The Mitigation Effect of Synthetic Polymers on ... - ACS Publications

Article pubs.acs.org/JPCC

The Mitigation Effect of Synthetic Polymers on Initiation Reactivity of CL-20: Physical Models and Chemical Pathways of Thermolysis Qi-Long Yan,† Svatopluk Zeman,*,† P. E. Sánchez Jiménez,‡ Tong-Lai Zhang,§ L. A. Pérez-Maqueda,‡ and Ahmed Elbeih∥ †

Institute of Energetic Materials, University of Pardubice, 53210 Pardubice, Czech Republic Instituto de Ciencia de Materiales de Sevilla, CSIC-Universidad de Sevilla, C. Américo Vespucio No. 49, 41092 Sevilla, Spain § State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, 100081 Beijing, China ∥ Military Technical College, 11787 Cairo, Egypt ‡

S Supporting Information *

ABSTRACT: In this paper, the thermal decomposition physical models of different CL-20 polymorph crystals and their polymer bonded explosives (PBXs) bonded by polymeric matrices using polyisobutylene (PIB), acrylonitrile butadiene rubber (NBR), styrene butadiene rubber (SBR), Viton A, and Fluorel binders are obtained and used to predict the temperature profiles of constant rate decomposition. The physical models are further supported by the detailed decomposition pathways simulated by a reactive molecular dynamics (ReaxFF-lg) code. It has been shown that both εCL-20 and α-CL-20 decompose in the form of γ-CL-20, resulting in close activation energy (169 kJ mol−1) and physical model (first-order autoaccelerated model, AC1). Fluoropolymers could change the decomposition mechanism of ε-CL-20 from the “first-order autocatalytic” model to a “three-dimensional nucleation and growth” model (A3), while the polymer matrices of Formex P1, Semtex, and C4 could change ε-CL-20 decomposition from a single-step process to a multistep one with different activation energies and physical models. Compared to fluoropolymers, PIB, SBR and NBR may make ε-CL-20 undergo more complete N−NO2 scission before collapse of the cage structure. This is likely the main reason why those polymer bases could greatly mitigate the decomposition process of ε-CL-20 from a single step to a multistep, resulting in lower impact sensitivity, whereas fluoropolymers have only a little effect on that. For ε-CL-20 and its PBXs, the impact sensitivity depends not only on the heat built-up period of their decomposition, but also on the probability of hotspot generation (defects in solid crystals and interfaces) especially when it decomposes in a solid state. °C could be fitted well with a power law mechanism function.20 It has been shown from fast-heat-and-hold/Fourier transform infrared (FTIR) spectroscopy that N−NO2 homolysis is the rate-limiting step at lower temperature range, and NO2 is one of the dominant gaseous products,21 which was confirmed by FTIR spectroscopic technique22 and simultaneous TG-DTAFTIR-MS experiments.23 In fact, monomolecular dissociation mechanisms of CL-20 ions was studied by electron impacting mass spectroscopy with metastable mass-analyzed ion kinetic energy spectroscopy (MIKES),24 which will be discussed together with our data later in this paper. There are several polymorphs for CL-20 (α-, β-, δ-, γ-, ε-),25,26 among which the ε- form is the most powerful and widely used one.27 The crystal structure plays an important role in the reactivity and sensitivity of CL-20, and the impact sensitivities of pure polymorph

1. INTRODUCTION In order to produce safe munitions capable of surviving unwanted mechanical stimuli such as shocks from explosions and impacts by projectiles, the initiation reactivity of explosives has to be significantly reduced. Such reactivity could be reduced by employing the secondary explosive fillers, typically cyclic nitramines, into polymeric matrices that are chemically stable and compatible with these explosives,1−3 forming one of the representative insensitive materials “polymer bonded explosives” (PBXs). As a relatively new profound explosive filler, 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL20) has been used and evaluated in several kinds of PBXs.4−10 CL-20 is a three-dimensional caged structure polycyclic nitramine, which has been extensively investigated during past decades,11−13 including its thermal behavior and initiation reactivity evaluated by using various experimental techniques,14−16 density functional theory (DFT),17 and molecular dynamics (MD) simulations.18,19 It decomposes at about 230 °C, and the rate curves in the temperature range of 190−240 © 2014 American Chemical Society

Received: June 16, 2014 Revised: September 9, 2014 Published: September 18, 2014 22881

dx.doi.org/10.1021/jp505955n | J. Phys. Chem. C 2014, 118, 22881−22895

The Journal of Physical Chemistry C

Article

method,43 and α-CL-20 usually exists as a hydrate where the centrosymmetric cavities in the crystal lattice are occupied.44 The preparations of the samples (CL-20-VA, CL-20-FL, CL-20SE, CL-20-FM, and CL-20-C4) are described in our previous papers.4−10 The sample names are based on involved polymers, where SE is the abbreviation of Semtex 10 binder (15% by weight), containing NBR plasticized by a inert plasticizer with an aromatic unit in its molecule; FM is the abbreviation of Formex P1 matrices (13% by weight) containing SBR plasticized by oily material; C4 is the abbreviation of Composition C-4 binder (9% by weight), containing PIB, plasticized by dioctyl sebacate (DOS) and HM46 oil; VA is the abbreviation of Viton A binder (Viton A 200, 9% by weight), which is a copolymer of vinylidene fluoride and hexafluoropropylene with a fluorine content of 66%; FL is the abbreviation of Fluorel binder (Dyneon FT 2481, 9% by weight), which is a terpolymer of tetrafluoroethylene, vinylidene fluoride, and hexafluoropropylene also containing nonfluorinated aromatic building units. 2.2. Experimental techniques. The nonisothermal decomposition kinetics and physical models of involved materials are studied by means of the TG technique. The experiments were carried out on a Netzsch 209F3 instrument (Al2O3 crucible) under linear heating conditions. Generally, a wider heating rate range is in favor of higher reliability of kinetic evaluation. However, for such high energy density compounds, relatively high heating rate (greater than 5 °C) would result in burning except Rs-CL-20-C4,4−10 which should be excluded for kinetic calculation, and hence the heating rate was limited to a small range (e.g., 0.6−5 °C). The test temperature range for TG was 30−400 °C, with the sample mass of 1.80−2.40 mg using 50 mL·min−1 dynamic nitrogen atmosphere.

modifications of CL-20 were specified as follows (in J): 13.2 for ε-, 10.1 for α-, 11.9 for β-, and 12.2 for γ-modification.28,29 As a matter of fact, the synthetic polymers including styrene−butadiene rubber (SBR), acrylonitrile- butadinene rubber (NBR) and polyisobutylene (PIB) are usually much more stable than explosive fillers.2 However, the matrices based on these polymers including Formex P1, Semtex 10 and C4 were found to have strong effect on the reactivity of ε-CL-20 by mitigating its decomposition from a fast one-step (or a complex completely overlapped step) reaction to a slow multistep (or partially overlapped) reactions.4−8 Such polymer matrices could, therefore, significantly decrease the impact sensitivity of ε-CL-20, while some others polymers (e.g., fluoropolymers) have only a little effect on that.30−32 As far as we know, the mechanisms for such mitigation effects are still not well understood at a molecular level. The decomposition activation energies, physical models, and chemical pathways of cyclic nitramines could be greatly changed by polymer matrices, and such changes may be responsible for the improvement of impact sensitivity.33,34 The microscopic processes involved in initiation by impact are complicated and still not well interpreted. In general, it is believed that ignition by impact starts at the pockets of hotspots generated from energy localization.35,36 Although the actual mechanism of hotspots formation has not been experimentally determined, a variety of mechanisms were proposed for this process, including adiabatic compression of trapped gas in voids, friction involving sliding or impacting surfaces, shear band formation caused by mechanical failure, sparks, triboluminescence, and heating at crack tips.37 It means the impact initiation originated from a runaway chemical reaction due to self-heating by exothermic decomposition started at the site of hotspots. Therefore, the accurate modeling of impact sensitivity for high explosives including ε-CL-20 based PBXs requires an understanding of their mechanical, thermal, and chemical responses during the impact and subsequent exothermic chemical reactions.38,39 In this case, it is better to compare their decomposition physical models and chemical pathways due to interaction between polymer chains and ε-CL-20 molecules. However, the decomposition physical models for most of the ε-CL-20 based PBXs are not available from the literature, and hence first this paper is principally concerned with the interpretation of the decomposition physical models obtained by both combined kinetic analysis40 and master plots methods.41 Then the kinetic triplets are used to predict the temperature profiles for their constant rate decomposition, which are also connected with the impact initiation process. In order to clarify the chemical process for impact initiation, the detailed decomposition pathways under fast heating (300 K/ps) are obtained directly from the MD trajectories using a reactive force field code (ReaxFF-lg), which enable one to directly probe the condensed phase chemistry under extreme conditions of temperature and pressure, identifying the key bimolecular radical reactions responsible for the low activation route.42 The connections among the decomposition pathways, physical models, and the impact sensitivities are proposed. On this basis, how polymer matrices improve the impact sensitivity of CL-20 crystals by mitigating its initial decomposition is clarified.

3. THEORETICAL BACKGROUNDS 3.1. Determination of Kinetic Triplets. In order to obtain a complete kinetic description of a solid-state reaction, the kinetic parameters (triplet), namely the apparent activation energy (Ea), pre-exponential factor (A), and kinetic model ( f(α)) of each individual process, should be determined, where α represents the extent of conversion, and f(α) describes the way reactants convert to products. Isoconversional (model-free) methods are standard procedures for determining the activation energy of a process, regardless of any previous knowledge of the kinetic model. Friedman’s isoconversional method has been used to evaluate the dependence of activation energy on extent of conversion, which is widely discussed in the literature.41,45 Moreover, we have used the so-called “combined kinetic analysis” method for determining the kinetic triplet in a straightforward manner.40 The combined kinetic method implies a simultaneous analysis of experimental data obtained under arbitrary heating history. This procedure is based on the fact that only the true kinetic model simultaneously fits all experimental data yielding a unique f(T) function. Here a modified Šesták-Berggren (SB) equation (eq 1) has been used to fit the experimental data. Equation 1 acts like an umbrella since it can fit every kinetic function corresponding to the ideal models used in the literature and even accommodate any possible deviations from the ideality.46,47

2. EXPERIMENTAL SECTION 2.1. Materials. α-CL-20 is a product of the Explosia pilot plant in Czech Republic, RS-ε-CL-20 and ε-CL-20 were prepared by our work group by a special recrystallization

f (α) = cα m(1 − α)n

or

f (α) = α M(1 − α)N

(the latter is for master plots method) 22882

(1)



Article

Figure 1. Equilibrated packing structure of ε-CL-20 crystal with SBR and Viton A polymer chains shown as examples (one ε-CL-20 crystal contains 20 molecules; each unit cell contains 15 molecules of polymers; sizes are in Å, and the minimum distance of molecules is 2.1 Å).

The PBX models could be built as follows: a ε-CL-20 crystal is placed in one polymer matrix (e.g., SBR and Viton A shown in Figure 1) where the crystal is directly contacted with the polymer. It is clear from Figure 1 that SBR has larger branches with lower packing density than Viton A, which is also the case in reality (SBR has density of around 1.0 g.cm−3, while Viton A has 1.64 g.cm−3). The periodic cell of ε-CL-20 was optimized separately using ReaxFF, based on which the PBX models were equilibrated at 100 K and 1 atm using an NPT conditions (constant pressure, constant temperature, and constant number of atoms). The energies of polymer/CL-20 systems were minimized and then equilibrated at 100 K using NVT conditions (constant volume, constant temperature, and constant number of atoms) for 10 ps with a time step of 0.2 fs to eliminate bad contacts at the interface. Then all the prepared models are subject to molecular dynamics under the same conditions specified in the discussion part. The temperature of the system was controlled by a Berendsen thermostat with damping constant of 50 fs. The input parameters are the coordinates of the molecules, temperature, and force field parameters, and the outputs include the trajectory of the reaction processes, potential energies, and total fragments.

The combined kinetic analysis is based on the following eq:19 ⎡ dα /dt ⎤ E ln⎢ m n ⎥ = ln(cA) − ⎣ α (1 − α) ⎦ RT

(2)

Evaluating the parameters of eq 2 requires one to simultaneously substitute all kinetic data α and dα/dt vs T. The best fit values of the parameters are obtained when the best linearity of a plot of the left-hand side of eq 2 against the reciprocal temperature. The yields m and n (M and N) have no physical meaning, and SB function is a mere fitting equation. However, the comparisons of the obtained SB function with ideal kinetic models yields valuable insight regarding the type of kinetic mechanism driving the process. Additionally, in all cases, the z(α) and y(α) master plots method48−50 has been used for confirming the kinetic models. 3.2. Molecular Dynamics Simulation by a Reactive Force Field (ReaxFF). There has been enormous experimental and theoretical progress in recent years in thermal initiation of high energetic materials (HME), but a wide variety of phenomena remains not well-understood. A particular challenge is to understand the decomposition and subsequent reactions of HEM under the extreme conditions due to limited experimental techniques for the complex and fast chemical processes. Molecular dynamics (MD) with reactive potentials that exhibit simple, fast, and exothermic chemistry has been very useful to characterize fast chemical reactions in an isolated system.51 ReaxFF is a new-generation MD code using bondorder-dependent reactive force field based on extensive ab initio quantum mechanical (QM) calculations. It enables the simulation of HEM under realistic conditions in a computationally efficient way.42 Thus, ReaxFF could describe both the gas phase chemistry of hydrocarbons and various C−H−N-O energetic systems, including the derived decomposition pathways of RDX.52,53 It has been widely used in simulation of chemical pathways of high energetic materials decomposition, where “HE” force field was especially configured and used for nitramines.54,55 Another more suitable force field called “CHONSSi-lg” was developed recently for explosives based on “HE” force field considering London Dispersion,56 which does not take into account of force field of fluorine. In order for a better comparison, all of the involved materials have been simulated under extremely fast heating (300 K/ps) by using a “TiOCHNCl”, which is suitable for both C−H−N−O based energetic materials and polymers including hydrocarbons and fluoropolymers.57 It may provide us some information on hotspot initiation of the involved materials during impact loading, which makes the connections among impact sensitivity, decomposition pathways, and physical models possible.

4. RESULTS AND DISCUSSION 4.1. Thermal Decomposition Kinetics. 4.1.1. Activation Energies for Separated Processes. Most kinetic analysis procedures were developed for studying single step reactions and usually fail when applied to complex or multiple overlapping processes. According to our previously published TGA/DTG results (See Figure S1 in the Supporting Information),4−8,41,42 multi steps have been involved in decomposition of α-CL-20 (2 steps), CL-20-FM (2 steps), CL-20-C4 (2 steps), and CL-20-SM (3 steps). It has been widely accepted to use a model-free method to evaluate the dependence of activation energy on the conversion rate for the sake of better understanding of the whole decomposition process.41 However, it has been verified that the evaluation by model-free analysis does not take into account the interaction of the individual steps, and might produce the same prediction for different situations.58 Thus, model-free methods do provide correct predictions only for non-overlapping peaks or for a well-separated peaks, while unreliable kinetic parameters would be obtained if there were interactions between overlapped steps.34 Therefore, before kinetic evaluation, the overlapped peaks for decomposition of involved materials have to be separated. Recently, Perejón et al. proposed the analysis of overlapped kinetic processes by separation of the individual peaks and subsequent kinetic analysis of each separated peak.59 22883



Article

Figure 2. The peak separation procedure for multistep decomposition of α-CL-20 (2 steps), CL-20-FM (2 steps), CL-20-C4 (2 steps), and CL-20SM (3 steps) at the heating rates of 2.0 or 3.0 K·min−1. (Open squares represent the experimental data; solid lines are overall fitted curves and open circles are separated peaks.)

Figure 3. Dependence of activation energy on the extent of conversion for involved materials (the shadow represents the error bars).

separated peak data (detailed peak fitting processes are omitted for simplicity), the activation energies have been calculated separately, and the dependences of activation energies on extent of conversion for both overall decomposition processes and separated peaks are plotted in Figure 3. The activation energies of CL-20-VA, CL-20-FL, and ε-CL20 are taken from our previous papers,9,10,34 which are almost constant throughout the single (or completely overlapped) decomposition processes. In order for better comparison, one reference line has been plotted in Figure 3, which represents the average activation energy of ε-CL-20 (169 kJ mol−1). The reported decomposition activation energies in solid state23,65,66 for ε-CL-20 are in the vicinity of 166−176 kJ mol−1 for nonisothermal conditions, and in the range of 190−222 kJ mol−1 under isothermal conditions without sublimation.23,67,68 The activation energies of fluoropolymer bonded explosives containing ε-CL-20 are almost independent of the extent of

In this deconvolution procedure, the Fraser−Suzuki (FS) function (eq 3) has been used because it allows simulating the asymmetrical nature of kinetic differential curves. This procedure has been successfully applied to a number of complex solid state processes.60−64 2⎤ ⎡ ⎛ ⎛ x − a1 ⎞ ⎞ ⎥ ⎢ y = a0 exp − ln 2⎜⎜ln⎜1 + 2a3 ⎟ /a3⎟⎟ ⎢ a 2 ⎠ ⎠ ⎥⎦ ⎝ ⎝ ⎣

(3)

where a0, a1, a2, and a3 are amplitude, position, half-width, and asymmetry of the peak, respectively. The typical separation processes are shown in Figure 2. According to Figure 2, the overlapped peaks are well separated with correlation coefficients higher than 0.99. During this procedure, one has to use the same asymmetry (a3) for the same steps at different heating rates, which, therefore, makes the separated peaks correlate with each other. Based on the 22884



Article

Table 1. Parameters for Reaction Models of CL-20 and Its PBXs Evaluated by Non-isothermal TG Experiments Combined kinetic method samples α-CL-20 α-CL-20second ε-CL-20 Rs-ε-CL-20 ε-CL-20-SEfirst ε-CL-20-SEsecond ε-CL-20-SEthird ε-CL-20-C4first ε-CL-20-C4second Rs-ε-CL20-C4first Rs-ε-CL20-C4second ε-CL-20-FMfirst ε-CL-20-FMsecond ε-CL-20-VA ε-CL-20-FL first

m 0.820 0.708 0.677 0.709 0.721 0.667 0.845 0.648 0.708 0.349 0.024 0.042 0.615 0.514 0.593

n

Ea

0.821 0.456 0.464 0.463 0.814 0.764 0.467 0.919 0.518 1.195 0.860 0.301 0.397 0.507 0.590

89.0 ± 0.2 178.7 ± 0.9 166.3 ± 1.3 184.1 ± 1.5 227.2 ± 0.2 195.6 ± 0.7 175.7 ± 1.1 177.8 ± 2.2 200.8 ± 2.1 142.5 ± 1.2 211.8 ± 1.1 241.8 ± 1.6 196.9 ± 1.4 200.5 ± 1.4 204.0 ± 1.4

Master plots method cA/min−1 9.0 1.7 1.1 1.5 4.8 1.7 3.7 4.6 2.1 6.2 3.7 2.4 1.9 3.8 1.4

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.5 × 10 0.4× 1018 0.4× 1017 2.8× 1019 0.3× 1022 0.3× 1019 0.9× 1017 1.6× 1017 1.1× 1020 1.8× 1014 0.9× 1021 1.1× 1024 2.7× 1020 0.9× 1020 0.3× 1021

6

m

M

N

Ea(ca)

Lg(A)(mf)

2.46 3.04 2.59 2.31 1.56 0.28 3.62 3.85

0.73 0.71 0.74 0.78 0.75 0.18 0.71 -

0.43 0.52 0.48 0.37 0.52 1.24 0.38 -

89 ± 1 173 ± 3 169 ± 2 178.3 228 ± 2 194 ± 6 171 ± 7 166 ± 5 230 ± 24 159 ± 8 177 ± 7 243 ± 8 194 ± 13 184 ± 4 206 ± 3

9.46 19.62 19.18 20.31 24.25 20.64 19.01 18.03 25.43 17.90 20.41 26.53 21.31 16.97 22.66

Notes: (1) superscripts 1st, 2nd, 3rd mean different steps of decomposition; ca, means the calculated values taken from the authors’ previous papers,4−10 which are the average values calculated by Friedman’s isoconversional method (0.3 < α < 0.7); mf, values are obtained by model fitting using the corresponding average activation energies and physical models; (2) Ea, activation energy, in kJ mol−1.

releasing of O2 and NO2, which will be clarified in the following sections. The second step of α-CL-20, CL-20-C4 and the third mass loss step of CL-20-SE are considered as fast autocatalytic decomposition process of CL-20 with cleavage of cage structure releasing HCN, N2, CO2 and H2O. The polymer matrices of Formex P1, Semtex 10 and C4 could greatly decrease the impact sensitivity of ε-CL-20.30−32 Their multistage decomposition due to mitigation effect of these polymers may be responsible for the high impact energy for initiation, which will be further clarified in the following sections. 4.1.2. Physical Models for the Decomposition Reactions. Based on the above-mentioned theory in section 3.2, the characteristic functions z(α) and y(α) have been plotted for the studied materials, but they are not presented here for saving space (see Supporting Information Table S1, Figure S3, and Figure S4). It has been found that the y(α) curves obtained for different heating rates almost overlap with each other, even though there are small shifts for those of Rs-ε-CL-20 and CL20-FL, which indicates that their decomposition processes are slightly dependent on the thermal history. The corresponding parameters for the JMA and AC models are listed in Table 1 (the detailed procedure for this method could refer to the authors’ previous paper48). If we apply the combined kinetic method (eq 2) to experimental data of the same sample (triplets of T, dα/dt, α), the plots of ln[(dα/dt)/f(α)] versus reciprocal of temperature (1/T) under arbitrary experimental conditions could be obtained (shown in Figure S5). In order to exclude the errors of the induction period, only the data in the range of 0.1< α < 0.9 has been considered. It has been shown in Figure S5, that all experimental data, regardless of the heating rate under which they were obtained, can be fitted simultaneously with acceptable correlation coefficient (>0.98), indicating that each of the one-step processes (for ε-CL-20, CL-20-VA and CL-20-FL) and separated processes (for α-CL20, CL-20-FM, CL-20-SE and CL-20-C4) can be described by a single kinetic triplet. However, there are still some deviations for Rs-ε-CL-20 and CL-20-FL, which are in agreement with the minor changes of αmax,y values for different heating rates. The kinetic triplets are thus obtained by such fitting procedures and

conversion, indicating the effect of Viton A and Fluorel is almost equivalent. The activation energy of CL-20-FL and CL20-VA is much higher than that of the pure ε-CL-20, resulting in lower rate constants and higher thermal stability. The average activation energy for the second step of α-CL-20 (173 kJ mol−1, and 176 kJ mol−1 was reported in the literature69) and the third step of CL-20-SE (171 kJ mol−1) are almost identical to that of pure ε-CL-20, showing the same rate-limiting chemical process. This fact proves that both ε-CL-20 and α-CL20 decomposes in the form of γ-CL-20,26 resulting in identical activation energy. However, such activation energy is much lower than that of the second step of CL-20-C4 (230 kJ mol−1). It means Semtex matrix has little effect on crystal morphology of ε-CL-20, while C4 has a great effect on that due to higher solubility of ε-CL-20 in polar plasticizer DOS.33,34 The modification of ε-CL-20 crystal surface by reducing the defect (Rs-ε-CL-20) could increase its thermal stability,43 resulting in higher activation energy (178 kJ mol−1) and lower impact sensitivity. However, the modified CL-20 crystals are less compatible with polar plasticizers (such as DOS), which would induce its polymorphic transition during heating,42 resulting in much lower thermal stability (e.g., in C4 polymer base; see Figure S2) and initial activation energy (159 kJ mol−1). C4 also makes Rs-ε-CL-20 less exothermic, where self-heating has been greatly decreased. Therefore, unlike the other PBXs, Rs-ε-CL20-C4 can undergo kinetically controlled decomposition without burning at higher heating rate (over 15 K·min−1).43 The activation energies of CL-20-FM, CL-20-SE, and α-CL20 are slightly different from our previously published results4−6,34 due to improvements of peak separation procedure and extended heating rate range for CL-20-FM (see Figure S1). For instance, the first step of α-CL-20 mass loss is due to dehydration, which contributes about 5% to the whole process according to the peak area. The activation energy of this process was calculated to be 225.3 kJ mol−1, which is too high for a dehydration process.34 This was caused by inappropriate peak separation, which has been reconsidered and the corresponding average activation energy was obtained herein as 89.4 kJ mol−1. The reason for the first step of CL-20-C4 and first two mass loss steps of CL-20-SE correspond to slow 22885



Article

Figure 4. A comparison of normalized curves of obtained kinetic models with some ideal models for RDX and its PBXs by combined kinetic methods. Notes: “D2: two-dimensional diffusion; R2: phase boundary controlled reaction (contracting area); R3: phase boundary controlled reaction (contracting volume); F1, first-order reaction, so-called unimolecular decay law, where random nucleation followed by an instantaneous growth of nuclei; A2, A3: random nucleation and two- and three-dimensional growth of nuclei through different nucleation and nucleus growth models; AC1: first-order autocatalytic (self-heating) model, where f(α) = α(1 − α)0.5.

comparison, all the obtained model curves are normalized at α = 0.5 and compared with those ideal physical models41,45−47 as described in the caption of Figure 4. It is clear from Figure 4, that the effects of the polymer matrices on the decomposition mechanism of ε-CL-20 are significant. According to the results, the decomposition reaction model of pure ε-CL-20 (as well as for Rs-ε-CL-20) is very close to AC1 (first-order autoaccelerated model), which is in agreement with the literature.14,22,23 Under the effect of fluoroelastomers, the reaction model shifts to a “threedimensional nucleation and growth model”, indicating slight mitigation of autocalytic effect of the overall rate limiting process. In fact, for thermal stable fluoroelastomers, they usually decompose over 400 °C separately, when even used as a binder for RDX or 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX),70 and they could stabilize RDX or HMX not only by decreasing its sublimation, but also by hindering its decomposition product releases. The polymer matrices containing mineral oil materials and plasticizers (Formex P1, Semtex, and C4) have stronger effect on the decomposition mechanism of CL-20, changing its decomposition process from a single step to a multistep with different activation energies and physical models. The chain structures of involved hydrocarbon polymers (SBR, PIB, and NBR) also decompose separately with ε-CL-20 at higher temperature, especially for PIB and SBR.71−74 There might be some interaction between the polymer chains and CL-20 during heating, making of CL-20 decompose slowly at the beginning. The corresponding chemical mechanisms will be specified in the final part of this paper. The initial steps of CL-20-FM, CL-20-C4, and CL-20-SE follow the nucleation and growth model (A2 and A3), but the mechanisms of their final steps are close to the first-order autocalytic model like pure ε-CL-20. For Rs-ε-CL-20, the interaction with C4 binder makes it decomposes much slower following reaction order models (R3 and F1). As mentioned above, the self-heating effect of Rs-ε-CL-20 during its decomposition has been greatly mitigated by C4 binder. 4.3. The Connection between the Kinetic Model and Impact Sensitivity. As mentioned in Introduction, the ignition by impact starts from pockets of hotspots generated from energy localization at the defects. Real explosive formulations are defective (plastic bonded, porous, etc.), and these heterogeneities lead to hotspots and a complex

compared with those obtained by master plots method in Table 1. It is clear from Table 1 that the activation energies obtained by combined kinetic method are very close to those obtained by isoconversional method. The exponential factors obtained by model fitting are more accurate than the calculated ones by isoconversional method, where the kinetic model has been neglected. The inclusion of polymers could greatly change the degradation mechanism of CL-20 by changing the decomposition models, rate constants, and activation energies. The attempted heating rates for ε-CL-20 and its PBXs are very limited due to burning at higher heating rate caused by selfheating as mentioned in our previous papers.4−6,9,10 In these cases, the kinetic predictions for higher heating rate (above 5 K· min−1) will not be reliable because it is impossible to completely exclude the self-heating effect in reality. It can be noticed that the parameters (m1 and n1 vs M and N) are very different depending on the method employed, and even the same SB model has been used. The obtained physical models have to be plotted and normalized for better comparison in the same graph, which will be done after reliability tests in the following section. 4.2. Reliability of Obtained Reaction Models and Their Physical Interpretation. The curve shape of the models obtained by combined kinetic method and master plots method are very close to each other even they have different mathematic expressions (see Figure 4 and Figure S6)). It means that both of the methods yield equivalent mechanism models, and one could select any one of them to reconstruct the experimental curves for the sake of reliability. The α−T curves have been simulated by using kinetic triplets determined by combined kinetic method (shown in Figure S7). It can be seen that the simulated curves agree well with experimental ones, especially in the cases of ε-CL-20, α-CL-20 and CL-20C4. However, there are minor deviations for CL-20-FL and CL20-VA and CL-20-SE at the initial stage of decomposition due to experimental errors caused by heat and mass transfer, which have been ignored during combined kinetic analysis (only data between 0.1< α < 0.9 are considered). Regardless, the reconstructed curves with such minor deviations are acceptable if considering the experimental errors. In this case, those physical models are reliable enough, and now they are ready for interpretation and further kinetic prediction. In order for better 22886



Article

multidimensional flow even in macroscopically unidirectional detonation. The initial temperature at the hotspot that could lead to a runaway chemical reaction is very important for the impact sensitivity of the explosives. The decomposition mechanism (reaction model) has been proved to connect with the impact initiation of the RDX-based PBXs.48 Considering the explosive initially decomposing at a constant rate in very short time scale due to fast heating by the hotspot, this could result in an internal thermal build-up (selfsustainable). Hence the constant rate thermal analysis may provide some information for the initial temperature of low decomposition rate that could lead to a thermal runaway. One could use the obtained kinetic triplets to simulate constant low reaction rate temperature profiles using the following equation:75 ⎛ −E ⎞ Cr = cA exp⎜ a ⎟α m(1 − α)n ⎝ RT ⎠

ance and sensitivity parameters of different CL-20 crystals and their PBXs are summarized in Table 2. There is another factor that might affect the activation energy of the mixtures, which is known as activation entropy (ΔS‡). It could be calculated on the basis of the following equation. ΔS‡ = R(ln A − ln Tb) − 205.86

(5)

where values of Tb and ΔS‡ are listed and interpreted in Table 2. The positive ΔS‡ values give evidence of homolytic primary process of the studied PBXs decomposition. According to Table 2, the detonation velocity as well as the decomposition heat could be slightly brought down for Viton A and Fluorel, whereas over 10% decrease occurred for C4, Formex, and Semtex matrices, due to great decrease of charge density and dilution of the energy. With regard to the impact sensitivity, the polymer could greatly increase the impact energy of ε-CL-20, making it much safer to handle.30−32 Interestingly, for RDX based PBXs, it has been shown that the material that has longer heat built-up period (ti) has greater impact energy.48 One has to note that the heat built-up period means the time from the very beginning of the decomposition to the conversion stage (Δα in %) where a minimum temperature is needed (ti=Δα/Cr, e.g., for ε-CL-20, ti = 52.5%/2.5%·min−1 = 21 min). However, if we compare the sensitivity data with the shape of constant rate decomposition curves for ε-CL-20 based materials (see Figure 5), it shows different trends. RDX decomposes in liquid state, and the heat transfer is more uniform, while ε-CL-20 decomposes in solid state, and the stabilization of the crystal lattice should be taken into account. Thus, the defects on the crystal surface, voids between polymers and the crystal have great effect on the impact initiation reactivity. In this case, for similar system, the trend is the same as RDX-based materials, e.g., Rs-ε-CL-20 has longer ti than ε-CL-20 and hence has greater impact energy. The heat built-up period of ε-CL-20-FL and ε-CL-20-VA is very close (around 17.6 min), which is smaller than those of the ε-CL-20-C4 and ε-CL-20-FM (around 18.7 min), resulting in lower impact energies (6.9 and 7.2 J). ε-CL-20-SE has the larger ti than ε-CL-20-C4 and ε-CL-20-FM, but it has lower impact energy due to greater porosity for easier generation of hotspots. This phenomenon reveals that the impact initiation depends greatly not only on the heat built-up period of thermal decomposition of energetic materials (chemical properties of the materials), but also depends on the probability hotspots

(4)

where Cr is the predetermined constant reaction rate, Ea is the apparent activation energy, and cA is the integrated preexponential factor. The decomposition processes under the rate of 2.5%.min−1 for involved materials have been simulated (see Figure 5). The corresponding charge perform-

Figure 5. A comparison of constant reaction rate profiles of CL-20 and its PBXs simulated by corresponding kinetic triplets obtained by combined kinetic method.

Table 2. Some Charge Performances and Sensitivity Parameters of CL-20 and Its PBXs sample

formula

Me

VoDexp

de/dp

Im

Hd

Tb

ΔS‡

αmin

ε-CL-20 Rs-ε-CL-20 α-CL-20 CL-20-FM CL-20-C4 CL-20-SE CL-20-VA CL-20-FL

C6H6N12O12 C6H6N12O12 C6H6N12O12 C10.47H14.54N12O11.88 C9.32H16.08N12O11.98 C9.85H12.88N12O12.55 C7.22H6.83F1.51N12O11.93 C7.18H6.65F1.57N12O11.97

438.0 438.0 438.0 498.6 243.7 502.2 481.3 479.5

9530 9800 9380 8355 8594 8228 9023 8855

1.98/1.77 2.01/1.95/1.70/1.39 1.77/1.42 1.64/1.25 1.94/1.53 1.92/1.62

4.2 12.5 21.4 21.1 17.9 6.9 7.2

1348 2303 1288 1583 2639 1757 1597 1893

205.5 209.4 205.6 195.7 214.4 227.4 224.0 230.2

128.6 121.2 113.5 181.2 166.0 123.8 150.5 161.9

52.5 55.5 52.9 46.7 46.9 54.8 44.0 44.3

Notes: (a) Values are tested by our workgroup and published elsewhere;30−32 de/dp, experimental charge density/packet density for MD simulation, in g·cm−3; Me, molecular weight, g mol−1; VoDexp, experimental detonation velocity, in m·s−1; Im, impact energy, in J; Hd, heat of decomposition from DSC peaks (shown in Figure S2), in J·g−1; Tb, the critical temperature extrapolated from the onset temperature of TGA curves when the heating rate decreases to zero, in Celsius; ΔS‡, activation entropy, in J·K−1.mol−1. αmin, the conversion rate where the minimum temperature is achieved during constant rate decomposition, in %. 22887



Article

Figure 6. Total fragments from cook-off simulation of the systems pure ε-CL-20 crystal and ε-CL-20 with different polymer chains (each model cell contains one ε-CL-20 crystal and 15 molecules of polymer chains with 10 repeating units).

Figure 7. Species analyses for the systems of ε-CL-20 crystal under a linear heating rate of 300 K·ps−1 until 2500 K at 8 ps, then isothermal for another 8 ps.

generation (crystal surface and bulk properties) especially when it decomposes in solid state. 4.4. The Effect of Polymer on the Decomposition Pathways of CL-20 under Fast Heating. As discussed above, the inherent rate limiting chemical reactions are responsible for runaway decomposition during impact initiation of energetic materials. It has been proven that the activation energy of nitro based explosives is determined by the unimolecular NO2 cleavage in gas phase, but in condensed phase, the route leading to NO2 cleavage goes through a series of radical mediated bimolecular reactions with highest barrier in the sequence.76 The condensed phase decomposition induced by localized hotspots is more relevant to impact initiation of PBXs and it is also the case for shock induced initiation, where a hot spot arises from shear localization at the convex polymer asperity.77 It has been shown that the hot spot could be attenuated by the polymer binder, reaching a steady temperature state involving NO2 dissociation and HONO formation for pentaerythritol tetranitrate (PETN), and the hot-spot propagation is highly dependent on the energy release resulting from chemical reactions. Hotspots that are unable to reach a critical radius of ∼1.5 nm would self-extinguish for initiation of PETN, whereas hotspots that exceed the critical radius continue to grow with a radial velocity that becomes supersonic following a JMA nucleation and growth kinetic model.78 In order to clarify how the polymers mitigate the decomposition of ε-CL-20 from a single step to a multistep process and hence decrease the impact sensitivity, reactive molecular dynamic simulations under fast heating are carried out. The materials are heated from 100 to 2500 K with a heating rate of 300 K·ps−1

and then maintain at that temperature for 8 ps based on the packing models shown in Figure 1. The temperature program and the total gases products of involved materials are shown in Figure 6. One could notice that the changes in total fragments are very different for ε-CL-20 and its PBXs. It is reasonable that the polymer bonded materials have more fragments than pure ε-CL-20 due to decomposition of the polymers. Fluoropolymers are more stable than the others, resulting in fewer fragments. When the temperature achieves 2500 K, their fragments almost maintain constant during isothermal period. During fast heating, all of the materials start to decompose slowly at around 1.5 ps, and the rapid decomposition starts after 3 ps with some new fragments. If we normalized the gases production curves, as shown in Figure 6b, it is interesting to notice that the gas releasing rates of fluoropolymer bonded materials are much faster at the beginning than those of the other polymers; these fluoroelastomers have a stabilization effect on the thermal stability of CL-20.13,30 It means that the decomposition of corresponding PBXs started at higher temperatures as it is in the case of the other studied PBXs with softened binders (it is well documented also by Tb values in Table 2). This fact leads to above-mentioned faster evolution of fragments. These fluoroelastomers have, according to the thermochemistry of detonation, positive effect on their PBXs performance.30−32 The detailed compositions of the main fragments are monitored and plotted in Figures 7−9. The generated fragments during fast heating of pure ε-CL-20 crystal are plotted in Figure 7. It has been shown that ε-CL-20 started to decompose at 2 ps (600 K, whereas it is 475 K at heating rate of 5 K min−1) by releasing some O radicals from 22888



Article

−NO2 groups producing intermediate C6H6O11N12 and O2, and then a large amount of NO2 was produced due to scission of N−NO2. From 4 ps, the cage structure started to collapse producing HCN and CH2ON, and at the same time HONO formation also occurred. From 5 ps, the content of NO2 started to decrease, which has been either partially oxidized to NO3 or attract hydrogen from CH2ON to produce HONO and CHON (CP-2 in Scheme 1). Later on, the N−NO bond stated to

represents hydrogen abstraction by NO2, resulting in HONO molecules, which is an interesting alternate way of HONO formation during the later stages interaction with the NO2 free radical and the intermediate. Such variation of hydrogen abstraction was first described in nitramines. Those observations are also consistent with the recent experimental study on the interaction of NO2 with hydrocarbon soot79 and the highlevel CCSD (T) study on hydrogen abstraction from monomethyl hydrazine by NO2. Under extreme conditions, the HONO molecule is not stable and dissociates into NO and OH radicals. It is the case according to our simulation, where the intermediate HONO is observed between 5 ps and 11 ps and transferred to NO and H2O later on (see Figure 7). The simulations by Isayev et al.82 also provided a detailed description of the chemical processes in the initial stages of thermal decomposition of condensed CL-20. It has been found that only N-NO2 bond homolysis (NO2 fission, producing CP1) is the distinct initial decomposition channel, where no HONO elimination was observed. Instead, NO2 fission was followed by ring-breaking reactions, which agree with the experimental results showing a dominance of NO2 fission in the early stages of thermal decomposition, and CH2O, CO2, CO, HCN, and N2O are the main gaseous products.80,81 The experimentally determined ratios of CO2, CO, and N2O are 3.3, 1.2, and 0.82, respectively, with calculated values of 1.25, 1.0, and 1.0, respectively.79 Such significant deviation in the concentration of CO2 can be attributed to the short time scale and small system size accessible to the AIMD simulations, which might be the case for our simulations. Only trace amounts of CO2 and CO are detected herein, which come from the secondary reactions. Okovytyy et al.83 confirmed the abovementioned decomposition pathway of CL-20 by DFT theory using a B3LYP functional correlation. They found that the homolytic NO2 group elimination from five-membered ring is least endothermic, and CL-20 unimolecular decomposition results in the formation of the aromatic compound 1,5dihydrodiimidazo [4,5-b:4′-5′-e]pyrazine (CP-3 in Scheme 1), which has been proved by Quasim et al.84 Under the effect of polymer matrices, the type of gaseous products of ε-CL-20 might not change, because it decomposes separately with those polymeric bases. However, the hydrogen radical released from these polymers might interact with the gaseous products of ε-CL-20 in an isolated system during simulation, and Figure 8 gives the fragments produced during cook-off simulations of ε-CL-20 with fluoropolymers. It has been observed that Fluoropolymer started to release H at around 3.0 ps, and the main chains stay stable throughout the simulation period. In reality, they are much more stable than εCL-20. The initial decomposition of ε-CL-20 yielding O2 has been postponed from 2.0 to 2.5 ps due to stabilization of the polymers via hydrogen bond between O in −NO2 and hydrogen on polymer chains. The quantity of O2 has been greatly decreased and remains constant after 3.5 ps throughout the simulation period for both CL-20-FL and CL-20-VA. The HONO started to form at 4.0 ps, which is the same as pure εCL-20. The production process of NO2 is almost the same for both materials, with a maximum rate at 5.0−5.5 ps, but the quantity is much larger than pure ε-CL-20 due to intramolecular hydrogen transfer from polymer chains to N on cage structure. The cage structure started to collapse immediately after N-NO2 scission at 3 ps with a larger number of HCN production compared with pure ε-CL-20. If we compare CL20-FL and CL-20-VA as shown in Figure 8b,d, CL-20-VA

Scheme 1. Proposed Chemical Pathways for Unimolecular Decomposition of CL-20 from Molecular Dynamic Simulations and Experimental Observations

break, producing NO gases. This finding is in contrast to the suggested concerted intramolecular mechanism of HONO formation in RDX/HMX, where departing NO2 groups capture hydrogen radical.79 After 8 ps (2500 K), the HONO was transferred to NO and H2O, whereas the CH2ON and CHON were gradually oxidized to more stable gases N2, H2O, and CO2, consuming a large amount of O2. The detailed chemical pathways of ε-CL-20 decomposition both in thermolysis and biodegradation conditions have been widely investigated. Xiao and Yang24,80 studied the ε-CL-20 ion dissociation mechanisms using mass analyzed ion kinetic energy spectrum (MIKE) and collision induced dissociation. The pyrolysis gas chromatography−mass spectrometry (Py-GC/MS) technique was also employed to study ε-CL-20 decomposition.81 They found that three major distinctive channels for subsequent decomposition of ε-CL-20 intermediates including C4H5N2+ (CP-4) and C4H4N2O+ ions with m/z of 81 and 96, respectively (see Scheme 1). The main secondary reactions are (a) fragmentation, (b) oxidation by a NO2 radical, and (c) hydrogen abstraction by a NO 2 radical. Fragmentation of the intermediates results in stepwise breaking into HCN molecules. Oxidation by NO2 yields N-methylenformamide derivatives and NO, leading to further decomposition. Methylenformamide derivatives undergo fast decomposition, eliminating one to two HCN molecules (depending on molecular length) and yielding the simplest methylenformamide carbenes.82 The third pathway 22889



Article

Figure 8. Species analyses for the systems of ε-CL-20 crystal with fluoropolymers under linear heating rate of 300 K·ps−1 until 2500 K at 8 ps, then isothermal for another 8 ps.

produced much more NO and less N2 than CL-20-FL. Only trace amounts of CO2 and CO have been produced for both materials. In general, both polymers have weak mitigation effects on decomposition of ε-CL-20, resulting in very close reaction physical models, activation energy, and impact sensitivity. In contrast, the effects of hydrocarbon polymers (PIB, SBR, and NBR) on initial chemical pathways of ε-CL-20 decomposition are significant, as shown in Figure 9. First, compared to fluoropolymers, much larger amounts of HONO, H2O, HCN, and HNO, together with the new product NH3 have been produced due to greater intramolecular hydrogen transfer from polymer chains to the gas products of CL-20 molecules. For CL-20-SBR, N2O was also produced during 2.5−7.0 ps as an intermediate. The situation for O2 production under effects of hydrocarbon polymers is almost the same as those of fluoropolymers, revealing that O releasing from −NO2 is the common initial mechanism during fast heating (impact initiation), which was not observed experimentally22,23 during thermal decomposition of CL-20. However, this process has been observed during slow biodegradation of CL-20, which will be discussed later.85,86 According to Figure 9, it seems that the hydrocarbon polymers decompose earlier than ε-CL-20, but it is not the case according to real-time observation during simulation. Although the chain structures of these polymers are much more stable than the ε-CL-20 skeleton, the NBR polymer could undergo cross-link reaction, while PIB and SBR would release one or two hydrogen radicals during initial cool-off. If we compare the HCN production caused by collapses of the cage structure for all involved materials, it is clear that under the effect of PIB and SBR polymers, the production of HCN increases very fast at 5− 6 ps and slowly decreases after 9−10 ps. However, for CL-20NBR, the content of HCN continuously increases until 14 ps, because part of the HCN comes from NBR rubber, which could

undergo an analogous dehydrocyanation prior to main chain breakdown.74 One may also notice that for fluoropolymer bonded materials and pure ε-CL-20, the process of N−NO2 scission (started at 1.5−2.0 ps) and collapse of cage structure by releasing HCN or CH2NO (started at 2.5−3.0 ps) are almost overlapped with each other. However, for hydrocarbon polymer-based materials, the collapse of cage structure has been greatly postponed (started at 4.5−5.0 ps), where more NO2 gases have been produced due to more hydrogen transfer from polymer chains to N on the cage skeleton. It has been proved that the NO2 gas could stabilize the ring structure of nitramines.22,24,87 In this case, under the effect of hydrocarbon polymers, ε-CL-20 may undergo more complete N−NO2 scission before collapse of cage structure (more CP-3 will be produced; see Scheme 1). This might be the main reason that hydrocarbon polymer could greatly mitigate the decomposition process of ε-CL-20, resulting in much higher impact energy, while fluoropolymers have only a little effect on that due to deficiency of hydrogen.13,30−32 This can be further proved by the change of potential energy during simulation (see Figure 10). The potential energy of ε-CL-20 is much higher than those of the polymer-based counterparts. It slowly increases due to heating and lasts 3 ps to reach the maximum until the decomposition reactions are initiated, which releases energy because of the exothermic formation of small gaseous products. The potential energies of ε-CL-20, CL-20-FL, and CL-20-VA start to decrease earlier than those of the other polymers, revealing earlier exothermic chemical reaction with stronger self-heating. However, the potential energies of all materials start to largely decrease from 8 ps to the end of our simulation due to secondary further reactions to more stable smaller molecules at constant high temperature. As a comparison, in addition to thermal decomposition, the biodegradation and hydrolysis of CL-20 are also widely investigated, including microbial (Pseudomonas sp.FA1),85 22890



Article

Figure 9. Species analyses for the systems of ε-CL-20 crystal with hydrocarbon polymers under a linear heating rate of 300 K·ps−1 until 2500 K at 8 ps, then isothermal for another 8 ps.

similar to slow photodegradation. It has been shown that photodegradation and Fe0-mediated degradation of CL-20 occurred via three different routes: N-denitration, Ndenitrohydrogenation, and formation of mononitroso derivative of CL-20 (shown as Scheme 2).90 In the second mechanism for biotransformation of CL-20, a hydride ion transfer occurs across an N−N bond, removing nitrite and generating the denitrohydrogenated intermediate (C6H7N11O10). This pathway has been observed with a dehydrogenase enzyme isolated from Clostridium sp. EDB291 and with a purified diaphorase enzyme.92 In the third pathway, the mononitroso derivative of CL-20 (C6H6N12O11) is formed via reduction with two redox equivalents. These initial enzymatic reactions were postulated to destabilize the CL-20 molecule and promote ring cleavage. However, recent experimental findings suggest that CL-20 degraded via at least two initial routes: one involving denitration, and the second involving sequential reduction of the N−NO2 to the corresponding nitroso (N−NO) derivatives prior to denitration and ring cleavage.93 It is also the case for εCL-20 under fast heating, where CP-9 and CP-10 have been formed as intermediates producing a large amount of O2, which has been slightly blocked by the effects of fluoropolymers and largely decreased due to effects of hydrocarbon polymers,

Figure 10. , Potential energy for the systems of ε-CL-20 crystal and εCL-20 with different polymer chains (right side Y axis corresponds to ε-CL-20 and the others share left side Y axis).

enzymatic (salicylate 1-monooxygenase and nitroreductase),87,88 and alkali hydrolysis89 degradation via an initial Ndenitration route. As mentioned above, the initial chemical pathway of CL-20 decomposition under fast heating is very 22891



Article

oil materials and plasticizers (Formex P1, Semtex, and C4) have strong effect on decomposition mechanism of CL-20, resulting in multistep mechanism with different activation energies and physical models. (3) For CL-20 and its PBXs, the initiation energy by impact depends not only on the heat built-up period of thermal decomposition of energetic materials (chemical properties of the materials), but also on the probability of hotspots generation (defects in crystals and interfaces) especially when it decomposes in solid state. According to simulations, under the effect of polymers, the process of O release from −NO2 group is decreased, which makes CL-20 more stable. Compare to Fluoropolymers (Viton A and Fluorel), the hydrocarbon polymers (PIB, SBR, and NBR) may make ε-CL-20 undergo more complete N−-NO2 scission before collapse of cage structure and hence these two steps have been partially separated. This could be used for interpretation of the experimental results that the hydrocarbon polymer could greatly mitigate the decomposition process of εCL-20 from a single step to a multistep, resulting in much higher impact energy, whereas fluoropolymers have only little effect on that.

Scheme 2. Proposed Chemical Pathway for the Initial Biotransformation of CL-20 Catalyzed by Salicylate 1Monooxygenase Followed by Secondary Decomposition

■

ASSOCIATED CONTENT

S Supporting Information *

resulting in more CP-7 and CP-8 intermediates and hence more NO2 products. However, as we could see in Figures 8 and 9, NO2 is not stable and can be transformed to other compounds because it stays in an isolated high temperature system. In reality, most of the NO2 gas are released to the surroundings under lower temperature and can be easily detected. Evolving gaseous products contribute to the mass transport between the sample and environment, and such transport properties are responsible for the physical models of thermal decomposition.

The Supporting Information includes the parameters for decomposition reaction models of CL-20 and its PBXs evaluated by nonisothermal TG experiments; nonisothermal TGA and DSC curves of involved materials; The y(α), z(α), as well as the combined kinetic analysis plots for their thermal decomposition; a comparison of normalized curves of obtained kinetic models with some ideal models by master plots method; and the reconstruction of the experimental curves by using kinetic triplets from combined kinetic analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

■

5. CONCLUSIONS The thermal decomposition kinetic triplets of CL-20 and its PBXs are obtained by both isoconversional and combined kinetic analysis methods, which are used to predict temperature profiles of the constant rate decomposition. The physical models are further supported by the detailed decomposition pathways under fast heating (300 K/ps) simulated from reactive molecular dynamics (ReaxFF-lg) code. The connections among the decomposition pathways, physical models and the impact sensitivities are proposed, and the following conclusions could be made: (1) Both ε-CL-20 and α-CL-20 decomposes in the form of γCL-20, resulting in identical activation energy of about 168 kJ mol−1. The modification of ε-CL-20 crystal surface by reducing the defect (Rs-ε-CL-20) could increase activation energy from 168 to 178 kJ mol−1 and impact energy from 4.2 to 12.5 J, resulting in higher thermal stability and lower sensitivity. However, the modified CL-20 crystals are less compatible with polar plasticizers (such as DOS), which would induce its polymorphic transition during heating, resulting in much lower initial activation energy (159 kJ mol−1) and worse thermal stability. (2) The effect of the fluoropolymers on the decomposition mechanism of CL-20 is significant and similar. They can change its decomposition mechanism from a “AC1, first-order autoaccelerated model” to a “A3, three-dimensional nucleation and growth model”; The polymer matrices containing mineral

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Phone: +420 466038503; Fax. +420 466038024. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The partial financial support from the ERASMUS program, and projects TEP-7858 from Junta de Andaluciá and CTQ201127626 from the Spanish Ministerio de Economiá y Competitividad and FEDER funds are acknowledged. Additionally, one of the authors (P.E.S.J.) is supported by a Juan de la Cierva grant. The authors also would like to thank Dr. H. v. Schoot from “Scientific Computing & Modeling” in Netherlands for his kind instructions on the molecular dynamics simulations by ReaxFF code in ADF software.

■

REFERENCES

(1) Nouguez, B.; Mahé, B.; Vignaud, P. O. Cast PBX Related Technologies for IM Shells and Warheads. Sci. Technol. Energ. Mater. 2009, 70, 135−139. (2) Yan, Q.-L.; Zeman, S.; Elbeih, A. Recent Advances in Thermal Analysis and Stability Evaluation of Insensitive Plastic Bonded Explosives (PBXs). Thermochim. Acta 2012, 537, 1−12.

22892



Article

(3) Vadhe, P. P.; Pawar, R. B.; Sinha, R. K.; Asthana, S. N.; Subhananda, R. A. Cast Aluminized Explosives (Review). Combust. Explos. Shock Waves 2008, 44, 461−477. (4) Yan, Q.-L.; Zeman, S.; Šelešovský, J.; Svoboda, R.; Elbeih, A. Thermal Behavior and Decomposition Kinetics of Formex-Bonded Explosives Containing Different Cyclic Nitramines. J. Therm. Anal. Calorim. 2013, 111, 1419−1430. (5) Yan, Q.-L.; Zeman, S.; Zhao, F.-Q.; Elbeih, A. Noniso-thermal Analysis of C4 Bonded Explosives Containing Different Cyclic Nitramines. Thermochim. Acta 2013, 556, 6−12. (6) Yan, Q.-L.; Zeman, S.; Elbeih, A.; Zbyněk, A. The influence of the Semtex matrix on the thermal behavior and decomposition kinetics of cyclic nitramines. Cent. Eur. J. Energetic Mater. 2013, 10, 509−528. (7) Zeman, S.; Elbeih, A.; Yan, Q.-L. Note on the Use of the Vacuum Stability Test in the Study of Initiation Reactivity of Attractive Cyclic Nitramines in Formex P1 Matrix. J. Therm. Anal. Calorim. 2013, 111, 1503−1506. (8) Zeman, S.; Elbeih, A.; Yan, Q.-L. Note on the Use of the Vacuum Stability Test in the Study of Initiation Reactivity of Attractive Cyclic Nitramines in the C4 Matrix. J. Therm. Anal. Calorim. 2013, 112, 433− 1437. (9) Yan, Q.-L.; Zeman, S.; Elbeih, A. Thermal Behavior and Decomposition Kinetics of Viton A Bonded Explosives Containing Attractive Cyclic Nitramines. Thermochim. Acta 2013, 562, 56−64. (10) Yan, Q.-L.; Zeman, S.; Zhang, T.-L.; Elbeih, A. Non-isothermal Decomposition Behavior of Fluorel Bonded Explosives Containing Attractive Cyclic Nitramines. Thermochim. Acta 2013, 574, 10−18. (11) Simpson, R. L.; Urtiev, P. A.; Ornellas, D. L.; Moody, G. L.; Scribner, K. J.; Hoffman, D. M. CL-20 Performance Exceeds That of HMX and Its Sensitivity is Moderate. Propellants, Explos. Pyrotech. 1997, 22, 249−255. (12) Bazaki, H.; Kawabe, H.; Miya, H. Synthesis and sensitivity of Hexanitrohexaazaisowurtzitane (HNIW). Propellants, Explos. Pyrotech. 1998, 23, 333−336. (13) Nair, U. R.; Gore, G. M.; Sivabalan, R.; Satpute, R. S.; Asthana, S. N.; Singh, H. Studies on Polymer Coated CL-20The Most Powerful Explosive. J. Polym. Mater. 2004, 21, 377−382. (14) Geetha, M.; Nair, U. R.; Sarwade, D. B.; Gore, G. M.; Asthana, S. N.; Singh, H. Studies on CL-20: The most powerful high energy material. J. Therm. Anal. Calorim. 2003, 73, 913−922. (15) Ryzhov, L. R.; McBride, J. M. Low-Temperature Reactions in Single Crystals of the Polycyclic Nitramine 15N-HNIW. J. Phys. Chem. 1996, 100, 163−169. (16) Dong, L.; Li, X.; Yang, R. Thermal Decomposition Kinetics of Hexanitrohexaazaisowurtzitane by Mass Spectrometry. Acta Phys. Chem. Sin. 2008, 24, 997−1001. (17) Okovytyy, S.; Kholod, Y.; Qasim, M.; Fredrickson, H.; Leszczynski, J. The Mechanism of Unimolecular Decomposition of Hexanitrohexaazaisowurtzitane, A Computational DFT Study. J. Phys. Chem. A 2005, 109, 2964−2970. (18) Xu, X.; Xiao, H.; Xiao, J.; Zhu, W.; Huang, H.; Li, J. Molecular Dynamics Simulations for Pure ε-CL-20 and ε-CL-20-Based PBXs. J. Phys. Chem. B 2006, 110, 7203−7207. (19) Sorescu, D. C.; Rice, B. M.; Thompson, D. L. Molecular Packing and NPT-Molecular Dynamics Investigation of the Transferability of the RDX Intermolecular Potential to 2,4,6,8,10,12-Hexanitrohexaazaisowurtzitane. J. Phys. Chem. B 1998, 102, 948−952. (20) Patil, D. G.; Brill, T. B. Thermal Decomposition of Energetic Materials 59: Characterization of Residue of Hexanitrohexaazaisowurtzitane. Combust. Flame 1993, 92, 456−458. (21) Patil, D. G.; Brill, T. B. Thermal Decomposition of Energetic Materials 53. Kinetics and Mechanism of Thermolysis of Hexanitrohexaazaisowurtzitane. Combust. Flame 1991, 87, 145−151. (22) Lobbecke, S.; Bohn, M. A.; Pfeil, A.; Krause, H. Thermal behavior and stability of HNIW (CL-20), 29th Int. Annual Conference of ICT, Karlsruhe, Germany, June 30−July 3,1998. (23) Turcotte, R.; Vachon, M.; Kwok, Q.; Wang, R.; Daved, E. Thermal Study of HNIW (CL-20). Thermochim. Acta 2005, 433, 105− 115.

(24) Xiao, H.; Yang, R. Hexanitrohexaazaisowurtzitane Ion Dissociation Mechanism Based on Mass-Analyzed Ion Kinetic Energy Spectrum. J. Propul. Power 2005, 21, 1069−1074. (25) Song, Z.-W.; Yan, Q.-L.; Li, X.-J.; Qi, X.-F.; Liu, M. Crystal Transition of ε-CL-20 in Different Solvents. Chin J. Energet Mater. 2010, 6, 648−653. (26) Li, J.; Brill, T. B. Kinetics of Solid Polymorphic Phase Transitions of CL-20. Propellants, Explos., Pyrotech. 2007, 32, 326−330. (27) Krause, H. H. New energetic materials. In Tiepel, U., Ed., Energetic Materials; Wiley-VCH GmbH & Co. KGaA: Weinheim, Germany, 2005; pp 1−25. (28) Ou, Y.; Wang, C.; Pan, Z.; Chen, B. Sensitivity of Hexanitrohexaazaisowurtzitane. Chin. J. Energ. Mater. 1999, 7, 100− 102. (29) Lee, M. H.; Kim, J. H.; Park, Y.Ch.; Kim, W.-S. Control of Crystal Structure and Its Defect of ε-HNIW Prepared by Evaporation Crystallization. Ind. Eng. Chem. Res. 2007, 46, 1500−1504. (30) Elbeih, A.; Zeman, S.; Jungova, M.; Vávra, P.; Akstein, Z. Effect of Different Polymeric Matrices on Some Properties of Plastic Bonded Explosives. Propellants, Explos., Pyrotech. 2012, 37, 676−684. (31) Elbeih, A.; Zeman, S.; Jungova, M.; Vávra, P.; Akstein, Z. Explosive Strength and Impact Sensitivity of Several PBXs Based on Attractive Cyclic Nitramines. Propellants, Explos., Pyrotech. 2012, 37, 329−334. (32) Elbeih, A.; Zeman, S.; Jungova, M.; Vávra, P. Attractive Nitramines and Related PBXs. Propellants, Explos., Pyrotech. 2013, 38, 379−385. (33) Yan, Q.-L.; Zeman, S.; Elbeih, A.; Song, Z.-W.; Málek, J. The Effect of Crystal Structure on the Thermal Reactivity of CL-20 and Its C4 Bonded Explosives (I): Thermodynamic Properties and Decomposition Kinetics. J. Therm. Anal. Calorim. 2013, 112, 823−836. (34) Yan, Q.-L.; Zeman, S.; Elbeih, A.; Svoboda, R.; Málek, J. The Effect of Crystal Structure on the Thermal Initiation of CL-20 and its C4 Bonded Explosives (II): Models for Overlapped Reactions and Thermal Stability. J. Therm. Anal. Calorim. 2013, 112, 837−849. (35) Sikder, A. K.; Sikder, N. A Review of Advanced High Performance, Insensitive and Thermally Stable Energetic Materials Emerging for Military and Space Application. J. Hazard. Mater. A 2004, 112, 1−5. (36) M, S.; Field, J. E.; Greenaway, M. W. Crystal Sensitivities of Energetic Materials (A Review). Mater. Sci. Technol. 2006, 24, 402− 413. (37) Lai, W.-P.; Lian, P.; Wang, B.-Z. New Correlation for Predicting Impact Sensitivities of Nitro Energetic Compounds. J. Energet. Mater. 2010, 28, 45−76. (38) Yan, Q.-L.; Zeman, S. Theoretical Evaluation of Sensitivity and Thermal Stability for High Explosives Based on Quantum Chemistry Methods: A Brief Review. Int. J. Quantum Chem. 2013, 113, 1049− 1061. (39) Kossoy, A.; Akhmetshin, Y. Identification of Kinetic Models for the Assessment of Reaction Hazards. Process Saf. Prog. 2007, 26, 209− 220. (40) Perez-Maqueda, L. A.; Criado, J. M.; Sanchez-Jimenez, P. E. Combined Kinetic Analysis of Solid-state Reactions: A Powerful Tool for the Simultaneous Determination of Kinetic Parameters and the Kinetic Model without Previous Assumptions on the Reaction Mechanism. J. Phys. Chem. A 2006, 110, 12456−62. (41) Vyazovkin, S.; Burnham, A. K.; Criado, J. M.; Pérez-Maqueda, L. A.; Popescu, C.; Sbirrazzuoli, N. ICTAC Kinetics Committee Recommendations for Performing Kinetic Computations on Thermal Analysis Data. Thermochim. Acta 2011, 520, 1−19. (42) Duin, van A.C.T.; Dasgupta, S.; Lorant, F.; Goddard, W. A., III. ReaxFF: A Reactive Force Field for Hydrocarbons. J. Phys. Chem. A 2001, 105, 9396−9409. (43) Ahmed, E.; Adela, H.; Zeman, S. Path to ε-HNIW with Reduced Impact Sensitivity. Cent. Eur. J. Energ. Mater. 2011, 8, 173−182. (44) Li, J.; Brill, T. B. Kinetics of Solid Polymorphic Phase Transitions of CL-20. Propellants, Explos., Pyrotech. 2007, 32, 326−330. 22893



Article

(45) Criado, J. M.; Sánchez-Jiménez, P. E.; Pérez-Maqueda, L. A. Critical Study of the Isoconversional Methods of Kinetic Analysis. J. Therm. Anal. Calorim. 2008, 92, 199−203. (46) Málek, J.; Criado, J. M.; Šesták, J.; Militký, J. The Boundary Conditions for Kinetic Models. Thermochim. Acta 1989, 153, 429− 432. (47) Málek, J. Crystallization Kinetics by Thermal Analysis. J. Therm. Anal. Calorim. 1999, 56, 763−769. (48) Málek, J. The Kinetic Analysis of Non-isothermal Data. Thermochim. Acta 1992, 200, 257−269. (49) Málek, J. Kinetic Analysis of Crystallization Processes in Amorphous Materials. Thermochim. Acta 2000, 355, 239−253. (50) Yan, Q.-L.; Zeman, S.; Sánchez Jiménez, P. E.; Zhao, F.-Q.; Pérez-Maqueda, L. A.; Málek, J. The Effect of Polymer Matrices on the Thermal Hazard Properties of RDX-Based PBXs by Using Model-Free and Combined Kinetic Analysis. J. Hazard. Mater. 2014, 271, 185− 195. (51) Politzer, P.; Boyd, S. Molecular Dynamics Simulations of Energetic Solids. Struct Chem. 2002, 13, 105−113. (52) Strachan, A.; van Duin, A. C. T.; Chakraborty, D.; Dasgupta, S.; Goddard, W. A., III. Shock Waves in High-energy Materials: The Initial Chemical Events in Nitramine RDX. Phys. Rev. Lett. 2003, 91, 098301. (53) Strachan, A.; Kober, E. M.; van Duin, A. C.T.; Oxgaard, J.; Goddard, W. A., III. Thermal Decomposition of RDX from Reactive Molecular Dynamics. J. Chem. Phys. 2005, 122, 054502. (54) Li, Z.; Lang, C.; Chen, W.; Jun-Ying, W. Molecular Dynamics Study of the Effect of H2O on the Thermal Decomposition of α-Phase CL-20. Acta Phys. Chim. Sin. 2013, 29, 1145−1153. (55) Zhang, L.; Zybin, S.; van Duin, A. C. T.; Dasgupta, S.; Goddard, W. A.; Kober, E. J. Carbon Cluster Formation During Thermal Decomposition of Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine and 1,3,5-Triamino-2,4,6-trinitrobenzene High Explosives from ReaxFF Reactive Molecular Dynamics Simulations. J. Phys. Chem. A 2009, 113, 10619−10640. (56) Liu, L.; Liu, Y.; Zybin, S. V.; Sun, H.; Goddard, W. A., III. ReaxFF-lg: Correction of the ReaxFF Reactive Force Field for London Dispersion, with Applications to the Equations of State for Energetic Materials. J. Phys. Chem. A 2011, 115, 11016−11022. (57) Chenoweth, K.; Cheung, S.; van Duin, A. C. T.; Goddard, W. A., III; Kober, E. M. Simulations on the Thermal Decomposition of a Poly(dimethylsiloxane) Polymer Using the ReaxFF Reactive Force Field. J. Am. Chem. Soc. 2005, 127, 7192−7202. (58) Moukhina, E. Determination of Kinetic Mechanisms for Reactions Measured with Thermoanalytical Instruments. J. Therm. Anal. Calorim. 2012, 109, 1203−1214. (59) Perejón, A.; Sánchez-Jiménez, P. E.; Criado, J. M.; PérezMaqueda, L. A. Kinetic Analysis of Complex Solid-State Reactions. A New Deconvolution Procedure. J. Phys. Chem. B 2011, 115, 1780−91. (60) Svoboda, R.; Málek, J. Applicability of Fraser−Suzuki Function in Kinetic Analysis of Complex Crystallization Processes. J. Therm. Anal. Calorim. 2013, 111, 1−12. (61) Erceg, M.; Jozic, D.; Banovac, I.; Perinovic, S.; Bernstorff, S. Preparation and Characterization of Melt Intercalated Poly(ethylene oxide)/Lithium Montmorillonite Nanocomposites. Thermochim. Acta 2014, 579, 86−92. (62) Koga, N.; Goshi, Y.; Yamada, S.; Perez-Maqueda, L. A. Kinetic Approach to Partially Overlapped Thermal Decomposition Processes. J. Therm. Anal. Calorim. 2013, 111, 1463−1474. (63) Sanchez-Jimenez, P. E.; Perez-Maqueda, L. A.; Perejon, A.; Criado, J. M. Nanoclay Nucleation Effect in the Thermal Stabilization of a Polymer Nanocomposite: A Kinetic Mechanism Change. J. Phys. Chem. C 2012, 116, 11797−11807. (64) Yoshikawa, M.; Yamada, S.; Koga, N. Phenomenological Interpretation of the Multistep Thermal Decomposition of Silver Carbonate to Form Silver Metal. J. Phys. Chem. C 2014, 118, 8059− 8070. (65) Lbbecke, S.; Bohn, M. A.; Pfeil, A.; Krause, H. Proceedings of the 29th International Annual Conference, ICT, Karlsruhe, 1998; p 145/1.

(66) Dumas, S. V.; Gauvrit, J. Y.; Lanteri, P. Determining the Polymorphic Purity of ε-CL-20 Contaminated by Other Polymorphs Through the Use of FTIR Spectroscopy with PLS Regression. Propellants, Explos., Pyrotech. 2012, 37, 230−234. (67) Lee, J.; Jaw, K. S. Thermal Decomposition Properties and Compatibility of CL-20, NTO with Silicone Rubber. J. Therm. Anal. Calorim. 2006, 85, 463−7. (68) Chovancova, M.; Zeman, S. Study of Initiation Reactivity of Some Plastic Explosives by Vacuum Stability Test and Non-isothermal Differential Thermal Analysis. Thermochim. Acta 2007, 460, 67−76. (69) Ostmark, H.; Bergman, H. Proceedings of the International Symposium on Energetic and Materials Technology, American Defense Preparedness Association, Meeting no. 680, Phoenix, AZ, September 1995; p. 76. (70) Felix, S. P.; Singh, G.; Sikder, A. K.; Aggrawal, J. P. Studies on Energetic Compounds-Part 33: Thermolysis of Keto-RDX and Its Plastic Bonded Explosives Containing Thermally Stable Polymers. Thermochim. Acta 2005, 426, 53−60. (71) Madorsky, S. L.; Mark, H. F.; Immergut, E. H. Thermal Degradation of Organic Polymers; Polymer Reviews; Interscience Publishers: New York, 1964; Vol. 7. (72) Jee, C. S. Y.; Guo, Z. X.; Stoliarov, S. I.; Nyden, M. R. Experimental and Molecular Dynamics Studies of the Thermal Decomposition of a Polyisobutylene Binder. Acta Mater. 2006, 54, 4803−4813. (73) Chen, K. S.; Yeh, R. Z. Kinetics of Thermal Decomposition of Styrene-Butadiene Rubber at Low Heating Rates in Nitrogen and Oxygen. Combust. Flame 1997, 108, 408−418. (74) Budrugeac, P.; Segal, E.; Ciutacu, S. Thermal Oxidative Degradation of Nitrile-butadiene Rubber. J. Therm. Anal. 1991, 37, 1179−1191. (75) Sanchez-Jimenez, P. E.; Perez-Maqueda, L. A.; Perejon, A.; Criado, J. M. Constant Rate Thermal Analysis for Thermal Stability Studies of Polymers. Polym. Degrad. Stab. 2011, 96, 974−981. (76) Furman, D.; Kosloff, R.; Dubnikova, F.; Zybin, S. V.; Goddard, W. A., III; Rom, N.; Hirshberg, B.; Zeiri, Y. Decomposition of Condensed Phase Energetic Materials: Interplay between Uni- and Bimolecular Mechanisms. J. Am. Chem. Soc. 2014, 136, 4192−4200. (77) An, Q.; Goddard, W. A., III; Zybin, S. V.; Jaramillo-Botero, A.; Zhou, T. Highly Shocked Polymer Bonded Explosives at a Nonplanar Interface: Hot-Spot Formation Leading to Detonation. J. Phys. Chem. C 2013, 117, 26551−26561. (78) Hu, Y.; Brenner, D. W.; Shi, Y. Detonation Initiation from Spontaneous Hotspots Formed During Cook-Off Observed in Molecular Dynamics Simulations. J. Phys. Chem. C 2011, 115, 2416−2422. (79) Oxley, J. C.; Kooh, A. B.; Szekeres, R.; Zheng, W. Mechanisms of Nitramine Thermolysis. J. Phys. Chem. 1994, 98, 7004−7008. (80) Yang, R.; Xiao, H. Dissociation Mechanism of HNIW Ions Investigated by Chemical Ionization and Electron Impact Mass Spectroscopy. Propellants, Explos., Pyrotech. 2006, 31, 148−154. (81) Naik, N. H.; Gore, G. M.; Gandhe, B. R.; Sikder, A. K. Studies on Thermal Decomposition Mechanism of CL-20 by Pyrolysis Gas Chromatography-mass Spectrometry (Py-GC/MS). J. Hazard Mater. 2008, 159, 630−635. (82) Isayev, O.; Gorb, L.; Quasim, M.; Leszczynski, J. Ab Initio Molecular Dynamics Study on the Initial Chemical Events in Nitramines: Thermal Decomposition of CL-20. J. Phys. Chem. B 2008, 112, 11005−013. (83) Okovytyy, S.; Kholod, Y.; Quasim, M.; Fredrickson, H.; Leszynski, J. The Mechanism of Unimolecular Decomposition of 2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazisowurtzitane, a Computational DFT Study. J. Phys. Chem. A 2005, 109, 2964−2970. (84) Quasim, M.; Furey, J.; Fredrickson, H. L. Semiempirical Predictions of Chemical Degradation Reaction Mechanisms of CL-20 as Related to Molecular Structure. Struct. Chem. 2004, 15, 493−499. (85) Bhushan, B.; Paquet, L.; Spain, J. C.; Hawari, J. Biotransformation of 2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane 22894



Article

(CL-20) by Denitrifying Pseudomonas sp. Strain FA 1. Appl. Environ. Microbiol. 2003, 69, 5216−5221. (86) Bhushan, B.; Halasz, A.; Spain, J. C.; Hawari, J. Initial Reaction(s) in Biotransformation of CL-20 is Catalyzed by Salicylate 1-monooxygenase from Pseudomonas sp. Strain ATCC 29352. Appl. Environ. Microbiol. 2004, 70, 4040−4047. (87) Tall, A.; Zeman, S. Determination of Heats of Decomposition of Some 1,3,5,7-Tetraazacylooctane and 1,3,5-Triazacyclohexane Derivatives Using Differential Scanning Calorimetry. J. Them. Anal. 1977, 12, 75−81. (88) Bhushan, B.; Halasz, A.; Hawari, J. Nitroreductase Catalyzed Biotransformation of CL-20. Biochem. Biophys. Res. Commun. 2004, 322, 271−276. (89) Balakrishnan, V. K.; Halasz, A.; Hawari, J. Alkaline Hydrolysis of the Cyclic Nitramine Explosives RDX, HMX, and CL-20: New Insights into Degradation Pathways Obtained by the Observation of Novel Intermediates. Environ. Sci. Technol. 2003, 37, 1838−1843. (90) Balakrishnan, V. K.; Monteil-Rivera, F.; Halasz, A.; Corbeanu, A.; Hawari, J. Decomposition of Polycyclic Nitramine Explosive, CL20, by Fe0. Environ. Sci. Technol. 2004, 38, 6861−6866. (91) Bhushan, B.; Halasz, A.; Hawari, J. Biotransformation of CL-20 by a Dehydrogenase Enzyme from Clostridium sp. EDB2. Appl. Microbiol. Biotechnol. 2005, 69, 448−455. (92) Bhushan, B.; Halasz, A.; Hawari, J. Stereo-specificity for pro-(R) hydrogen of NAD(P)H during Enzyme-catalyzed Hydride Transfer to CL-20. Biochem. Biophys. Res. Commun. 2005, 337, 1080−83. (93) Bhushan, B.; Halasz, A.; Thiboutot, S.; Ampleman, G.; Hawari, J. Chemotaxis-Mediated Biodegradation of Cyclic Nitramine Explosives RDX, HMX, and CL-20 by Clostridium sp. EDB2. Biochem. Biophys. Res. Commun. 2004, 316, 816−821.

22895


The Mitigation Effect of Synthetic Polymers on ... - ACS Publications

Recommend Documents