Graphene-Material-Modified PMMA Coated with 1,3,5,7-Tetranitro-1,3,5,7-tetraazacyclooctane

Abstract

Since the energetic material 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX) has potential safety hazards during its application, it was chosen to solve this problem by coating the surface of HMX through the self-polymerization reaction of methyl methacrylate (MMA). However, its mechanical properties were poor for further application, so graphene oxide (GO), hydroxylated graphene (GO_-OH), and reduced graphene oxide (rGO) were chosen to be doped into PMMA for coating modification. The properties were also investigated. The composite microspheres were regular in shape. Furthermore, it was observed that graphene materials were present on the surface of the microspheres, and no crystal transformation of HMX occurred during the process. The thermal stability of the composite microspheres was improved, and the activation energies of the HMX/PMMA/GO, HMX/PMMA/rGO and HMX/PMMA/GO_-OH composite microspheres were increased compared with those of the HMX/PMMA microspheres. At the same time, the high-energy dropout characteristics of the composite microspheres were improved, and the impact sensitivity of all microspheres was reduced, compared with that of the HMX/PMMA microspheres. The compressive strength of pillars pressed with composite microspheres increased by 1.91, 0.92 and 3.13 MPa, respectively. The mechanical properties of the composite microspheres were improved. As a result, HMX/PMMA composite microspheres have better properties.

1. Introduction

1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX) is an organic compound, a kind of violently explosive white crystalline powder. There are four kinds of crystal type—α, β, γ, δ—but actual applications use the stable β type at room temperature. With a melting point of 281 °C, a density of 1.96 g/cm³, no moisture absorption, and a high detonation speed, it is one of the best single-mass explosives, because of its high density and detonation speed and is widely used in military and civilian fields [1,2,3,4]. In the context of the rapid development of blunt projectile technology, HMX shows poor sensitivity to shock, friction, shock wave, thermal energy and electric spark, and does not meet the safety requirements of blunt projectile charge, thus limiting the application of HMX [5,6]. Out of these studies, the selection of excellent-performing binders to coat energy-containing materials has been shown to be an effective way to address the safety of energy-containing materials. Our research group found that PMMA-coated HMX can increase the safety of HMX, but there are also certain problems, such as the difficulty of casting the coated particles [7]. The main reason is that its mechanical properties are not suitable when it is used as a coating material for energy-containing materials. Therefore, modifiers were added to the PMMA to optimize the performance of the microspheres.

Polymethyl methacrylate (PMMA) is a spontaneous polycondensation of methyl methacrylate (MMA). Polymethyl methacrylate (PMMA) is a thermoplastic resin with a linear polymer structure that deforms when heated and retains a certain shape when cooled [8,9]. PMMA has smooth surface, low specific gravity, high strength, good mechanical properties, good stability, good chemical safety and aging resistance [10,11]. Jia et al. [12] prepared energy-containing microspheres with HMX as the inner core and PMMA as the outer wall. However, the high hardness of PMMA resulted in microspheres that could not be easily compressed and molded for application. Xinlei Jia et al. [13] achieved PMMA/PVA coating of three ammonium nitrate explosives (RDX, HMX, CL-20) through a novel molecular co-assembly model. Experimental results showed that the safety and thermal properties of the three ammonium nitrate explosives were effectively improved, and the addition of PMMA effectively improved the thermal stability of the high-energy materials, effectively solving the problems of exposed leakage and low coverage of the explosive particles.

Yuan yuan Cui [8] experimentally obtained thin PMMA films with graphene sheets attached and investigated the effect of oxidation of graphene nanofillers on the interfacial shear strength of a monolayer graphene/PMMA nanocomposite system and to assess the effect of the degree of functionalisation of graphene on the interfacial shear strength. It was shown that the optimisation of the degree of functionalisation of graphene nanofillers is important to maximise the strengthening effect of the composite. Skountzos et al. [14] modified PMMA with graphene (G) and graphene oxide (GO) as fillers. Results showed that adding GO increased composites’ elastic and shear modulus by 63% and 72%, respectively, compared with adding G. Graphene is a two-dimensional carbon nanomaterial with high electrical, thermal, and mechanical conductivity, being also dispersible in a polymer matrix [15]. Moreover, it is often used as an ideal filler for polymer modification. It contains carboxyl groups (-COOH) and hydroxyl groups (-OH) on its surface, which are more active than graphene. GO, a derivative of graphite, is also a two-dimensional substance, with excellent mechanical properties and that can form ordered and layered structures [16,17]. A functionalized graphene produced by functionalizing GO could perform better [18]. Among them, functional fossil graphene filling shows excellent mechanical properties in materials. It has both the flexibility of organic material and the mechanical strength of graphene [19]. Among them, hydroxylated graphene (GO_-OH) contains several hydrophilic functional groups (-OH), which improves the dispersibility of hydroxylated graphene. The GO_-OH structure has a complete aromatic ring structure and contains more sp² graphene domains, enhancing its hardness [20]. Reduced graphene oxide (rGO) is a two-dimensional monolayer graphite sheet composed of a single atomic layer of carbon atoms connected into a remeasured hexagonal crystal with a rudimentary structure, in that the sp2 orbital of each carbon atom forms three covalent bonds with three adjacent carbon atoms, forming 3σ bonds. The remaining P electron remains perpendicular to the surface of rGO, forming π bonds with surrounding atoms [20,21,22]. This material’s physical, surface area, and electrical properties are excellent [23].

There are also certain applications of graphene materials in energetic material. Li Rui et al. [24] prepared HMX/GO composites by a solvent-nonsolvent method. The results indicated that coating GO on the HMX surface could significantly reduce the overall mechanical sensitivity and improve the material’s activation energy and thermal stability. Wang Jingyu et al. [4] used GO and fluoroelastomer (Viton) to coat HMX and also prepared HMX/Viton/G and HMX/Viton composites. The results indicated that the HMX/Viton/GO composite particles’ thermal stability was higher than those of HMX and HMX/Viton. The frictional susceptibility of HMX/Viton/GO composite particles was also lower than that of HMX/Viton, and the analysis demonstrated that GO has a better susceptibility reduction effect than G.

In this paper, in order to solve the problem of poor mechanical properties of HMX/PMMA microspheres, which cannot be widely used, we chose to perform a self-polymerization reaction by adding graphene material into MMA, relying on the interaction force between MMA and graphene material, and realizing the rapid polycondensation on the surface of HMX crystals under the catalytic effect of the initiator to achieve the coating. HMX/PMMA microspheres and HMX/PMMA/GO, HMX/PMMA/rGO and HMX/PMMA/GO_-OH composite microspheres were prepared by emulsion polymerization. Various aspects of their properties were tested and analyzed. It was shown that the incorporation of the three graphene materials had a modifying effect on the PMMA-coated HMX, which effectively improved the performance of the microspheres. The overall results provide a reference for the study of PMMA-coated energetic material.

2. Experiment

2.1. Molecular Dynamics Simulations

All molecular dynamics simulations were carried out on the forcite module of Materials Studio Software (MS 20.1.0.2728). HMX, PMMA and graphene materials were arranged in cubic periodic boxes. The stabilized constructions in the equilibrium tracking file were elected from the COMPASS perspective to calculate the binding energy. First of all, using the morphology module, the main growth crystal plane of HMX in a vacuum was forecast to be (0,1,1), chosen as the adsorption plane to construct a 4 ∗ 2 ∗ 3 supercell. According to the mass ratio of the binder to HMX (M_(HMX):M_(PMMA) = 98:2), a PMMA binder model was added to the simulation system to build a structural model (Figure 1). Then, the structure was optimized in the NPT environment using the amorphous cell module and forcite module in MS(20.1.0.2728) software. Molecular dynamics simulations were run on the model. After thermodynamic equilibrium was reached, the binding energy data between the different components were obtained by analyzing and calculating the single-point energy of the equilibrium system.

2.2. Materials

The company Gansu Yinguang Chemical Industry Group (Baiyin, China). provided the HMX; Methyl methacrylate (MMA) came from Shanghai Maclean Biochemical (Shanghai, China). Graphene oxide (GO) was provided by Suzhou Hengqiu Technology (Suzhou, China). Azobiso-butyronitrile (AIBN) was provided by Tianjin Guangfu Chemical Research Institute (Tianjin, China). Polyvinyl alcohol (PVA) was obtained by Qingdao Yusuo Chemical Technology (Qingdao, China) Twain 80 was purchased from Tianjin Damao Chemical Reagent Factory. Span-80 came from Tianjin Tianli Chemical Reagent. (Tianjin, China) Taiyuan Iron and Steel. (Taiyuan, China) supplied pure water.

2.3. Preparation of HMX Composite Energy-Bearing Microspheres

HMX was coated by an emulsion polymerization method through PMMA. The mass ratio of HMX to PMMA was controlled at 95:5, in which the amount of graphene modifier was 2% of PMMA. First, the HMX suspension was prepared: a certain quantity of HMX, graphene materials (mass fraction of 2%), compound emulsifier 0.02 g (M_{Twain 80}: M_{Division 80} = 5:5), and deionized water, poured into a beaker, stirred, and ultrasonically treated for 30 min, dispersing the HMX suspension evenly.

In the second step, a certain amount of MMA monomer was poured into the beaker with the dispersant (PVA) and initiator (AIBN). The as-obtained mixture was treated with ultrasound until the initiator was completely dissolved.

The HMX suspension was then poured into a flask. The prepolymer solution obtained in the second step was added to the flask using a constant-pressure funnel, the stirring speed was controlled at 300 rpm and the temperature was set at 70 °C. After a period of reaction, the black particles precipitated from the flask and the solution became transparent. Finally, the resulting solution was filtered and dried for 6 h to obtain the microsphere composites. Figure 2 illustrates the formation scheme and mechanism of these microspheres.

2.4. Sample Characterization and Performance Test

2.4.1. Scanning Electron Microscopy (SEM)

Samples prepared by Czech field emission scanning electron microscopy were characterized with a voltage of 10 kV, N₂ was used as a protective gas, and gold was sprayed on the samples for filming.

2.4.2. X-ray Powder Diffraction Test (XRD)

The DX-2700 X-ray powder diffractometer was used to test the crystal shapes of the raw material and the prepared samples. The operating voltage was 40 kV, the current was 30 mA is 30 mA and the Cu target was chosen. The scanning range was 5°~55°, the step angle was 0.05°, and the sampling time was 0.5 s.

2.4.3. Thermal Performance Test (DSC)

The thermal decomposition performance of the sample was tested by a French DSC131 differential scanning calorimeter, and the test conditions were as follows: N₂ of 30 mL/min was used as the protection gas, the sample mass was (0.4 ± 0.1) mg, the test temperature range was 50 °C~340 °C and the heating rates were 5 °C/min, 10 °C/min, 15 °C/min and 20 °C/min, respectively.

2.4.4. Impact Sensitivity Test

The shock susceptibility of the sample was tested with 12 drop-hammer instruments. Test conditions: the test temperature was 10~35 °C, the weight of a falling ball was (2.500 ± 0.002) kg, the weight of the dosage was (35 ± 1) mg, the relative humidity was less than 80% and each group contained 25 rounds.

2.4.5. Static Mechanical Properties of the Column

Each microsphere was pressed into a Φ10 ∗ 15 mm drug column, and the mechanical properties of the drug column were tested With a Changchun Qilin Instrument Equipment Co., Ltd., (Nanjing, China) WSM-10KN high- and low-temperature electronic universal test machine. The test parameters were: Temperature of 20 °C, test speed of 0.5 mm/min and humidity of 14%

3. Results

3.1. HMX/PMMA Composite Microspheres Simulate Binding Energy

The binding energy could provide an opportunity to assess the compatibility between the binder and the main energetic material in the composite microspheres and the stability of the associated simulation system. The larger the value of the binding energy, the better the compatibility and stability in the simulation system. The formula for calculating the binding energy is as follows: E_bind = −(E_total − E_explosive − E_poly). The binding energy of the HMX/PMMA composite microsphere system was calculated as shown in

Table 1. Simulated binding energies of different HMX/PMMA composite microspheres (kJ/mol).

Sample	Etotal	Eexplosive	Epoly	Ebind
HMX/PMMA	−10,917.88618	−11,239.6831	358.674404	36.877492
HMX/PMMA/rGO	−10,369.4602	−11,223.22927	945.94211	92.173036
HMX/PMMA/GO	−10,749.50348	−11,216.60923	586.315372	119.209619
HMX/PMMA/GO-OH	−10,870.49072	−11,210.80889	462.019731	121.701564

below.

Based on the comparison of the binding energies calculated by the four analog systems, it was found that compared with the HMX/PMMA model, the binding energy of the graphene-material-added models increased in the following sequence: HMX/PMMA < HMX/PMMA/rGO < HMX/PMMA/GO < HMX/PMMA/GO_-OH. This indicated that the models with added graphene materials were improved, among which the model with added GO_-OH was the most stable. The addition of GO_-OH causes HMX and PMMA molecules to bind tightly, resulting in a stronger interaction between the three materials.

3.2. Morphology Analysis

In Figure 3a, the graphene material binds to PMMA and co-covers the HMX surface. The specific spherical formation process of HMX/PMMA composite microspheres is as follows: in the blending system of MMA and graphene materials, the addition of emulsifier reduces the original surface tension of water molecules, so that MMA exists in a dispersed form in solution, and after the addition of further initiator AIBN, MMA rapidly carries out junction and agglomeration on the surface of HMX under the catalytic action, due to the existence of intermolecular interaction between MMA and graphene materials, so that the graphene material and the PMMA formed by polycondensation are coated on the surface of the HMX molecule together. The SEM micrographs of the composites are displayed in Figure 3.

In Figure 3b. The raw material HMX particles are irregular in shape and unevenly distributed. Figure 3c–e present the GO, rGO and GO_-OH, respectively. In the coating process, the MMA monomer polymerizes under the action of the initiator to form PMMA-coated core HMX, forming a dense coating layer. Here, the coating layer grows thicker as the process continues, thus obtaining spherical HMX/PMMA particles. The composite microspheres were regular, and the particle size distribution was narrow. The dispersion of the particles is detailed in Figure 3f. Compared with the smooth and regular particles in Figure 3f, a stacking phenomenon occurred due to the different interaction forces of the surface functional groups of graphene modifiers. Moreover, the weak interaction force of GO surface functional groups cannot support the graphite oxide lamellae, and the stacking phenomenon occurs, resulting in the rough surface of the composite particles shown in Figure 3g–i and the existence of the cladding layer. The obvious layered folds indicated that different graphene modifiers were prepared in flake morphology on HMX/PMMA microspheres. As shown in Figure 3, the graphene material was combined with PMMA and co-covered the HMX surface. Also, comparing Figure 3g–i and the composite microspheres prepared in Figure 4a, the HMX/PMMA/GO_-OH composite particles had fewer surface defects. The spheres were dense without obvious leakage, and the cladding effect were more satisfactory, compared to Figure 3g,h.

3.3. Crystal Structure Analysis

From Figure 4, it can be observed that raw material HMX showed diffraction peaks at 14.708°, 16.029°, 20.543°, 23.040°, 31.913°, corresponding to the (0 1 1), (0 2 0), (−1 0 2), (−1 2 0), (−1 3 2) crystal surfaces of β-HMX (PDF#00-44-1620).

Figure 4. XRD spectra and FT-IR spectra of raw HMX and prepared composite samples: (a) XRD spectra of samples; (b) FT-IR spectra of samples.

Figure 4. XRD spectra and FT-IR spectra of raw HMX and prepared composite samples: (a) XRD spectra of samples; (b) FT-IR spectra of samples.

Meanwhile, the diffraction peaks of the sample correspond to those of the raw material HMX one-to-one. However, PMMA is amorphous, and the formation of cladding on the surface of HMX intensified the dispersion of the X-rays of HMX: the height of the diffraction peaks was weakened, and the peak width was broadened. In addition, GO and GO_-OH displayed diffraction peaks at 13.3° and 27.5°. Furthermore, the low graphene content caused the characteristic peaks of graphene materials in the XRD patterns of composites to be not obvious. The overall analysis showed that the crystal structure of HMX did not change during the wrapping process, and the PMMA and graphene materials were successfully wrapped on the HMX surface.

The structure of the raw materials HMX, graphene modifier and the HMX-based composite microspheres were further studied by FT-IR. It can be seen from Figure 4 that the HMX-based composite energy-containing microspheres prepared by the emulsion polymerization method include nearly all the tensile vibration peaks of HMX and the binder PMMA, which is consistent with the XRD test results. The raw material HMX has consecutive absorption peaks in the absorption region of 2900–3100 cm⁻¹, which are contained in the telescopic vibrational peaks of -CH₂ on HMX and the resonance peaks generated by the hydrogen bond formed between O and H in -NO₂, 1522 cm⁻¹ and 1297 cm⁻¹, which are the vibrational characteristic peaks of telescopic N-NO₂, and 1572 cm⁻¹ and 1267 cm⁻¹, respectively, for the characteristic peaks of the stretching vibration of N-NO₂, 1460–1490 cm⁻¹ with the characteristic peaks of the deformation vibration of -CH₂- and -CH₃, respectively, and 1390 cm⁻¹ and 1290 cm⁻¹ are the peaks of the stretching vibration of C-H. Further analysis found that the spectral bands of the four composite microspheres in the FT-IR spectrum contain the characteristic telescopic peaks of HMX and PMMA, indicating that PMMA covered the surface of HMX. Compared with HMX/PMMA microspheres, the FTIR curves of the four composite particles prepared with graphene material addition has changed. For HMX/PMMA/GO samples, the vibration characteristic peaks of GO appeared at 3000~4000 cm⁻¹ and 1620 cm⁻¹, respectively, indicating that GO was present in composite particles. Also, in HMX/PMMA/GO_-OH, the peak intensity of the vibration characteristics of -OH increased, indicating that the content of hydroxyl groups was increased, further indicating the existence of GO_-OH in composite particles. Compared with HMX/PMMA, the peak strength of the HMX/PMMA/rGO spectrum was weaker, and the peak type remained unchanged. This may be because the telescopic vibration peaks in rGO’s FTIR curve were so weak that they would not affect the surface of the prepared sample. Therefore, XRD and FT-IR results confirmed the successful preparation of HMX-based composite microspheres.

3.4. Thermal Analysis

Thermal stability is an essential trait of energetic materials [25]. Therefore, a DSC-131 differential scanning calorimeter was used to analyze the thermal performance of raw HMX and the as-prepared HMX-based composite microspheres. Figure 5 shows the DSC curves measured at four heating rates. It was found that the HMX and the HMX-based samples showed similar trends at different heating rates. In the decomposition curve, there is an insignificant small heat absorption peak near 200 °C, where the HMX crystal transitions from β to δ crystalline form. Comparison of Figure 5a with Figure 5b–d yields that, due to the significant autothermal autocatalytic effect of HMX, the incorporation of PMMA coating weakens the thermal feedback and redox reaction of HMX decomposition, which leads to the weakening or even disappearance of the heat-absorbing peaks of the crystallization transition in the composite microspheres. It can be seen from Figure 5 and

Table 2. Decomposition peak temperatures of raw HMX and HMX/PMMA composite microspheres.

Sample	5 °C/min	10 °C/min	15 °C/min	20 °C/min
Raw HMX	280.82	284.74	286.86	289.59
HMX/PMMA	283.92	286.66	289.22	291.32
HMX/PMMA/GO	287.24	291.32	293.25	294.97
HMX/PMMA/GO-OH	282.75	286.34	289.13	290.75
HMX/PMMA/rGO	285.55	288.74	290.06	293.25

that HMX and HMX-based PBX samples have similar change trends at different heating rates, and the peak temperature of thermal decomposition increases correspondingly with the increase of heating rates for the same samples.

At the same time, according to the heat fraction liberation of samples at different heating rates, the Kissinger formula, Rogers formula and Arrhenius formula were used to calculate each peak temperature T_p0, and thermal decomposition apparent activation energy E_a, the pre-exponential factor A and HMX heating rate approaching 0, respectively [26,27]. The critical temperatures of thermal explosion T_b are shown in

Table 3. Thermal decomposition kinetic parameters of different samples.

Sample	Ea/(kJ·mol−1)	Log(A)	Tp0/°C	Tb/°C
Raw-HMX	409.25	38.51	272.69	274.22
HMX/PMMA	423.57	39.72	277.28	278.81
HMX/PMMA/GO	468.74	43.64	279.07	280.47
HMX/PMMA/rGO	436.39	40.95	278.72	280.22
HMX/PMMA/GO-OH	474.30	44.34	276.75	278.11

.

$$\ln (\frac{{\beta }_i}{T_{pi}{}^2})=\ln (\frac{AR}{E_a})-\frac{E_a}{R{T}_{pi}}$$

(1)

$$T_{pi}=T_{po}+\mathrm{a}{\beta }_i+b{\beta }_i{}^2+c{\beta }_i{}^3$$

(2)

$$T_b=\frac{E-\sqrt{E^2-4RE{T}_{p0}}}{2R}$$

(3)

$$A\exp \frac{-Ea}{RT}=\frac{k_{B}T}{\mathrm{h}}\exp \frac{-\Delta G^{\ne }}{RT}$$

(4)

$$\Delta H^{\ne }=E_{\mathrm{a}}-RT$$

(5)

$$\Delta S^{\ne }=\frac{\Delta H^{\ne }-\Delta G^{\ne }}{T}$$

(6)

β is the heating rate (K/min or K/s). T_p0 is the decomposition peak temperature of explosive at heating rate β, K; A is the the pre-exponential factor, min⁻¹ or s⁻¹; R is the gas constant, 8.314 J/(mol·K); E_a is the apparent activation energy, J/mol.

It can be observed from Table 3 that the apparent activation energies and the pre-exponential factors of HMX-based composite microspheres were elevated to different degrees. Free energy of activation (ΔG≠), activation enthalpy (ΔH≠) and activation entropy (ΔS≠) can also be obtained from Equations (4)–(6). The thermodynamic parameters of thermal decomposition for all samples were calculated and analysed, and the results are shown in Figure 6. The ΔG≠, ΔH≠ and ΔS≠ of the three samples increased to different degrees but the ΔG≠ of the samples did not change much, which could be attributed to the strong self-heating and autocatalytic effect of HMX, which was less affected by the coated composite, resulting in a small change in ΔG≠ [13]. Compared with HMX, the thermal properties and activation energies of the composite particles were improved. Also, we can see in Figure 6 that there is a “kinetic compensation effect” between E_a and InA for the three elemental explosives, with all data points falling on the regression line. This shows that when HMX was covered as a core material, the binder PMMA decomposed and produced many free radicals at increasing temperatures. Many free radicals could increase the thermal stability of composite particles.

Moreover, the thermal stabilities of the composite microspheres prepared by adding samples GO, rGO, and GO_-OH were further strengthened compared with that of HMX/PMMA. That was because, after the addition of graphene material, the graphene material combined with PMMA to form a protective layer on the surface of HMX, and this cladding layer hindered the heat transfer between HMX crystals and alleviated their thermal degradation rate. At the same time, the surface of graphene material contained a large number of oxygen-containing functional groups (phenolic, hydroxyl, epoxide, carboxylic acid), which could undergo a strong exothermic decomposition reaction during heating. This is because during the decomposition process, the graphene material affects the heat transfer process between HMX crystals, which mostly exists in a flake structure, which may hinder the heat transfer process. Therefore, among the three modifiers, the thermal performance of HMX/PMMA/GO_-OH composite microspheres has higher thermal stability.

3.5. Impact Sensitivity Analysis

To test the safety performance of HMX-based composite energy-containing microspheres, the samples were tested for impact sensitivity. The results of the characteristic drop test of HMX-based composite energy-containing microspheres are detailed in Figure 7.

The characteristic drop heights of composite particles were increased and the sensitivity of HMX was decreased. As already detailed, PMMA wrapped around the HMX surface completely, thus reducing the stress concentration and deformation of HMX under the external mechanical stimulation of the falling hammer, which decreases the probability of hot spot appearance and propagation. After adding graphene substances, the contact between HMX and the surrounding media was reduced under external forces, and the heat generated was roughly distributed in the sheet structure of graphene substances, resulting in a decrease in the number of hot spots. At the same time, because GO_-OH has more edge hydroxyl groups, it showed a strong mechanical interlocking ability with the binder PMMA, making the cladding material PMMA/GO_-OH more complete. Therefore, under mechanical stimulation, the cushioning effect of the cladding layer was more obvious, so the impact sensitivity of the HMX/PMMA/GO_-OH. microsphere has been greatly improved.

3.6. Static Mechanical Test Analysis

To gain more insight into the mechanic performances of HMX-based composite microspheres, the microspheres were pressed into a pillar column, and the compression performance of the pill was tested. The pressure-elongation curve and stress-strain curve are shown in Figure 8a,b, and the static mechanical property parameters of composite energetic microspheres in Figure 8c.

In Figure 8a,b, it is shown that the mechanical property curves of HMX-based composite microspheres exhibited a trend of first increasing and then decreasing. The pressure and compressive strength maximum can be obtained from the above two curves (Figure 8). Further combined with the SEM results, this indicates that the HMX-based composite microspheres have a regular shape so that more microspheres can be inserted into the same volume of columns when press-fitting the microspheres. The compactness of the pressed pillar was also enhanced; this required stronger force for debonding and thus improved the pillar’s mechanical properties. Analyzing the mechanical properties of the pressure columns, it was found that there are contact and interaction between composite microspheres during the static mechanics test. For HMX/PMMA microspheres, although PMMA was viscoelastic and could bond HMX particles and transfer stress under pressure, PMMA showed the disadvantage of being hard and brittle, resulting in HMX/PMMA not having very much pressure resistance under continuous pressure [21]. From Figure 8c compared with the HMX/PMMA column, the maximum tolerance of the HMX/PMMA/GO, HMX/PMMA/rGO and HMA/PMMA/GO_-OH columns was improved. Among them, the improvement in the HMX/PMMA/GO_-OH pillar was the most obvious, with a maximum strength of 443.40 N and a compressive strength of 5.21 MPa.

Moreover, using three modifiers, GO, rGO and GO_-OH, dispersed in PMMA, resulted in a link between the graphene-based material and PMMA, inhibiting cracks when PMMA was compressed externally, resulting in greater pressure required to deform the drug column. In PMMA and rGO composites, the polymerisation of MMA occurs between the rGO lamellar structures. PMMA and rGO form an interlayer structure, which increases the distance between rGO lamellar structures, but the uneven dispersion of rGO in PMMA leads to a reduction in the surface area for interaction between rGO and PMMA and a decrease in stress transfer efficiency, so the mechanical properties of the composite microspheres are less effective when rGO is used as a modifier [28]. In GO and PMMA composites, hydrogen bonds are formed between the functional groups of PMMA and GO, and the interfacial strength increases to a certain extent. The stretching of PMMA chains under pressure and the subsequent gradual breaking of the hydrogen bonds between GO and PMMA help to increase ductility and toughness, but the number of hydrogen bonds formed is limited because only ester functional groups in PMMA can act as hydrogen bond acceptors, so the addition of GO can only improve the mechanical properties of the composite microspheres to a certain extent. Among the three graphene substances, GO and GO_-OH have more functional groups on the surface, the functional groups can strengthen the interlocking effect between the graphene substance and PMMA [29] and GO_-OH retains the original continuous conjugated structure of graphene while introducing hydroxyl groups, and its mechanical properties are well preserved. Thus, the mechanical properties of HMX/PMMA/GO_-OH and HMX/PMMA/GO were relatively significantly improved.

4. Conclusions

In this paper, using the self-polymerization reaction of MMA, three graphene materials were added to the PMMA-coated energetic material HMX. Compared with the PMMA-coated HMX alone, the overall microsphere performance was significantly improved, and the addition of GO_-OH improved the safety, thermal and mechanical properties of the energetic material, solved the problem that the HMX/PMMA microspheres could not be compressed and molded and provided a basis for the research and further application of PMMA-coated energetic material. Comprehensive testing and analysis showed that HMX/PMMA/ GO_-OH composite energy-containing microspheres have better safety properties. Due to their thermal and mechanical properties, HMX/PMMA/GO_-OH composite energy-containing microspheres can be widely used in the field of energy-containing materials.