## 3 Results and Discussion  
### Morphology and Characteristics  
The polymer content in the composites was tested by high-performance liquid chromatography (HPLC). Table 1 shows that the polymers concentrations for \(\varepsilon\)-CL-20/PDA, \(\varepsilon\)-CL-20/PLD, \(\varepsilon\)-CL-20/PNE, and \(\varepsilon\)-CL-20/POHDA composites were 1.6, 0.7, 1.3, and 1.4 wt.%, respectively. The morphologies of raw \(\varepsilon\)-CL-20 and four core-shell structures (\(\varepsilon\)-CL-20/PDA, \(\varepsilon\)-CL-20/PLD, \(\varepsilon\)-CL-20/PNE, and \(\varepsilon\)-CL-20/POHDA) are shown in Figure 3. For the raw \(\varepsilon\)-CL-20 particles prepared via a recrystallization process, the crystal shape was a more regular polyhedron (Figure 3f). The shape of the passivated \(\varepsilon\)-CL-20 crystal particles  
Figure 2: The schematic diagram of the preparation method of the core-shell structure. remains polyhedral. To compare the surfaces of raw \(\varepsilon\)-CL-20 and core-shell structures, SEM images with high magnification are shown in Figure 3a-e. The white CL-20 crystals gradually turned brown-black with progressing polymerization. After 24 h of reaction, the color of the crystal particles did not change, indicating the polymerization finished. As shown in Figure 3a-d, four polymers in a dense protective film formed on the surface of the \(\varepsilon\)-CL-20 particles. In comparison with the protuberance and cracks appearing on the surface of \(\varepsilon\)-CL-20 crystals (Figure 3e) caused by the electron beams during the SEM test, similar phenomena were significantly improved in the well-coated samples (Figure 3a-d). This reveals that even though the raw explosive crystals are extremely sensitive to electron beams, dense polymer films coated on the crystals could effectively improve the resistance to electron beams [25]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & \(\varepsilon\)**-CL-20 Content (wt \%)** & **Content (wt \%)** \\ \hline epsilon-CL-20 (\(\varepsilon\)-CL-20) & 100 & 0 \\ \(\varepsilon\)-CL-20/polydopamine (PDA) & 98.4 & 1.6 \\ \(\varepsilon\)-CL-20/poly-L-dopamine (PLD) & 99.3 & 0.7 \\ \(\varepsilon\)-CL-20/poly-norepinephrine (PNE) & 98.7 & 1.3 \\ \(\varepsilon\)-CL-20/poly-6-hydroxydopamine (POHDA) & 98.6 & 1.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Composition of core-shell microcapsules. Figure 3: Scanning electron microscopy (SEM) images for (**a**) epsilon-CL-20 (\(\varepsilon\)-CL-20)/polydopamine (PDA), (**b**) \(\varepsilon\)-CL-20/poly-L-dopamine (PLD), (**c**) \(\varepsilon\)-CL-20/poly-norepinephrine (PNE), (**d**) \(\varepsilon\)-CL-20/poly-6-hydroxydopamine (POHDA), and (**e,f**) \(\varepsilon\)-CL-20. The change in the electronic binding energy of the elements on the surface of \(\varepsilon\)-CL-20 determined by the XPS test provided valuable insights into the costing effectiveness. The C1s, O1s, and N1s spectra for \(\varepsilon\)-CL-20, PDA, and \(\varepsilon\)-CL-20/PDA samples are shown in Figure 4. The spectrum data of the other three groups (\(\varepsilon\)-CL-20/PLD, \(\varepsilon\)-CL-20/PNE, and \(\varepsilon\)-CL-20/POHDA samples) are shown in Figures 5-7. The percent contribution of the C1s, O1s, and N1s atom to \(\varepsilon\)-CL-20, PDA, and \(\varepsilon\)-CL-20/PDA samples were integrated and are presented in Table 2. The coating effect of explosive particles can be accessed via analyzing the N/C ratio on the particle surface [25]. The N/C ratio on the surfaces of PDA, PLD, PNE, and POHDA are 0.11, 0.13, 0.12, and 0.12, respectively. The N/C atomic ratio of CL-20 (1.06) is relatively higher, which is due to the existence of various \(-\)NO\({}_{2}\) and \(-\)NH\({}_{2}\) groups. After being coated with PDA, PLD, PNE, or POHDA, the N/C ratio decreased from 1.06 to 0.51,  
From the spectrum of PDA in Figure 4, four fitted C1s peaks centered at 284.5, 285.8, 287.9, and 291.2 eV are attributed to C-H or C-C, C-O, or C-N, C=O, and \(\pi\)-\(\pi\)* shake up.