###### Abstract  
2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) is a widely used high-energy explosive for the application of energetic materials. However, the phase transformation from \(\varepsilon\)-CL-20 to \(\gamma\)-CL-20 restrains its further application in polymer bonded explosives (PBXs) and propellants. To inhibit the phase transition of CL-20, dopamine was first used in an efficient and facile method of in situ polymerization to passivate CL-20 crystals. The core-shell microcapsule particles were obtained, and the morphological characterization demonstrates the formation of a dense core-shell structure. The differential scanning calorimetry (DSC) and in situ X-ray diffraction (XRD) test results show that the compact and dense coating delays the \(\varepsilon\)-CL-20 crystal transformation temperature by about 30 \({}^{\circ}\)C, which enhances thermal stability. In addition, with the coating via polymers, the friction sensitivity of \(\varepsilon\)-CL-20 crystals decreases significantly. The findings indicate a successful application of dopamine chemistry in high-energy explosives, which provides an attractive method to modify the properties of CL-20 crystals. energetic crystal; phase transition; CL-20; dopamine; core-shell structure 2020 10 2663 10.3390/app10082663 10.3390/app10082663 10.3390/app10082663 10.3390/app10082663 10.390/app10082663 10.  
sensitization of explosives to deflagration. The internal defect structure of explosive crystals adversely influences the mechanical and thermal sensitivity of polymer bonded explosives (PBX) [7; 8]. Therefore, the phase transition of CL-20 must be investigated. In recent years, the research on crystal phase transition of \(\varepsilon\)-CL-20 has mainly focused on two aspects: (1) the phase transition of \(\varepsilon\)-CL-20 during the recrystallization in a liquid-phase environment. Crystallization parameters, such as temperature, solvents, agitation rate, and the methods of adding solvents, can significantly affect the polymorphs and transition of \(\varepsilon\)-CL-20 [9; 10; 11; 12; 13; 14]. (2) Additives are incorporated via conventional coating or physical mixing to investigate the phase transformation of \(\varepsilon\)-CL-20 crystals. The additives usually have a high dipole moment or slightly dissolve CL-20, which can easily polarize \(\varepsilon\)-CL-20, thus promoting its transition to \(\gamma\)-CL-20 [15; 16]. Neutral polymers, such as glycidyl azide polymer (GAP), isocyanate desmodur (N100), ethylene propylene diene monomer (EPDM), and butadiene rubber (BR), inhibit the crystal change of CL-20 [6; 17]. Despite the conventional coating method improving the coverage of the explosive particle surface to a certain extent, this method cannot completely remove the defects of the particle surface coating [10; 11; 12]. Thus, new materials must be found with strong adhesion for modifying energetic crystal particles. Recently, mussel coatings inspired by dopamine in situ polymerization attracted interest and are usually used to passivate large amounts of substrates such as glass, metals, polymers, and nano-carbon materials [18; 19; 20; 21; 22; 23; 24; 25]. Dopamine chemistry was reported for crystal modification of energetic materials, such as HMX, RDX, and 1,3,5-triamino-2,4,\(\delta\)-trinitrobenzene (TATB) [11; 26; 27; 28]. However, few studies have reported the application of dopamine chemistry technology for CL-20 solid-solid phase transition studies. In this study, a convenient in-situ polymerization method was applied to prepare the core-shell \(\varepsilon\)-CL-20/polydopamine (\(\varepsilon\)-CL-20/PDA) microparticles. The obtained core-shell structure significantly improves the thermal stability, phase transition stability, and safety of \(\varepsilon\)-CL-20 crystal. Four kinds of dopamine materials were selected and employed for coating experiments. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used to confirm the firmness and compactness of the core-shell structure.