3.2.4. Mechanical Properties

Tensile test was performed for ChNF, CNF and ChNF–CNF composites and the results are shown in Table 1. The tensile strength of the composite at first was 174.5 MPa (no ChNF), reached 224.0 MPa when the ChNF content was 10% and then decreased thereafter. Note that mechanical properties of the pristine ChNF is much lower than the CNF, i.e. more flexible than the CNF. Similarly, the yield strength of the composite was initially 111.7 MPa and reached its maximum of 149.0 MPa when the ChNF content was 10%. It was shown that the elongation at break of the composite increased from 2.02% to 4.17% and then decreased. It was shown that, when reinforced by ChNF, the tensile strength, yield strength and elongation at break of the composite improved. However, Young's modulus of the composite was not improved and slightly decreased due to the low modulus of ChNF. The modulus of ChNF is 7.3 GPa, while that of CNF is 16.9 GPa.

**Table 1.** Tensile test results of ChNF, CNF and their composites.


\* when number of ACC passes is 30.

Nevertheless, the overall mechanical properties of the composite were improved, which might be due to the reinforcement of ChNF to CNF in the composite. Most previous studies produced cellulose–chitosan composites by dissolving chitosan or cellulose and blending them [2,32]. However, in this research, we adopted the non-dissolving method to prepare ChNF–CNF composites, and a remarkable improvement of the mechanical properties of the ChNF–CNF composite was observed. There could be several mechanisms that can explain the mechanical properties improvement of the composite. The first one is improvement of the bonding sites between ChNF and CNF due to different size of ChNF and CNF. Note that the ChNF size was larger than that of CNF. The long length of ChNF in the CNF matrix and high surface areas of CNF created many reaction sites and bonding areas on their surfaces. Furthermore, the CNF is relatively stiff and ChNF is flexible. The mismatched mechanical properties of ChNF and CNF could give a room for managing the composite mechanical properties. Figure 8 shows the possible concept of reinforcement in the composite. By blending these two heterogeneous polysaccharide nanofibers, we can manage the mechanical properties of the composite. It is hoped that this idea could be a way to produce cellulose–chitosan composites with better mechanical properties without dissolving process of chitosan or cellulose.

**Figure 8.** A possible reinforcement mechanism of ChNF and CNF composite.
