, the cellulose content in NA should be kept under 10 wt %. Moreover, 10 wt % cellulose can also be completely dissolved in NA at room temperature after two h of dissolution, but this regenerated cellulose film exhibits both cellulose I and II crystal structures (CrI = 22%, CrII = 15%). This indicates that moderate heating enhances the phase transformation of cellulose from type I to type II in the dissolution process.