## 3 Results and Discussion
### Effect of Inorganic Nanofillers on CNC/Photopolymer Composites  
#### 3.3.1 Mechanical Properties of Photopolymer Composites with CNC and/or Inorganic Nanofillers  
In comparison with the tensile strength of a pure photopolymer composite, the tensile strengths of the composites filled with inorganic nanofilers showed 32 MPa (28.6% increase) for nano-PCC and 35 MPa (40% increase) for nanoclay at 0.25 wt% loading level, respectively (Figure 16). It means that the effect of inorganic nanofilers was greater than that of CNC particles on the tensile strengths of composites at the same filler loading level, which indicated that the reinforcing effect of the inorganic nanofilers was excellent. In other words, theoretically the particle size and size distribution of initial cellulose nanocrystal materials are pretty good enough to reinforce DLP 3D-printed composites because of the length, diameter, and aspect ratio of rod-like CNC dispersions were 100-150 nm, 9\(\sim\)14 nm, and around 10, respectively. However, after freeze-drying process of the CNC dispersions, the lyophilized CNC forms were turned into micrometer scale as they were aggregated or agglomerated. Hence, the freeze-dried CNC particles did not act as good reinforcing nanofilers in filled composites. Also, the properly dispersed nano-PCC or nanoclay particles exhibited better compatibility with the polymer matrix by organo-modifying the surface of each particle, which prevented the self-aggregation of nanoscale particles. These kinds of things subsequently led to better mechanical strength properties. In particular, the nanoclay/photopolymer composite showed the highest tensile strength and tensile modulus compared to the nano-PCC/photopolymer composite (Figures 16 and 17), which can be explained by the shapes and dispersions of the inorganic nanofilers in the matrix. Nano-PCC and nanoclay particles have very small nano-units, and when they are evenly dispersed in composites, they provide remarkable toughness and rigidity to polymer resins, enabling the improvement of mechanical properties. However, these reinforcing effects resulted in somewhat different mechanical properties between nano-PCC/photopolymer and nanoclay/photopolymer composites. In the case of the nano-PCC/photopolymer composite, nano-PCC particles were evenly distributed in the polymer matrix, and these good dispersions were confirmed by SEM images. Also, the nano-PCC particles can increase the density of photopolymer composites and efficiently transfer loads or stresses to give the composites rigidity and toughness, resulting in better mechanical properties. Similar to the nano-PCC/photopolymer composite, the nanoclay particles in the nanoclay/photopolymer composite were also found to be evenly dispersed by SEM images, but the nanoclay had a plate-like shape (diameter <6 \(\upmu\)m) different from the nano-PCC (diameter 40 nm) featuring a cubic particulate shape. Hence, the nanoclay particles seemed to play a role in obstructing the direction of stress transfer occurring during tensile failure. In other words, the nano-PCC/photopolymer composite received an appropriate load and then quickly transferred it to other nearby nano-PCC particles, distributing the load and inducing a reinforcing effect. However, in the case of plate-shaped nanoclay, the received stress was high, but the residual stress to be delivered was low. In addition, the amount of stress dissipation within the composite was higher because the stress detoured to the platelet structure of the nanoclay, and it was repeated inside the composite. This is the structural difference between nano-PCC/photopolymer and nanoclay/photopolymer, which may be attributed to the higher aspect ratio of nanoclays [37]. The composites with both CNC and inorganic nanofillers had lower tensile strength values than the composites with inorganic nanofiller only. This is because the size and shape of the added CNCs was not as small as nanoscale and the CNCs was not evenly dispersed.