A Novel Spherulitic Self-Assembly Strategy for Organic Explosives

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A Novel Spherulitic Self-assembly Strategy for Organic Explosives: Modifying the Hydrogen Bonds by Polymeric Additives in Emulsion Crystallization Xiaoqing Zhou, Qi Zhang, Rong Xu, Dong Chen, Shilong Hao, Fude Nie, and Hongzhen Li Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00044 • Publication Date (Web): 16 Mar 2018 Downloaded from http://pubs.acs.org on March 18, 2018

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Crystal Growth & Design

A Novel Spherulitic Self-assembly Strategy for Organic Explosives: Modifying the Hydrogen Bonds by Polymeric Additives in Emulsion Crystallization Xiaoqing Zhou, Qi Zhang,* Rong Xu, Dong Chen, Shilong Hao, Fude Nie, and Hongzhen Li* Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang, Sichuan 621900, China Supporting Information Placeholder

ABSTRACT:A novel strategy to prepare spherical crystals of 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) has been developed by introducing polymeric additives into the antisolvent emulsion crystallization. And the spherical crystals are induced by modifying the intermolecular hydrogen bonds network of LLM-105. The results show that the concentration of polyvinyl pyrrolidone (PVP), which acts as a polymeric additive, is a crucial factor to get quite different morphologies of LLM-105 crystal products. X-like shaped crystals have been produced in the absence of PVP. In contrast, spherical crystals have been obtained in the presence of PVP. Importantly, LLM-105 spherulites with a mean particle size of 78.0 µm can be obtained by adding a proper amount of PVP, which has a narrow size distribution (CV = 31.2). In addition, the time-resolved morphological evolution processes of X-like shaped and spherical crystals have been performed. Meanwhile, 1H NMR experiments also have been conducted to understand the intermolecular hydrogen bonds between LLM-105 molecules and the polymer. Inspired by the result of above experiments, LLM-105 spherulitic formation mechanism has been proposed. Furthermore, LLM-105 spherulites exhibit more excellent mechanical and safety properties. It is suggested that these spherulites have a great potentiality in the military application. Therefore, this polymer-induced spherical crystallization method is significantly important for the design and fabrication of organic small molecules with spherical shapes.

1. INTRODUCTION Controlling crystallization is of great interest in many fields because it is crucial for the crystal morphology that can influence the physicochemical properties and downstream operations of crystalline products.1-4 For explosives, crystallization procedure is especially important since the crystal morphology of explosives can not only determine their mechanical and safety properties, but also influence their formulation and manufacturing processes such as extrusion or melt casting.5,6 For instance, on the same level of size, the spherical shaped explosives are more mechanically and thermally insensitive than nonspherical (needles/plates) shaped ones. Moreover, the former have obvious advantages such as higher ratio of surface volume, better free-flowing properties and higher bulk density.7,8 Hence, it is significant to explore an effective crystallization methodology to obtain spherical shaped crystals to improve the safety of explosives and expand its application. Spherical crystallization, developed by Kawashima and coworkers9, is an emerging method to get spherical crystals through an agglomeration process of microcrystal. Spherical agglomeration (SA) and quasi-emulsion solvent diffusion (QESD) method10 are two kinds of typical spherical crystallization technique. In these methods, the emulsion droplets are produced firstly, and then the crystals precipitate inside the droplets due to the counter diffusion between good solvent and poor solvent. Simultaneously, the crystals are transformed into spherical aggregates. To date, these approaches have been widely used in the preparation of spherical organic drug and become an outstanding strategy for designing the desired products with improved physical and mechanical properties.1119 However, the formation mechanism of spherical crystals has

not been fully understood, because the fast kinetics resulted by the antisolvent crystallization make the observation of the morphological evolution details difficult.20 Moreover, compared with the organic drug molecules, only a few literatures6,21,22 have discussed the methods to prepare the spherical crystals of organic explosive molecules, especially for those molecules containing strong intra- and intermolecular hydrogen bonds, such as 2,4,6-trinitrobenzene-1,3,5-triamine (TATB), 3,5-dinitropyrazine-2,6-diamine-1-oxide (LLM-105), and 2,2-dinitroethylene-1,1-diamine (FOX-7). Although these strong hydrogen bonds are useful to keep the extraordinary thermal stability and low sensitivity of these explosives,23-25 they also result in extremely low solubility of explosive molecules in most organic solvents making the crystallizations of these explosives great difficult. In addition, due to the intermolecular strong hydrogen bonds existing in these organic molecules, the explosive crystals grow faster along certain faces and result in the formation of needle-like or plate-like morphology.26-28 Therefore, it is very significant and tremendous challenging to investigate spherical crystallization of such explosive molecular with strong hydrogen bond network. In this work, LLM-105 was selected as a model organic explosive molecular, which has potential applications in insensitive boosters, detonators, and main charges in specialty munitions because of its excellent thermal stability and safety.29-32 LLM105 exhibits abundant intermolecular and intramolecular hydrogen bonds in its crystal structure (Figure 1), which might be responsible for the excellent thermal stability and safety to shock, friction and spark. However, LLM-105 crystals are manufactured by the conventional crystallization method as long needle-like shapes, which will produce some problems such as poor flow properties, low bulk density, and poor me-

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chanical properties. These problems make LLM-105 very difficult to use in explosive manufacturing processes. Therefore, obtaining spherical LLM-105 crystals is essential and significant. Herein, we developed a facile polymer-induced antisolvent emulsion crystallization method to prepare LLM-105 spherulites through modifying the hydrogen bond network of LLM-105, which is a novel approach so far never mentioned in the existing researches of spherical crystallization of explosive organic small molecules. X-like shaped and spherical crystals of LLM-105 were self-assembled in the absence and presence of polymeric additives, respectively. Detailed characterization of particle morphology, microstructure, size, and size distribution were tested. Moreover, the evolution processes of X-like shaped crystals and spherulites of LLM-105 were monitored by SEM, and the possible mechanism of LLM-105 spherulite formation was also elucidated by the results of morphology evolution experiments. 1HNMR experiments were also conducted to understand the intermolecular hydrogen bonds interactions between LLM-105 and PVP. Finally, the mechanical and safety properties of spherical particles were examined and compared with those of needle-like crystals.

was constant at 15:1. After 30 min, the crystallization process was stopped. Next, the resulting precipitates were filtered and washed several times with distilled water. The obtained LLM105 microstructures were dried in a vacuum drying oven at 80 o C for 6 h. 2.3 Characterization. Scanning electron microscopy (SEM, Sigma-HD, Zeiss) was employed to character the morphology of the LLM-105 crystals. A polarized optical microscope (POM, Axio Scope. Al, Carl Zeiss) was used for the optical microscopic observation in transmission mode. Powder X-ray diffraction (PXRD, D8 advance, Bruker) patterns were recorded with the Cu−Kα radiation ((λ = 1.5418 Å) generated at 30 mA and 40 kV. Fourier transform infrared spectra (FT-IR, Thermo Scientific, UT, USA) were collected from 400 to 4000 cm−1. The laser diffraction particle size analyzer (Malvern Mastersizer 2000) was used to characterize the crystal size and the size distribution of crystal particles. The value of CV, which is used to denote the size distribution of crystal particles, is calculated by the equation CV = 100(PD90 − PD10)/2PD50. PD90, PD10, and PD50, are the particle sizes when the volume fraction reaches 90%, 10% and 50%, respectively. Thermogravimetric and differential scanning calorimetry (TGA−DSC 2, Mettler Toledo) were performed to record the thermal decomposition properties. The proton nuclear magnetic resonance spectra (1H NMR) were conducted by a Bruker AV400 NMR spectrometer operating at 400 MHz for 5 mM LLM105/DMSO-D6 solutions without or with polymeric additives at 25 °C. The impact sensitivity tests were carried out to evaluate the safety properties by a WL-1 type impact sensitivity instrument. Samples of 35 mg (±1%) were tested by a freefalling 5 Kg drop weight from variable heights, and the impact sensitivity was expressed by the drop height of 50% explosion probability (H50). The mechanical properties were evaluated by a confined, quasi-static uniaxial compression method, named as the compressive stiffness test (CST)33,34, in a materialstesting machine (Instron 5582). The compressing speed of test machine is 0.05 mm/min.

3. RESULTS AND DISCUSSION Figure 1. Molecular packing of LLM-105 looking down the direction. Hydrogen bonds are indicated by green dashed lines.

2. EXPERIMENTAL SECTION 2.1 Materials. Needle-like LLM-105 crystals (Figure S1) were obtained according to the reported method.28 Analytical grade dimethyl sulfoxide (DMSO) and acetic ether, which were purchased from Tianjin Chemical Industry Co., were used as solvent and antisolvent, respectively. Polyvinyl pyrrolidone (PVP K29) was AR grade and purchased from Alfa Aesar. 2.2. Fabrication of LLM-105 Spherulites by antisolvent Crystallization. In this paper, polyvinyl pyrrolidone (PVP) was selected as optimization additives after an initial study of screening the effect on the morphology of a series of soluble polymers. A certain amount of LLM-105 with or without PVP was dissolved in DMSO. Then the LLM-105 solution was quickly poured into a stirred (three-bladed propeller) glass crystallizer, containing a certain amount of ethyl acetate. The crystallization temperature was 5 oC, and the speed of the stirrer was 700 rpm. The volume ratio of ethyl acetate and DMSO

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3.1. Spherulite Formation. Interestingly, the morphologies of LLM-105 obtained in the presence and absence of PVP show obvious dissimilarity, which suggested that the PVP has played a significant effect on the morphology of the resulting crystals. First, in the absence of the polymeric additive, LLM105 crystals were self-assembled into X-like shaped microstructures (Figure 2a) via the antisolvent crystallization by using DMSO and ethyl acetate as good and poor solvents, respectively. As a comparison, in the presence of PVP, spherical crystals were obtained. The morphology and size of spherical crystals were be controlled by varying the value of nPVP/nLLM105 (nPVP is the monomer molar content of PVP, and nLLM-105 is the molar content of LLM-105). When the ratio value of nPVP/nLLM was 0.01, loose microspheres were produced (Figure 2b). In this case, these microspheres are composed of radially aligned microfibers as the structural subunit. When the nPVP/nLLM-105 value was 0.1, compact LLM-105 spherulites were achieved. The SEM images with low magnification and the POM images of the spherulites (Figure 2c) show the uniform particle size distribution and the high spherical degree, which are like a ball. Moreover, The high magnification SEM images reveal the structural details of LLM-105 spherulites


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(Figure 2d). It is clearly shown that the surface of LLM-105 spherulites is smooth and compact. The SEM images (Figure S2) of the broken spherulites exhibitthat the spherulites are made up of the radially aligned nanofibers, which indicates that the growth of spherulite is elongated along a radial orientation toward the surface. These results highlight the excellent effect of PVP as a crystal morphology modifier to induce the spherulitic self-assembly of LLM-105. In addition, the morphology and size of the LLM-105 spherulites can be tailored by changing the concentration of PVP.

Figure 3. PXRD diffraction patterns of (a) simulated, (b) X-like shaped crystals obtained in the absence of PVP, and (c) spherulites of LLM-105 produced in the presence of PVP.

Figure 2. Images of LLM-105 crystals: (a) X-like shaped crystals obtained without the polymeric additives. (b) Loose microspheres obtained in the presence of PVP with low concentration. (c) Spherulites obtained in the presence of PVP with high concentration. (d) SEM images with higher magnification of (c) Showing the structural details of spherulites. The reaction conditions are as follows. (a): [LLM-105] = 0.014 mol·L-1, Vethyl acetate/VDMSO= 15; (b) : [LLM-105] = 0.014 mol·L-1, nPVP/nLLM-105 = 0.01, Vethyl ace-1 tate/VDMSO = 15; (c, d) : [LLM-105] = 0.014 mol·L , nPVP/nLLM-105 = 0.1, Vethyl acetate /VDMSO = 15.

As shown in Figure 3, the powder X-ray diffraction patterns of the X-like shaped and spherulitic crystals exhibit good agreement with the simulation results, respectively, confirming both LLM-105 crystals are in the same crystal structure of monoclinic unit cell occupied by four molecules in space group P21/n.30 These results indicate that the PVP additive just affect the crystal habit of LLM-105, but doesn’t lead to the polymorphism changed. Moreover, the remains of contamination and polymorphic changes of LLM-105 spherulites were also confirmed by FTIR spectra and DSC-TG analysis as shown in Figure S3 and Figure S4, respectively. The curves of particle size distributions of LLM-105 crystals with different morphology display obvious distinguish, as shown in Figure 4. Apparently, the size distribution of the Xlike shaped crystals particles (CV = 127.0) is much wider than the size distributions of spherical crystals (CV = 41.3 and 31.2

Figure 4. Particle size distributions of the self-assembled LLM105 microstructures obtained with and without PVP.

for the loose microspheres and spherulites, respectively). The mean particle sizes of the X-like shaped crystals, loose micro spheres and spherulites were 6.3, 49.6 and 78.0 µm, respectively. The X-like shaped crystals show a broad double-peaked distribution, by which it could be concluded that some freshly nucleated LLM-105 crystals were continuously produced in the crystallization process, and nucleation and subsequent growth of new polycrystalline LLM-105 particles occured in the absence of PVP. In contrast, the loose microspheres exhibit a narrow double-peaked distribution, and the spherulites present a sharp single-peaked distribution. The clear difference between three kinds of obtained crystals is the lack of small crystals in the size range of 0 − 10 µm for the spherical products, which suggests that second nucleation did not occur and only the primary crystal nucleus continuously grew in such rapid crystallization process in the presence of PVP. These results indicate that the PVP additive play a significant role in the nucleation and growth of LLM-105 spherulites. Therefore, it can be seen that the spherulitic growth strategy via adding polymeric additives would be a feasible method for controlling particle size of energetic materials.



Figure 5. The morphology evolution process studies of two pathways of the LLM-105 crystallization in the absence (the Pathway 1 in images a-f) and presence (the Pathway 2 in images g-m) of PVP cases. The crystallization conditions are as follows. [LLM105] = 0.014 mol·L-1, Vethyl acetate/VDMSO= 15 (Pathway 1), [LLM105] = 0.014 mol·L-1, Vethyl acetate/VDMSO= 15, nPVP/nLLM-105 = 0.1 (Pathway 2).

3.2. Formation Mechanism of LLM-105 Spherulites. In order to fully understand the role of PVP on the self-assembly

of LLM-105 spherulites, the morphological evolution of LLM105 microstructures was carefully captured by off-line using SEM (Figure 5). In the absence of PVP, the micro-rod with a length of about 4µm was first produced (Figure 5a), then transformed into a cross-shaped structure with an included angle of about 45° (Figure 5c), and gradually changed to X-like shaped microstructure (Figure 5d-f). On the other hand, in the presence of PVP, the similar cross-shaped structure (Figure 5g) was also first obtained. However, the subsequent morphology evolution course was significantly distinct. The cross-shaped structures transformed into bowknot-like superstructures (Figure 5h), then gradually changed to bird’s nest-like ones (Figure 5i, j) and finally assemble into spherulites (Figure 5l). These results clearly display the different morphology evolution process of LLM-105, and a possible self-assembled mechanism is proposed as schematically illustrated in Scheme1. As shown in Scheme 1, initially the immediate mixture of the saturated DMSO solution of LLM-105 and the antisolvent causes the formation of LLM-105 emulsion droplets. Subsequently, the solubility of LLM-105 inside the droplets reduced due to the counter diffusions of solvents, then nucleation and growth of LLM-105 crystals occurred, resulting in crystals precipitation. Finally, the different microstructures quickly produced by self-assembling. According to the crystal structure of LLM-105 (Figure 1), there are abundant intermolecular hydrogen bonds within a layer, and LLM-105 crystals can grow faster along certain faces providing the micro-rods with a high aspect ratio. Subsequently, these micro-rods were selfassembled into cross-shaped structures with a fixed included angle of about 45o, then other micro-rods grew from the center of the cross-shaped structure, and finally X-like shaped aggregation crystals were observed with a certain thickness. However, in the presence of PVP, the aspect ratio was remarkably reduced and nano-fibers of LLM-105 crystals formed, then they were woven with each other to form the bowknot-like superstructures with a certain thickness. Next, these superstructures were further self-assembled in the three-dimensional space to form a space-filling particle with a number of multidirectional small-angle branches, which finally transformed into spherulitc structures. The different crystallization pathways induced by PVP can be mainly attributed to the intermolecular interactions between LLM-105 and PVP. Thus, 1H NMR spectroscopic analyses were performed to reveal the intermolecular interactions. As shown in Figure 6, in the absence of PVP, the signals of NH2 were double-peak, because one hydrogen atom (H1) participates in H-bonds with O of NO2, and the other one (H2) participates in H-bonds with O of N-O, leading to the different chemical environments of hydrogen atoms on the bonded NH2 groups. Thus, there were two different chemical shifts in the 1 H NMR spectrum, which belonged to 9.04 and 8.80 ppm, respectively. However, in the presence of PVP, the chemical shifts of NH2changed by the different concentration of PVP. With the loading of PVP increasing, the two different chemical shifts of NH2 gradually moved toward the center between 9.04 and 8.80 ppm, and eventually resulted in one single-peak with a chemical shift of 8.92 ppm. The single peak of NH2 indicated that the protons


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Scheme1. Illustration of the formation process of X-like shaped crystals and spherulites of LLM-105 obtained in the absence and presence of PVP, respectively.

Figure 6. 1H NMR spectra of (a) LLM-105, (b) PVP /LLM-105 (nPVP/nLLM-105 = 0.01), (c) PVA /LLM-105 (nPVP/nLLM-105 = 0.1).

of LLM-105 were in the homogeneous chemical environments, suggesting the original N-H …O (between NO2 and N→ O group) hydrogen bonding networks in LLM-105 molecular were replaced by stronger intermolecular N-H…O (C=O on polymer chain) hydrogen bonds interactions between LLM105 and PVP due to the O of the carbonyl group in PVP molecular acting as a strong hydrogen bond acceptor. These results clearly confirmed that the hydrogen bonds in LLM-105 molecular were significantly modified by PVP, which could be responsible for the nucleation and crystal growth inhibition. For instance, the nucleation time were influenced by the polymer additive PVP, which is proved by the fact that in the absence of PVP the precipitations of LLM-105 immediately occurred once the mixing procedure, but the precipitations of LLM-105 were be observed at least for 5 seconds after the mixing procedure in the presence of PVP. Meanwhile, the sharp single-peaked distribution of LLM-105 spherulites mentioned previously also indicated that the nucleation has been inhibited by PVP. Moreover, the crystal growth of LLM-105 also was inhibited by PVP according to the fact that microfibers with high aspect ratio were obtained in the absence of PVP, while nano-fibers with low aspect ratio were obtained in

the presence of PVP. All the above experiments are used to explain that the real factor that inducing the growth of spherulites is PVP. From the literature, several mechanisms have been reported to explain the development of spherulitic particles.35 The evolution of the LLM-105 spherulite morphology discovered in our study is remarkably similar to the simulations of Granasy and co-workers35. Firstly, according to their views, the highly undercooled complex liquids in this emulsion crystallization produce a large driving force to result in a highly branched polycrystalline crystallization pattern with an average circular shape, which is the necessary condition for the formation of spherulites. However, only X-like shaped microstructures were obtained without the PVP additives in our crystallization system. Therefore, we consider that the PVP additives also play a crucial role in the growth of LLM-105 spherulites. Combining with the spherulitic growth mechanisms proposed by Granasy et al., on the molecular scale we speculate that the adsorption of PVP additives on some surfaces of LLM-105 crystals makes the grain boundary energies changed, leads to the disorder of these microstructures and reduces the orientational mobility. Consequently, LLM-105 crystals are constructed by innumerable nano-fibers growth radially from a central precursor and eventually form polycrystalline spherulites. Hence, this emulsion crystallization technique cooperating with the modified hydrogen bonds in LLM-105 molecular by PVP additives in our work is an extremely effective method for the formation of organic explosive spherulites. 3.3. Safety and Mechanical Properties of Spherulites. The safety of an explosive is usually evaluated by sensitivity, which is the response of an explosive to an external stimulation such as impact, fraction, and shock. In this work, the sensitivity was measured by impact drop testing to characterize the safety of LLM-105 crystals with different morphologies. Using a WL-1 type impact sensitivity instrument, the impact sensitivity was expressed by the drop height of 50% explosion probability (H50). The value of H50 is higher, the impact sensitivity is lower. LLM-105 samples of approximately 35 mg (± 1%) were tested by a free falling 5 Kg drop weight from heights of variable distances. The spherulites exhibited a H50 of above 112.2cm (the maximum height of the WL-1 type instrument). In compared, the needle-like shaped crystal, X-



like shaped crystals and loose microspheres showed the values ofH50 of 78.4, 82.5, and 77.3 cm, respectively. These results suggested that LLM-105 spherulites show the best safety property than the other three kinds of particles, which demonstrates that this novel spherulitic self-assembly emulsion crystallization procedure is especially important for the application of LLM-105. To gain insight into the effect of the morphology of LLM-105 crystals on their mechanical properties, the compressive stiffness tests (CST) were carried out in an Instron 5582 materialstesting machine, with a 0.05 mm/min compressing speed. Three samples of each product were tested to validate the reproducibility of the measurement. The initial secant modulus (EISM) was used to characterize the mechanical properties of materials. The high value of the EISM means that the material has a high resistant to the compressive deformation. There is a clear difference in the compaction curves of different particles as shown in Figure 7. The spherical particles appear much “harder” or “stiffer” compared to those of nonspherical particles (needle-like and X-like shaped crystals). Then the values of the EISM of needle-like crystals, X-like shaped crystals, loose microspheres and compact spherulites, which were computed based on the compaction curves, are 37.59, 32.51, 77.35 and 113.17 MPa, respectively. It shows obvious differences in the values of the EISM. The EISM of the spheruites particles is about three times as that of the needle-like particles, and approximately double as that of the incompact microspheres particles. It can be concluded that the spherulites show promising mechanical strength improvements over the needle-like crystals formed by this conventional crystallization method.

self-assembled. The presented crystallization methodogy with modifying the intermolecular hydrogen bonds by a polymer additive in this work could be employed not only in energetic organic small molecular but also in some other organic small molecular materials with abundant hydrogen bond network.

4. CONCLUSIONS In summary, spherulitic particles of LLM-105 were successfully self-assembled by a polymer-induced emulsion crystallization method. By this method, the size of LLM-105 spherulites can be tuned by changing the concentration of PVP. The obtained LLM-105 spherulites have much better mechanical and safety properties than common needle-like crystal products. Therefore, the military application field of LLM-105 might be extended because of spherulites with excellent properties. Moreover, the morphological evolution processes of LLM-105 crystals obtained in the presence and absence of PVP were successfully captured in spite of the quickly crystallization process, which are crucial for the rational understanding the underlying self-assembled mechanism. And 1H NMR spectroscopic results also indicated that the intermolecular hydrogen bonds of LLM-105 were broken by PVP. Based on these experiments, a reasonable spherulite growth mechanism is proposed. In short, this emulsion crystallization technique cooperating with modifying the hydrogen bonds in LLM-105 molecular by polymeric additives is an excellent method for the formation of LLM-105 spherulites. Furthermore, this novel crystallization method will be useful to design and fabrication of spherical particles of other organic explosives with abundant strong intermolecular hydrogen bonds.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. SEM images, OM images, FT-IR spectra, DSC and TG curves (PDF)

AUTHOR INFORMATION Corresponding Author *Email: Q. Zhang, [email protected]; [email protected]; H. Z. Li, [email protected].

Notes The authors declare no competing financial interest. Figure 7. Particle size distributions of (a) needle-like crystals, (b) X-like shaped crystals obtained in the absence of PVP, (c) loose microspheres and (d) spherulites obtained in the presence of PVP of LLM-105.

ACKNOWLEDGMENT

The particle shape, particle size, and size distribution are generally responsible for the physicochemical properties of powder products. From our study in this paper, it can be concluded that the physicochemical properties of spherulites are much better than that of needle-like crystals of LLM-105. The polymer-induced spherical crystallization method enables preparing energetic organic small molecular materials with better physicochemical properties. To the best of our knowledge, it is the first time that LLM-105 spherulites were

REFERENCES

We greatly appreciate the financial support from the National Natural Science Foundation of China (no.11302199 and no.11672273).

(1) Shekunov, B. Y.; York, P. Crystallization Processes in Pharmaceutical Technology and Drug Delivery Design. J. Cryst. Growth 2000, 211, 122−136. (2) Myerson, A. S. Handbook of Industrial Crystallization, 2nd ed. Butterworth-Heinemann: Boston, 2002. (3) Variankaval, N.; Cote, A. S.; Doherty, M. F. From Form to Function: Crystallization of Active Pharmaceutical Ingredients. AIChE J. 2008, 54,1682−1688.


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(4) Modi, S. R.; Dantuluri, A. K. R.; Puri, V.; Pawar, Y. B.; Nandekar, P.; Sangamwar, A. T.; Perumalla, S. R.; Sun, C. C.; Bansal, A. K. Impact of Crystal Habit on Biopharmaceutical Performance of Celecoxib. Cryst. Growth Des. 2013, 13, 2824−2832. (5) Van der Steen, A. C.; Verbeek, H. J.; Meulenbrugge, J. J. Influence of RDX Crystal Shape on the Shock Sensitivity of PBXs, Proceedings of the 9th International Detonation Symposium on Detonation, Portland, Oregon, USA, August 28 − September 1; Office of the Chief of Naval Research: Arlington, VA, USA, 1989, 83−88. (6) Li, J.; Wu S.; Lu K.Study on Preparation of Insensitive and Spherical High Bulk Density Nitroguanidine with Controllable Particle Size. Propellants, Explos., Pyrotech. 2016, 41, 312–320. (7) Zhang, C.; Peng, Q.; Wang, L.; Wang, X. Propel. Thermal Sensitivity of HMX Crystals and HMX-Based Explosives Treated under Various Conditions. Propellants, Explos., Pyrotech. 2010, 35, 561−566. (8) Mandal, A. K.; Thanigaivelan, U.; Pandey, R. K.; Asthana, S.; Khomane, R. B.; Kulkarni, B. D. Preparation of Spherical Particles of 1,1-Diamino-2,2-dinitroethene (FOX-7) Using a Micellar Nanoreactor. Org. Process Res. Dev. 2012, 16, 1711−1716. (9) Kawashima, Y.; Okumura, M.; Takenaka, H. Spherical Crystallization: Direct Spherical Agglomeration of Salicylic Acid Crystals During Crystallization. Science 1982, 216, 1127−1128. (10) Kawashima, Y.; Niwa, T.; Handa, T.; Takeuchi, H.; Iwamoto, T.; Itoh, K. Preparation of Controlled-release Microspheres of Ibuprofen with Acrylic Polymers by a Novel Quasi-emulsion Solvent Diffusion Method. J. Pharm. Sci. 1989, 78, 68−72. (11) Roelands, C.; Ter Horst, J. H.; Kramer, H. J.; Jansens, P. J. Precipitation Mechanism of Stable and Metastable Polymorphs of LGlutamic Acid. AIChE J. 2007, 53, 354−362. (12) Kawashima, Y.; Cui, F.; Takeuchi, H.; Niwa, T.; Hino, T.; Kiuchi, K. Improved Static Compression Behaviors and Tablettabilities of Spherically Agglomerated Crystals Produced by the Spherical Crystallization Technique with a Two-solvent System. Pharm. Res. 1995, 7,1040−1044. (13) Paradkar, A.; Pawar, A.; Chordiya, J.; Patil, V.; Ketkar, A. Spherical Crystallization of Celecoxib. Drug Dev. Ind. Pharm. 2002, 28, 1213−1220. (14) Patra, C. N.; Swain, S.; Mahanty, S.; Panigrahi, K. C. Design and Characterization of Aceclofenac and Paracetamol Spherical Crystals and Their Tableting Properties. Powder Technol. 2015, 274, 446−454. (15) Thakur, A.; Thipparaboina, R.; Kumar, D.; Gouthami K. S.; Shastri, N. R. Crystal Engineered Albendazole with Improved Dissolution and Material Attributes. CrystEngComm 2016, 18, 1489−1494. (16) Kovacic, B.; Vrecer, F.; Planinsek, O. Spherical crystallization of drugs. Acta Pharm. 2012, 62, 1−14. (17) Tahara, K.; O’Mahony, M.; Myerson, A. S. Continuous Spherical Crystallization of Albuterol Sulfate with Solvent Recycle System. Cryst. Growth Des. 2015, 15, 5149−5156. (18) Javadzadeh, Y.; Vazifehasl, Z.; Dizaj, S. M.; Mokhtarpour, M. Spherical Crystallization of Drugs. Intech open, 2015, 85−104. (19) Jitkar, S.; Thipparaboina, R.; Chavan, R. B.; Shastri, N. R. Spherical Agglomeration of Platy Crystals: Curious Case of Etodolac. Cryst. Growth Des. 2016, 16, 4034−4042. (20) Yoreo, J. J. D.; Gilbert, P. U. P. A.; Sommerdijk, N. A. J. M.; Penn, R. L.; Whitelam, S.; Joester, D.; Zhang, H. Z.; Rimer, J. D.; Navrotsky, A.; Banfield, J. F.; Wallace, A. F.; Michel, F. M.; Meldrum, F. C.; Cölfen, H.; Dove, P. M. Crystallization by Particle Attachment in Synthetic, Biogenic, and Geologic Environments. Science 2015, 349, 6760. (21) Kim, K. J. Spherulitic Crystallization of 3-nitro-1,2,4-triazol5-one in Water+N-methyl-2-pyrrolidone, J. Crys. Growth 2000, 208, 569-578. (22) Heintz, T.; Fuhr, I. Generation of Spherical Oxidizers Particles by Spray and Emulsion Crystallization. VDI-Ber. 2005, 1901, 471−476.

(23) Zhang, C.; Wang, X.; Huang, H. π-Stacked Interactions in Explosive Crystals: Buffers against External Mechanical Stimuli. J. Am. Chem. Soc. 2008, 130, 8359−8365. (24) Liu, H.; Zhao, J.; Du, J.; Gong, Z.; Ji, G.; Wei, D. Highpressure Behavior of TATB Crystal by Density Functional Theory. Phys. Lett. A 2007, 367, 383–388. (25) Dobratz, B. M. The Insensitive High Explosive Triaminotrinitrobenzene (TATB): Development and Characterization —1888 to 1994. LA-13014-H. 1995. (26) Lovette, M. A.; Doherty, M. F. Needle-Shaped Crystals: Causality and Solvent Selection Guidance Based on Periodic Bond Chains. Cryst. Growth Des. 2013, 13, 3341−3352. (27) Ma, R.; Sasaki, T. Nanosheets of Oxides and Hydroxides: Ultimate 2D Charge-Bearing Functional Crystallites. Adv. Mater. 2010, 22, 5082-5104. (28) Rogers, J. A.; Lagally, M. G.; Nuzzo, R. G. Synthesis, Assembly and Applications of Semiconductor Nanomembranes. Nature 2011, 477, 45-53. (29) Pagoria, P. F.; Mitchell, A. R.; Schmidt, R. D.; Simpson, R.; Garcia, F.; Forbes, J. W.; Swansiger, R.; Hoffman, D. Synthesis, Scale-up and Characterization of 2,6-diamino-3,5-dinitropyrazine-1oxide. Technical Report No. UCRL-JC-130518, Lawrence Livermore National Laboratory, 1998. (30) Averkiev, B. B.; Antipin, M. Y.; Yudin, I. L.; Sheremetev, A. B. X-ray Structural Study of Three Derivatives of Dinitropyrazine, J. Mol. Struct. 2002, 606, 139–146. (31) Gilardi, R. D.; Butcher, R. J. 2,6-Diamino-3,5-dinitro-1,4pyrazine 1-oxide. Acta Crystallogr., Sect. E: Crystallogr. Commun. 2001, 57, 0657-0658. (32) Pagoria, P.; Zhang, M.; Zuckerman, N.; Lee, G.; Mitchell, A.; DeHope, A.; Gash, A.; Coon, C.; Gallagher, P. Synthetic Studies of 2,6-Diamino-3,5-Dinitropyrazine-1-Oxide (LLM-105) from Discovery to Multi-Kilogram Scale. Propellants, Explos., Pyrotech. 2017, 42, 1-14. (33) Li, M.; Huang, M.; Kang, B.; Wen, M.; Li, H.; Xu, R. Quality Evaluation of RDX Crystalline Particles by Confined Quasi-Static Compression Method. Propellants, Explos., Pyrotech. 2007, 32, 401−405. (34) Li, M.; Wen, M.; Huang, Ming.; Xu, R.;, Li, H.; Xu, R. Evaluation of Coherence Strength of Energetic Crystalline Granules by Compressive Stiffness Method. Chinese Journal of Energetic Materials, 2007, 15, 244-247. (35) Gránásy, L.; Pusztai, T.; Tegze, G.; Warren, J. A.; Douglas, J. F. Growth and Form of Spherulites. Phys. Rev. E 2005, 72, 011605.



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A Novel Spherulitic Self-assembly Strategy for Organic Explosives: Modifying the Hydrogen Bonds by Polymeric Additives in Emulsion Crystallization Xiaoqing Zhou, Qi Zhang,* Rong Xu, Dong Chen, Shilong Hao, Fude Nie, and Hongzhen Li*

A novel facile spherical emulsion crystallization strategy for organic explosive small molecule has been developed through modifying the hydrogen bonds by a polymeric additive. By this strategy, spherulites of 2,6-diamino-3,5-dinitropyrazine-1oxide (LLM-105) have been successfully prepared with the assistance of polyvinyl pyrrolidone (PVP). The obtained LLM-105 spherulites exhibit better mechanical and safety properties than common needle-like crystals. Consequently, the military application field of LLM-105 might be extended because of this novel spherical crystallization process.


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A Novel Spherulitic Self-Assembly Strategy for Organic Explosives

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