**4. Conclusions**

High yields of NC with different aspect ratios were obtained efficiently from cationic MD-MCC by acid hydrolysis, high-pressure homogenization, and high-intensity ultrasonication and the effects of AH-NC, HH-NC, and US-NC on the fundamental properties of starch films were compared. The cationic modification of NC was confirmed by FTIR analysis. Both HH-NC and US-NC prepared from MD-MCC were stable suspensions because of their higher zeta potential compared to NC samples

prepared from MCC. The AH-NC prepared from MD-MCC tended to aggregate because of the presence of sulfate groups and, thus, lack of surface charge. The XRD analysis revealed that the crystallinity of NC decreased for all preparation methods whereas the main chemical structure of fibers remained unchanged. The thermostability of the three types of NC decreased relative to that of MCC. The HH-NC exhibited the best dispersion in the starch matrix and demonstrated the best enhancement to the water vapor barrier properties of starch films. Cationic modification of MCC will be a promising strategy to improve the yields and dispersion of NC and ultimately enhance the properties of starch films. The starch/NC nanocomposite films, which are completely biodegradable and biocompatible, have immense potential for food-packaging applications.

**Author Contributions:** L.H. conceived, designed, and performed the experiments; L.H. and W.W. analyzed the data and wrote original draft; R.Z., H.D. and J.L. wrote review and edited the original draft; L.K. and H.H. contributed reagents/materials/analysis tools; H.H. managed the project and provided financial support.

**Funding:** This work was supported by Great Innovation Program of Agricultural Application Technology in Shandong province, the Key Research and Development Program of Shandong province (2018GNC113004, 2019GNC106035), Natural Science Foundation of Shandong province (ZR2017BC018), the Foundation (No. KF201808) of State Key Laboratory of Biobased Material and Green Papermaking in Qilu University of Technology (Shandong Academy of Sciences), and Funds of Shandong "Double Tops" Program.

**Conflicts of Interest:** The authors declare no conflict of interest.
