## 3 Results and Discussion  
### Effect of CNC Lyophilization Concentrations  
#### 3.1.1 Morphological Properties of CNCs Processed at Different Lyophilization Concentrations  
The SEM images showed that the morphology of the CNC powder, which was obtained by lyophilization processes, was a large-sized agglomerate at low and high concentrations (Figures 3 and 4). Cellulose nanocrystals with nanoscale units have a very large specific surface area and high surface energy, resulting in CNC self-agglomerations during processing [31]. Also, in the rapid-freezing process of CNC suspension, water quickly \begin{table}
\begin{tabular}{c c} \hline \hline
**Printer** & MiiCraft U125 \\
**XY Resolution** & 65 μm \\
**Wavelength** & 405 μm LED \\
**Base Layer** & 1 Layer \\
**Base Curing** & 5000 ms \\
**Buffer Layer** & 5 Layers \\
**Gap Adj.** & 100 μm \\
**Layer Curing** & 1500 ms \\
**Layer Thickness** & 50 μm \\
**Cleaning** & Tap water \& Ethyl alcohol \\ \hline \hline \end{tabular}
\end{table} 
Table 3: 3D printing parameters and scale values. Figure 2: Pictures of DLP 3D printer (exterior and interior photos) and 3D-printed composites. releases heat and solidifies to a crystalline state at \(-80\)\({}^{\circ}\)C. In this process, the molecular arrangement of water forms ice crystals, and the growth direction of ice crystals goes from the outside of the container to the inside. This is consistent with the previous findings that as the crystals grow, the arrangement of water molecules in the vessel changes, the volume of the solvent expands, and subsequently, the CNCs suspended in the solvent are subject to compressive stress due to the volume expansion of the surrounding ice crystal growth [32]. The CNCs are then aligned in the chiral-nematic phase, forming and concentrating lamellae. As a result, CNC lamellae bundles are formed by the self-aggregations of CNCs and the compressive stresses generated during cryogenic freezing. Figure 5 shows a schematic diagram of the above-described mechanism. Figure 4: SEM images of lyophilized CNCs processed at a high concentration: (**a**) \(\times\)100 and (**b**) \(\times\)200. Figure 3: SEM images of lyophilized CNCs processed at a low concentration: (**a**) \(\times\)100 and (**b**) \(\times\)300. This aspect of CNC microlamellae formation depended on the concentration of the CNC suspension being frozen. The higher the concentration of the CNC suspension, the shorter the physical distance between the CNCs and the higher the possibility of self-aggregation. Thus, when frozen, the ice crystal growth caused more condensation due to the physical compressive force, forming a larger and thicker lamella (Figure 4). SEM images (\(\times\)100) showed that CNC particles formed at a high CNC lyophilization concentration (2.2%) were \(>\)500 \(\upmu\)m, whereas CNC particles formed at a low CNC lyophilization concentration (1%) were 100-200 \(\upmu\)m. In addition, the high magnification SEM image (\(\times\)300) showed that single-layered thin lamellae were observed in the CNC particles lyophilized at the low concentration. However, the CNCs lyophilized at the high concentration were thicker, formed particles consisting of multiple layers, and showed CNC agglomerations on the lamella surface. These thick and large CNC particles, which were formed during the lyophilization process, could be the reason for the relative reduction of the filler effect in the composite matrix. 1.2 Morphological Properties of CNC/Photopolymer Composites Processed at Different CNC Lyophilization Concentrations  
The SEM images of the fracture surfaces of CNC/photopolymer composites showed that the composites with CNCs processed at a low CNC lyophilization concentration (1%) had smoother surfaces than the composites with CNCs processed at a high CNC lyophilization concentration (2.2%) (Figures 6-9). Additionally, comparing agglomerated CNC particles on the matrix, the composites with CNCs processed at the low concentration showed smaller and more uniform shapes of CNC particles.