## 3 Results and Discussion
### Phase Transition Stability of \(\varepsilon\)-CL-20 Composite Structures  
Figure 9 shows the standard spectra of four polymorphs of CL-20 and the X-ray spectra of \(\varepsilon\)-CL-20 at various temperatures. In the diffraction angle (\(2\theta\)) ranging from 12\({}^{\circ}\) to 14\({}^{\circ}\), the two crystal forms of CL-20 are significantly different. In comparison with the peaks at 12.8\({}^{\circ}\) and 13.8\({}^{\circ}\) of \(\varepsilon\)-CL-20, the new peaks appear at 12.9\({}^{\circ}\), 13.3\({}^{\circ}\), and 14.2\({}^{\circ}\) for \(\gamma\)-CL-20 [6]. According to Section 2.2, four kinds of CL-20 samples coated with different polymers were prepared, and the phase transition behavior of CL-20 in the composite system was characterized by in situ XRD [4]. Figure 10 shows the X-ray patterns of \(\varepsilon\)-CL-20/PDA, \(\varepsilon\)-CL-20/PLD, \(\varepsilon\)-CL-20/PNE, and \(\varepsilon\)-CL-20/POHDA at different temperatures. The phase composition of the \(\varepsilon\)-CL-20 crystals was calculated via the Rietveld refinement method through the Topas Academy procedure, which is standardless X-ray diffraction [29]. The refinement fit test results are presented in Figure 11.  
Figures 9 and 10 show that with increasing sample temperature, the characteristic peak of \(\varepsilon\)-CL-20 gradually weakened, and the characteristic peak of \(\gamma\)-CL-20 appeared and increased, indicating more \(\varepsilon\)-CL-20 changed to \(\gamma\)-CL-20. The XRD patterns did not change when the temperature decreased from 190 to 30 \({}^{\circ}\)C, which demonstrated an irreversible process for the crystal form change in this situation. As shown in Figure 11, the initial crystal transformation temperatures of \(\varepsilon\)-CL-20, \(\varepsilon\)-CL-20/PDA, \(\varepsilon\)-CL-20/PLD, \(\varepsilon\)-CL-20/PNE, and \(\varepsilon\)-CL-20/POHDA are 145, 175, 160, 160, and 170, \({}^{\circ}\)C, respectively. With the temperature increasing to 190 \({}^{\circ}\)C, we found that almost all \(\varepsilon\)-CL-20 of \(\varepsilon\)-CL-20, \(\varepsilon\)-CL-20/PLD, and \(\varepsilon\)-CL-20/PNE converted to \(\gamma\)-CL-20. However, the \(\gamma\)-CL-20 contents of \(\varepsilon\)-CL-20/PDA and \(\varepsilon\)-CL-20/POHDA were 37.9% and 73.3%, which indicated that PDA and POHDA could inhibit the CL-20 crystal transformation well. Figure 9: X-ray diffraction (XRD) patterns of (**a**) the conformers from four polymorphs of CL-20 and (**b**) \(\varepsilon\)-CL-20 at various temperatures. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{5}{c}{**Endothermic Peak**} \\ \cline{2-5}  & \(T_{o}\) (\({}^{\circ}\)C) & \(T_{p}\) (\({}^{\circ}\)C) & \(T_{\epsilon}\) (\({}^{\circ}\)C) & \(\Delta H\) (\(\mathbf{Jg^{-1}}\)) \\ \hline \(\varepsilon\)-CL-20 & 144.5 & 156.3 & 171.4 & \(-\)41.3 \\ \(\varepsilon\)-CL-20/PDA & 172.3 & 179 & 187.1 & \(-\)28.1 \\ \(\varepsilon\)-CL-20/PLD & 157.6 & 165.3 & 174.7 & \(-\)35.8 \\ \(\varepsilon\)-CL-20/PNE & 164.8 & 172.5 & 184.5 & \(-\)32.9 \\ \(\varepsilon\)-CL-20/POHDA & 173.4 & 176 & 185.2 & \(-\)30.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Thermal analysis data of epsilon-CL-20 (\(\varepsilon\)-CL-20) and different composites. Different from the crystallization from a solution, no polymorphic transition occurs when the medium is a solid matrix with a solvent. The activation energy for forming a new crystal phase in a solid environment is much higher than in a liquid environment, leading to a more complicated polymorphic transformation process in the solid matrix. The intense interfacial interaction between the PDA and the explosive crystal results in the high polycrystalline transition temperature. The polymorphic conversion could be reduced greatly via PDA coating, especially when the nucleation processes is the rate-limiting step [33]. Figure 11: Phase analysis of \(\varepsilon\)-CL-20 and different composites.