## 3 Results and Discussion
### Effect of CNC Contents on the Photopolymer Composites  
#### 3.2.1 Mechanical Properties of CNC/Photopolymer Composites with Various CNC Loading Levels  
The tensile strength of 0.25 wt% CNC/photopolymer composite was 30 MPa, indicating a 20% increase in tensile strength compared to the photopolymer composite without CNC particles (Figure 12). However, the incorporation of more than 0.25 wt% of CNCs did not show an improvement in tensile strength of the composites, but rather reduced the strength values. The tensile modulus of 0.25 wt% CNC/photopolymer composite was 2474 MPa, and it had 20% higher tensile modulus than the neat photopolymer composite (Figure 13). However, CNC/photopolymer composites with higher CNC loading levels (i.e., 0.5 and 1.0 wt%) showed lower tensile modulus values compared to the neat photopolymer composite, resulting in 5.7% and 10.3% reductions in modulus values, respectively.  
This tendency seemed to be due to the difference in the reinforcement effect of the CNC dispersion. The high tensile strength value at a relatively low CNC loading level (i.e., 0.25 wt%) more effectively accommodated the effects of the dispersion mechanism. This is probably because the loaded CNC particles had smaller and more evenly distributed size and shape, leading to even and uniform dispersions, and consequently served as an efficient stress carrier. However, elongation at break values tended to decrease with an increasing amount of CNCs, indicating that the excessive CNC additions resulted in the lowered ductility and increased stiffness of the photopolymer composites (Figure 14). When a stress was transferred to the CNC particles, the rigid CNCs were subjected to a significant amount of stress. This effect allowed the less stiff photopolymer to accept a small percentage of residual load through the CNC particles. With 180\({}^{\circ}\) beta-acetal bonding of carbon atoms, the CNC is inherently rigid, which provides adequate structural stability to mechanical resistance and meets the demand for intermolecular hydrogen bonding for higher stability [33; 34; 35].  
Figure 11: Tensile modulus of CNC/photopolymer composites as a function of lyophilized CNC concentration and CNC loading.  
Figure 14: Elongation at break of CNC/photopolymer composites with various CNC loading levels.  
Figure 12: Tensile strength of CNC/photopolymer composites with various CNC loading levels.  
Figure 13: Tensile modulus of CNC/photopolymer composites with various CNC loading levels.  
this very high stiffness, the CNC particles can sustain the additional stress transferred from the polymer matrix. The variation in mechanical performance between 0.25 wt% CNC and higher CNC loading levels (i.e., 0.5 and 1.0 wt%) can be explained by the free volume principle of the composites. As the CNC concentration increases, the volume fraction of the CNC increases, but the free volume decreases and the distance between adjacent CNCs decreases. As a result, they may accept inadequate polymer chains with limited mobility, which significantly affects the fracture mechanism of the photopolymer composites. In addition, the various mechanical performances observed in the photopolymer composites with more than 0.25 wt% CNC may affect the CNC orientation in the photopolymer resin network. Depending on the completeness of the CNC alignment in the resin polymer, its contribution to mechanical strength can vary, which is a factor that can be drastically changed by using DLP 3D printing.