Preparation and Characterization of a Novel Cocrystal Explosive

ARTICLE pubs.acs.org/crystal

Preparation and Characterization of a Novel Cocrystal Explosive Jin P. Shen,† Xiao H. Duan,† Qing P. Luo,† Yong Zhou,‡ Qiaoliang Bao,§ Yong J. Ma,† and Chong H. Pei*,† †

State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology, Mianyang 621010, P. R. China ‡ Eco-materials and Renewable Energy Research Center (ERERC), School of Physics, and National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China § Department of Chemistry, Faculty of Science, National University of Singapore (NUS) 3 Science Drive 3, Singapore 117543, Singapore ABSTRACT: On the basis of our previous Molecular Dynamics (MD) Simulation (Wei, C. X.; Huang, H.; Duan, X. H.; Pei C. H. Propell. Explos. Pyrot. 2009, 67, 2822
2826), a cocrystal explosive (CCE) consisting of octahydro-1,3,5,7-tetranitro1,3,5,7-tetrazocine (HMX) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) has been prepared with a solvent/nonsolvent (S/NS) process. Scanning electron microscopy (SEM) demonstrated that crystal morphology of the CCE was significantly improved in contrast to the crystal of HMX and TATB. Differential scanning calorimetry (DSC) showed that the CCE exhibited the enhancement of thermal stability and became less sensitive to impact, compared with the HMX. Raman spectroscopy, terahertz time-domain spectroscopy (THz-TDS), and X-ray photoelectron spectroscopy (XPS) provided characterization at the molecular level. The results indicated that the main mechanism of cocrystal originates from the N
O 3 3 3 H hydrogen bonding between
NO2 (HMX) and
NH2 (TATB).

’ INTRODUCTION Insensitive High Explosives (IHEs) have attracted considerable interest in the past three decades due to their potential application in various propellants and military warheads.1
4 Exploration of the explosive of low impact sensitivity and high explosive performance is the fundamental problem in the energetic materials, which has not been solved yet. HMX, a sensitive explosive, is a typical highly energetic material that has been widely used in national defense industries since the 1940s. The explosion performance of HMX is high; however, it is sensitive to heat and shock. Meanwhile, TATB belongs to a class of IHEs that are insensitive or difficult to detonate. The unique high thermal stability and its resistance to physical shock are desirable properties of TATB in many applications.5 Two methods are mainly developed to reduce the sensitivity of HMX,6,7 one is the control of the crystal shape and quality of I-RDX (insensitive 1,3,5-trinitro-1,3,5 -triaza-cycrohexane)8 and I-HMX,6 and the other preparation of HMX-based plastic bonded explosive (PBX). I-RDX and I-HMX changed from a macroscopic shape and crystal quality of the crystals, but cocrystal explosive (CCE) will further improve at the molecular level. Under same circumstance, the explosion performance of PBX is reduced, whereas that of CCE remains unchanged. Novel approaches to reduce the sensitivity of HMX are now considered widely. Cocrystal technology9
14 are so important for improving the solubility, bioavailability physical and chemical stability properties of drugs without changing their chemical structure that it is widely used for the pharmaceutical chemicals.15
18 The cocrystal is generally accepted to be neutral complexes composed of two components bonded by hydrogen r 2011 American Chemical Society

bonding, π-stacking, and Van der Waal’s forces.19,20 Hydrogen bond is essential for the structural stability of many important cocrystals.21
24 Reports of CCE are currently very limited. Michael patented cocrystals of HMX and AP (ammonium perchlorate).25 Zhou et al. studied on the cocrystallized explosive of urea nitrate and RDX.26 While all of those works provide a preliminary exploration of CCE, there is still lack of the convincing characterization to confirm whether the cocrystal was formed. Our co-worker Wei et al. recently reported the theoretical designing calculation of cocrystal HMX/TATB (molar ratio 1:1) by molecular dynamics (MD) simulation.27 The calculation indicates that the mechanical properties and stability of the explosive can be effectively improved. In this paper, we prepare a novel HMX/TATB (mass ratio 9:1 and its molar ratio is about 8:1) CCE with a so-called solvent/ nonsolvent (S/NS) process. The results demonstrate that the crystal quality and sensitivity of the prepared CCE are considerably improved compared with the HMX.

’ EXPERIMENTAL SECTION Materials and Sample Preparation. HMX and TATB (Figure 1) were supplied by the Institute of Chemical Materials, Chinese Academy of Engineering Physics (CAEP). DMSO and distilled water are used as solvent and nonsolvent, respectively. Received: December 23, 2010 Revised: February 21, 2011 Published: March 15, 2011 1759

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Figure 1. Chemical structures of HMX, C4H8N8O8, and TATB, C6H6N6O6.

First, the cocrystal explosive was prepared by the S/NS process. 0.45 g HMX and 0.05 g TATB (mass ratio 9:1) was put into 420 mL DMSO at 45
C. Then, the distilled water was slowly dropped into the solution at definite temperature, stirring for several hours. The solution was then filtrated. The product was washed 5 times with distilled water to remove the solvent. The sample was finally obtained by freeze-drying. The HMX and TATB were recrystallized by the same method, respectively. Molecular Dynamics (MD) Simulation. All the calculations are run with the Discover module of the commercial molecular modeling software package Materials Studio 3.0,28 using the Compass force field.29 MD simulation is carried out in an NVT ensemble with the Anderson constant temperature method at the temperature of 298K.30 For the equilibration stage, the time step for the MD simulation is 1 fs with a period of 100 ps. The Coulomb and van der Waals interactions are calculated by employing the atom-based and standard Ewald method. And the final structure of MD simulation is used as the balance structure of the HMX/TATB cocrystal model. Scanning Electron Microscopic (SEM). The SEM images were observed with FEI-Nova NanoSEM600. Dynamic Scanning Calorimetry (DSC). A TA Instruments SDT Q600 was employed. 2
5 mg of sample was weighed into crimped aluminum pans, pierced to allow vapor to escape, and pressed to increase contact between the pan and sample. Measurements were recorded with 100 mL min
1 air purge flow at 20
C min
1 from 25 to 500
C for each sample. Raman Spectroscopy. The Raman spectra were carried out with a WITEC CRM200 Raman system. The excitation source is a 532 nm laser (2.33 eV) with a laser power below 0.1 mW on the sample to avoid laser-induced local heating. A 100 objective lens with a numerical aperture (NA) of 0.95 was used in the Raman experiments, and the spot size of a 532 nm laser was estimated to be 500 nm. The spectra resolution of our Raman system is 1 cm
1. Terahertz Time-Domain Spectroscopy (THz-TDS). A modelocked Ti: sapphire femtosecond laser is used as light source. It has 100 fs pulse duration and central wavelength is from 770 to 840 nm. The laser beam is split into pump light and probe light by a cubic beam splitter (CBS). The pump pulse passes through a variable time-delay stage and illuminates the Æ100æ InAs at an angle of about 45 degrees THz emitter for generating THz wave. Two pairs of gold-coated parabolic mirrors are used to collimate and focus the generated THz beam. A high resistance silicon wafer is placed after the fourth parabolic mirror to combine the probe beam with the THz beam. This allows the probe beam to travel collinearly with the THz beam inside the 1 mm thick 110 ZnTe crystal, where the probing beam is modulated by the electric field of the THz radiation via the electro-optic effect. A quarter-wave plate (QWP), a Wollaston prism (PBS), and a pair of photodiodes are assembled for the balanced detection of the probe beam. The signal is read by a lock-in amplifier (LIA) from the detector at 1 kHz rate and is feed into a computer. X-ray Photoelectron Spectroscopy (XPS). The XPS were observed with ESCALAB250, employing a monochromatic Al Ksource (1486.69 eV). The instrument was operated in constant analysis energy

Figure 2. HMX/TATB cocrystal model.

Figure 3. Magnification of a part of the image shown in Figure 2. (CAE) mode with the pressure below 2  10
8 mbar. In the text, the atom of interest is indicated by being underlined. Impact Sensitivity Properties. The testing conditions of impact sensitivity were that the weight of dropping hammer was 10 kg, the height of dropping hammer was 25 cm, and the quantity of sample was 50 mg. Twenty samples were tested and the firing percent was calculated.

’ RESULTS AND DISCUSSION Results of Computational Studies. According to the preparation experiment of the cocrystal, HMX/TATB cocrystal model (3  3  2 unit cell) was constructed by substituted on (011) crystal face with TATB in HMX super cell. Four HMX molecules (HMX molecule is shown as stick-and-stick model) 1760

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Figure 5. DSC curves of (a) TATB, (b) HMX, and (c) cocrystal. And the iconograph is the magnification endothermic peak of b and c from 160 to 240
C.

Table 1. Summarized Results for DSC Experiments of HMX, TATB and Co-crystal phase transition

melting peak

decomposition peak

temperature (
C)

temperature (
C)

temperature (
C)

HMX

200.8

278.8

282.0

cocrystal

199.1

277.5

284.9

sample TATB

Figure 4. SEM of (a) HMX, (b) TATB, and (c) HMX/TATB cocrystal from S/NS method.

are substituted by TATB molecule (TATB molecule is shown as ball-and-stick model). The optimized structure of the HMX/ TATB cocrystal model (molar ratio 8:1) is showed in Figure 2. Obviously, according to the hydrogen-bond rules,24 there are three patterns (Figure 3) hydrogen bonds in the HMX/TATB super cell. And the three patterns have graph sets 1:R21(4), 2: R22(6), and 3: R12(4),31,32 respectively. The bond length is shown

388.3

in Figure 3, which shows that the intermolecular interaction is very strong between TATB and HMX. Of course, the bond length of hydrogen bond is one factor of the intermolecular interactions. Another one is binding energy which can well reflect the intermolecular interactions. Wei et al.27 studied the binding energy of TATB dimer, HMX dimer and HMX/TATB. The result revealed that the stability of supramolecular structure is HMX/TATB > HMX dimer > TATB dimer. In addition, other unpredictable and nonspecific lattice forces may be present in the solid state.24 Therefore, it is possible to form cocrystal in experiments between HMX and TATB. SEM. Crystal quality (crystal shape, crystal surface, and crystal defects) is one of the major factors to play an important role during safer storage, transport, and handling of munitions items and explosives while (at least) maintaining their performance.8 These physicochemical parameters may affect the detonation initiation spots of the explosive.7 Figure 4a
c reveals the morphological differences of HMX, TATB, and the synthesized HMX/TATB cocrystal. The HMX of about particle size 10 μm possesses a prismatic type microstructure without apparent crystal edges; nevertheless, many cracks were observed to exist on the crystal surface. The crack may result from the removal of solvent molecules from the lattice, thus leaving gaps in the regular HMX particles. The TATB is of hexagonal flake crystal (Figure 4b) with many crystal defects such as dislocations and grain boundaries. The diameter of the flake ranges from 5 to 20 μm, and the thickness is about 2 μm. In contrast, the surface of HMX/TATB cocrystals is smooth and the particle shape is regular shown in Figure 4c. The particle size of majority of the cocrystals are about 20
30 μm. The above results indicate that the S/NS methods can change not only the particle size of the crystals but also the shape. DSC. DSC thermograms of HMX, TATB and cocrystal product are shown in Figure 5 and the experimental data are 1761

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Figure 6. Raman spectra of (a) HMX, (b) TATB, and (c) micro-Raman spectroscopy (MRS) of HMX/TATB cocrystal. Spectum c was performed on the crystal surface pictured in image d.

Table 2. Assignments of the Major Bands from 50 to 3500 cm
1 of the Raman spectra of HMX, TATB, and HMX/TATB Cocrystal assignments

HMX

cocrystal

TATB

assignments

NO2 deformation

156.5

160.5

ring deformation

360.5

359.9 382.9

NO2 deformation

835.9

830.9

ring stretching

882.5

881.4

ring stretching

953.7

949.0 1168.1

1164.8

C
N stretching (C
NH2)

N
N stretching þ NO2 symmetric stretching

1310.6

1313.7

1318.6

NO2 symmetric stretching

NO2 asymmetric stretching

1567.6

1569.9 1609.2 2992.0

1606.2

C
N stretching (C
NO2)

2996.3

3221.0

3225.2

NH2 stretching

3033.6

shown in Table 1. TATB and HMX show exothermic peaks at 388.3 and 282
C, respectively. The exothermic decomposition peak of the cocrystal reaches 284.9
C and increases by 2.9
C comparing with HMX, which can be attributed to the hydrogen bonding in the structure. HMX gives the endothermic peak at 200.8
C, corresponding to the phase transition β f δ. The endothermic peak of the HMX/TATB cocrystal shifts to lower temperature at 199.1
C, which indicates some changes may have occurred in the crystal phase. We are investigating the detailed thermal property of CCE by DSC in different heating rate, and the results will be published in the future. Raman Spectroscopy. Brittain,33
35 Elbagerma,36 and so on have found that Raman spectroscopic is a very useful tool for characterization of cocrystals. The Raman spectra of HMX, TATB, and cocrystal are presented in Figure 6. The assignments for the most characteristic vibrational bands are listed in Table 2. As shown in Figure 6 and Table 2, the results indicated that both HMX and TATB are detected in a grain of crystal from Micro-Raman spectroscopy (MRS). The HMX has bands at 156.5, 835.9, 953.7, 1310.6, and 1567.6 cm
1. However, these bands in the cocrystal shift to 160.5, 830.9, 949.0, 1313.7, and 1569.9 cm
1, respectively. Simultaneously, some peak shift also takes place for TATB. These shifts can be attributed to the hydrogen-bonding, which changed the symmetry characteristic in cocrystal structure. THz-TDS. THz-TDS is attractive as a means to monitor intermolecular interactions, such as hydrogen bonds and related

387.4

ring deformation

882.5

ring stretching

3037.4

Figure 7. THz-TDS of (a) HMX (red trace), (b) TATB (green trace), and (c) cocrystal (black trace) from 0.1 to 2.6 THz.

supramolecular structure.37,20 Consequently, THz-TDS has attracted interest in fields where distinguishing organic supramolecular structures is important, including the pharmaceutical industry38 and explosives identification.39
42 The experimental THz spectrum of HMX, TATB and cocrystal is provided in Figure 7. The result reveals that the cocrystal exhibits a distinctive absorption peak at 2.4 THz (Figure 7c), corresponding to the 1762

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Figure 8. XPS N 1s spectra of (a) HMX (red trace), (b) TATB (green trace), and (c) cocrystal (black trace). Refer to Table 3 for binding energy positions of the fitted peak components.

Figure 9. XPS O 1s spectra of (a) HMX (red trace), (b) TATB (green trace), and (c) cocrystal (black trace). Refer to Table 4 for binding energy positions of the fitted peak components.

Table 3. XPS N 1s Peak Assignments and Positions samples

binding energy (eV)

assignments

HMX

400.1

C
N
C

TATB

405.5 398.5

N
NO2 C
NH2

404.2

C
NO2

cocrystal

401.5

C
N - C

406.7

N
NO2

398.7

C
NH2

404.5

C
NO2

N
O 3 3 3 H between -NO2 (HMX) and -NH2 (TATB). In addition, due to the symmetry of molecule broken, this absorption peak even suggests that crystal lattice vibrations have changed in the unit cell. We will later develop lattice dynamics calculations to achieve a molecular-level understanding of THz spectra. XPS. Figure 8 shows the XPS N 1s spectra of HMX, TATB and cocrystal. The assignments of the peak positions are shown in Table 3. According to Yukari Sato et al’s investigation,43 hydrogen bonding interactions of monolayer-base species can also be identified with XPS. The N 1s spectrum of HMX (Figure 8a) shows two peak components arising from C
N
C and N
NO2 nitrogen environments, appearing at 400.1 and 405.5 eV, respectively. For TATB (Figure 8b), two peaks are identified at 398.5 and 404.2 eV, attributing to C
NH2 and C
NO2, respectively. This is in good agreement with Sharma et al’s results.44 The peaks become complex in the HMX/TATB cocrystal (Figure 8c). Four peaks are observed at 398.7, 401.5, 404.5, and 406.7 eV. Significant shift from 400.1 to 401.5 eV and from 405.5 to 406.7 eV of the two peak of HMX was observed. For TATB, the two peak shifted to a slightly higher (0.2 and 0.3 eV) binding energy. The O1s spectra of HMX, TATB, and cocrystal are shown in Figure 9 and Table 4. The shapes of all the O1s peaks are similar, as all of them arise from the chemical environments of -NO2. The O1s of TATB shows a satellite peak at 535.0 eV, which may be assigned to water.45 In Figure10a, the peak at 286.2 is assigned to N
C
N in HMX. According to Wang45 et al., the Cls spectrum (see Table 5) of TATB should be of an intense asymmetrical peak at approximately 286.0 eV, which were resolved into two

Table 4. XPS O 1s Peak Assignments and Positions samples

binding energy(eV)

assignments

532.6 531.4

N
NO2 C
NO2

535.0

satellite

533.2

NO2

HMX TATB cocrystal

Figure 10. XPS C 1s spectra of (a) HMX (red trace), (b) TATB (green trace), and (c) cocrystal (black trace). Refer to Table 4 for binding energy positions of the fitted peak components.

Table 5. XPS C 1s Peak Assignments and Positions samples

binding energy(eV)

assignments

HMX

286.2

N
C
N

TATB

284.6

C
NH2, C
NO2

cocrystal

284.6

C
NH2, C
NO2, N
C
N

components, one is believed to be due to C-NH2 at 285.0 eV, the other C
NO2 at 286.6 eV. However, only one peak at 284.6 eV is observed in the present case, and the width is larger than N1s and O1s peaks. It may be a result of some hydrocarbon contamination.44 Moreover, some satellite peaks were observed 1763

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Table 6. Impact Sensitivity of Different Proportion HMX/ TATB Compound sample

mass ratio of TATB (%)

impact sensitivity (%)

1

0.0

100

2 3

2.5 5.0

60 30

4

10.0

0

5

15.0

10

6

20.0

10

from 282.0 to 284.0 eV (Figure 10b) and from 285.3 to 287.0 eV (Figure 10c). As a consequence, the XPS results allow characterization of molecular structure of the cocrystal, clearly revealing the interactions between HMX and TATB molecules. Impact Sensitivity Properties. Impact sensitivity is largely dependent on the physical property and the chemical properties.46 Results of the impact sensitivity are listed in Table 6. Compared with the HMX, the impact sensitivity of the cocrystal with addition of 10% of TATB in HMX was decreased from 100% to 0%. However further addition of 15.0% or 20.0% of TATB leads to a little increasing effect. Thus, it may be concluded that there are obvious desensitization effects when the mass ratio of HMX and TATB is 9:1. To further confirm how strongly the sensitivity really changed, more reliable experimental on impact sensitivity will conduct later.

’ CONCLUSIONS HMX/TATB cocrystal explosive was successfully prepared by a simple method at room temperature. According to SEM observation, the S/NS process improved the crystal quality which may play a key role in the detonation initiation spots of the explosive. Raman spectra and THz-TDS indicated that the N
O 3 3 3 H hydrogen bonding intermolecular interactions between
NH2 (TATB) and
NO2 (HMX) was major driven force for formation of the cocrystal. XPS spectra gave good indication of the nature of H-transfer at the molecular level. The data of drop weight impact showed that the cocrystal explosive was relative insensitive in contrast to HMX. The results maybe make a great contribution to exploring the mechanism of solidstate cocrystal. Further investigation into the mechanism will provide a foundation for exciting future work. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: þ86-816-2419492. Tel: þ86-816-2419280.

’ ACKNOWLEDGMENT This work was supported by the Postgraduate Innovation Fund sponsored by Southwest University of Science and Technology (09ycjj18). We are also grateful for the apparatus support of the Analytical and Testing Center of Southwest University of Science and Technology and the Key Laboratory of Terahertz Optoelectronics, Ministry of Education, Department of Physics, Capital Normal University.

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