J. Phys. Chem. 1982, 86,4260-4265
4260
r / 2 , and two weak signals at the positions corresponding to t9 = 0. The former pair is separated by D/(g,P), and the latter by 20/(g,@). Thus,in the spectra seen in Figures 3a and 4a, the doublet flanking the normal NO signal was recognized as the perpendicular (6 = 7r/2) components of the triplet species. In Figure 4a a weak negative dip is indicated by an arrow at 3750 G. This was recognized as the high-field member of the parallel (6 = 0) components. This assignment then places the low-field parallel component at 3200 G where it would be masked by the lowfield perpendicular signal. The g tensor and the crystal field parameter of the triplet species were thus determined as follows: g, = 1.976 g,, = 1.912
The crystal field parameter determined above and eq 2 yield R of 4.6 A. It has been shown that the largest deviation of the g tensor of NO from the spin only value (2.0023) occurs along the N-0 bond dire~tion.',~ The assignment given above then states that the symmetry axis for the dipole-dipole interaction must closely parallel the N-0 bond direction. A plausible structure of the NO-NO triplet formed in Na-A is thus envisaged as follows:
D/(g,P) = 288 G (or D = 0.0266 cm-')
Zeolite A has a cubic structure. Each unit cell contains a roughly spherical cavity having a diameter of 11A,defined by eight sodalite units placed at each corner of the unit cell. Each cavity thus has 8 walls of six-member ring, 12 walls of four-member ring, and 6 windows of eightmember ring shared with the adjacent cavities. In Na-A Na+ ions are located near the centers of all the six-member rings, and about half of the eight-member rings.1° Although the exact location and the orientation of the triplet species cannot be ascertained from the present study, there is certainly an ample space and there are enough cations within each cavity so that the formation of such a NO-NO triplet as depicted above would be possible.
Figure 6a, and b show respectively the computer-simulated spectra based upon the parameters determined above for the NO monomer, and the NO-NO triplet in the NO/Na-A system. A Lorentzian line shape with the line width of 20 G was assumed for the computation. Superposition of the spectra with the weighted ratio of 1:l is shown in Figure 6c. It is in an excellent agreement with the observed spectrum (Figure 4a). The powder pattern of the triplet species seen here is unusual in that it is extremely askewed by the anisotropy of the g tensor (gL> g,,). Interestingly, though, the g tensor of the triplet given above is very similar to that of NO monomer detected in the same NO/Na-A system. It strongly substantiates the notion that the triplet state in question is a radical pair comprising two NO molecules.
I
I
(10)R. Y. Yanagida, A. A. Amaro, and K. Seff, J. Phys. Chem., 77,805 (1973).
Solid Phase Transition Kinetics. The Role of Intermolecular Forces In the Condensed-Phase Decomposition of Octahydro- 113,5,7-tetranitro- 1,3,5,7-t etrazocine T. B. Brill' and R. J. Karpowlcz Department of Chemistty, University of Delaware, Newark, Delaware 19711 (Received: May 73, 1982; I n Final Form: July 13, 1982)
The fundamental basis of the interconversions between the polymorphs of the monopropellant octahydro1,3,5,7-tetranitro-l,3,5,7-tetrazocine (HMX) has been established. A Fourier transform infrared method was developed to study the rates of solid-solid phase transitions on the molecular level. Approximate first-order rates were observed. The activation energy (kJ mol-') and frequency factor (log A ) for the transitions are /3 -,6 (204 f 14; 19.9 f l),a -,6 (208 f 18; 19.9 f 2), and y 6 (219 f 20; 21.8 f 2). These Arrhenius data explain why the 6 transformation is the predominant thermally induced phase transition of P-HMX. Of much greater importance is the close resemblance of the Arrhenius data for the phase transitions to those for the condensed-phase event to the Arrhenius data. It is proposed that rupture of the intermolecular forces rather than cleavage of covalent bonds within the molecule is what largely controls the rate of thermal decomposition of HMX. With this new interpretation a number of heretofore unclear facets of HMX decomposition come into sharper focus. The implications for altering the rate of decomposition are noted.
-
-
Introduction Octahydro-1,3,5,7-tetranitro1,3,5,7-tetrazocine, shown in Figure 1, is known as HMX and is one of the most important energetic nitramines. In spite of extensive work on HMX, the initial step in the thermal decomposition of the condensed phase remains uncertain. Differential scanning calorimetry,l differential thermal analysis,2mass (1) Rogers, R. N. Thermochim. Acta 1972, 3, 437. 0022-3654/82/2086-4260~01.25/0
spectrometry: and gas evolution4measurements yield rates from which frequency factors, A , and activation energies, E,, can be calculated from the Arrhenius equation: log k (s-l) = log A - E,/RT. log A = 18-20 and E , = 209-220 (2) Sinclair, J. E.; Hoondee, W . Proc. S y m p . Explos. Pyrotech. 1971, 7, 1; Chem. Abstr. 1972, 76, 143050~.
(3) Goshgarian,B. B. AFRPL-TR-78-76;Air Force Rocket Propulsion Laboratory: Edwards, AFB, CA, 1976. (4) Robertson, A. J. B. Trans. Faraday SOC.1949, 45, 85.
0 19A7
American Chamical Snrietv
The Journal of Physical Chemistry, Vol. 86, No. 27, 7982 4261
Solid Phase Transition Kinetics
SJP
0 sec BETA
DELTA W
161 sec
0 2
U
200 sec
m
a 0
2 7 6 sec
u)
m
U
326 sec 374 sec
Flgure 1. Molecular conformation change during the phase transition from &HMX to 6-HMX.
4 2 3 sec
kJ mol-' for decomposition in the condensed phase above 243 "C. However, the link between these data and specific reactions in the solid and liquid phases is at best speculative because a common experimental measurement has not connected them. Covalent-bond-breaking steps of one kind or another have always been assumed to be the initial chemical event in thermal decompcaition,5but it is unclear which bonds are most often involved. In addition to condensed-phase decomposition, HMX exhibits extensive polymorphism. There are four known polymorphs each with a particular stability range? These have been characterized by micro~copy,~~' X-ray powder diffraction? and spectros~opy.~J~ The enthalpy changes during the reconstructive transformations which occur in the 165-205 "C range do not explain the particular pattern of conversions among the p~lymorphs.l'-~~ This paper describes the first kinetic study of the solid-solid phase transitions of HMX. The results reveal why certain polymorph conversions are observed while others are not. The stability and conversion scheme originates in kinetic factors rather than the enthalpy changes. The activation energies of the phase transitions reflect the energy needed to disrupt the electrostatic forces in the crystal lattice of HMX. However, a vastly more important implication of the results lies in the comparison of the Arrhenius data for these phase transformations to those reported for the condensed-phase decomposition process. The initial rate-controlling step for the decomposition in the condensed phase appears to involve mostly the disruption of the strong intermolecular electrostatic forces between HMX molecules (and between the HMX molecule and its decomposition products). This contrasts with all previous interpretations of the condensed-phase decom(5) The work in this area is extensive; see references in: (a) McCarty, K. P. AFRPL-TR-76-59;Air Force Rocket Propulsion Laboratory;Edwards AFB, CA, 1976. (b) Schroeder, M. A. CPIA Publication 308,16th JANNAF Combustion Meeting, Monterey, CA, Sept 1979, Vol. 11, pp 17-34; CPIA Publication 329, 17th JANNAF Combustion Meeting, Hampton, VA, Sept 1980, Vol. 11, pp 498-508, CPIA Publication 347,18th JANNAF Combustion Meeting, Pasadena, CA, Oct 1981, Vol. 11, pp 395-413. (c) Shaw, R.; Walker, F. E. J. Phys. Chem. 1977, 81, 2572. (6) Teetsov, A. S.;McCrone, W. C. Microsc. Cryst. Front 1965,15, 13. (7) McCrone, W. C. "Physics and Chemistry of the Organic Solid State"; Fox, D., Labes, M. M., Weissberger, A,, Eds.; Wiley: New York, 1965; Vol. 11, pp 726-66. (8) Cady, H. H.; Smith, L. C. LAMS-2652; Loa Alamos Scientific Laboratory: LOBAlamos, NM, May 3, 1962. (9) Goetz, F.; Brill, T. B. J. Phys. Chem. 1979, 83, 340. (10) Goetz, F.; Brill, T. B.; Ferraro, J. R. J. Phys. Chem. 1978,82,1912. 1971, 67, 556. (11) Hall, P. G. Trans. Faraday SOC. (12) Krien, G.;Licht, H. H.; Zierath, J. Thermochim. Acta 1973,6,465. (13) Rylance, J.; Stubley, D. Thermochim. Acta 1975, 13, 253.
x
-066
040
002
WAVENUMBERS
Flgure 2. Spectral change in the infrared during a 6 6 solid phase transition at 185.5 'C. The decrease in the absorbance of the stared band of 0-HMX was followed during the kinetic runs. +
position kinetics which assume that internal covalent bond breaking controls the process. Chemical reactions would be expected to occur in the same time frame as the breakdown of the intermolecular forces, but they are not necessarily rate determining in the conditions used for thermal decomposition studies of the condensed phase of HMX.
Experimental Section The kinetics of the solid phase transitions of HMX have been studied by using a Nicolet 7199 FT IR spectrometer with an MCT detector. A spectral region of 4000-700 cm-l was scanned at 4-cm-' resolution. In situ solid phase transition kinetic studies of the type reported here are largely unprecedented. A detailed description of the experimental procedure and its application to inorganic and organic solids will appear e1~ewhere.l~ The 0 polymorph of HMX containing a very low concentration of RDX (hexahydro-1,3,5-trinitro-s-triazine) impurity was used. The a polymorph was prepared by recrystallization of 0-HMX from glacial acetic acid.8 The sample was carefully washed and dried and then ground to the finest possible particle size. The y polymorph was prepared by dissolving 0-HMX in acetone and pouring the solution over cracked ice.8 The particle size of y-HMX obtained by this procedure was suitable for the kinetics study. A sample cell was built from an aluminum block so as to provide efficient heat transfer to the sample.14 The temperature could be controlled to within f0.2 "C with a quick response time. In a typical experiment, nominal 8 pM 6-HMX was dusted as a very thin film between two polished NaCl plates. A drop of silicone fluid was used to ensure good thermal contact between the HMX and the NaCl plates. Although kinetic data can be obtained without the silicone fluid, its presence reduces the scatter in the rate data. The (14) Karpowicz, R. J.; Brill, T. B. Appl. Spectrosc., in press.
Brill and Karpowicz
The Journal of Physical Chembtty, Vol. 86, No. 21, 1982
4262
-6.61
t ' t 185'
100
200
400
300
I / T ( K ) x 1000
Time ( s e c . ) version.
6-HMX solid phase transformation. The coefflcient of correlation is 0.972.
TABLE I : Arrhenius Data for th,e Three Thermally Induced Solid Phase Transformations o f HMX at 1 atm
TABLE 11: Some Distinguishing Features of the Polymorphs o f HMX
Flgure 3. Firstorder plots of the isothermal yHMX
transition
o+a a+ 6 7-6
E,, kJ mol-'
204 + 14 2 0 8 k 18 2 1 9 + 20
log A 19.9 + 1 19.9i 2 21.8+ 2
-+
6-HMX con-
Flgure 4. Arrhenlus plot for the y-HMX
temp, "C 166-194 188-200
171-185
silicone fluid and the configuration of the NaCl plates eliminated sublimation of the sample during the experiment. A heating rate of 2 "C min-' was used to reach the temperature range of the phase transition. The temperature was then quickly stabilized with a temperature regulator. Four IR spectra were recorded as a function of time at each temperature. The loss of absorbance in bands characteristic of the starting polymorph measured at specific time intervals afforded the rate of change of concentration as a function of time. Figure 2 shows the 6 transition with time at spectral change during a B 185.5 "C. A plot of In concentration vs. time for each temperature produced straight lines characteristic of a first-order reaction: d[B-HMX]/dt = -k[P-HMX]. For example, the data for the y 6 transition are plotted in Figure 3; the resulting Arrhenius plot is shown in Figure 4.15 The extent of decomposition is not large during these experiments and we m u m e that it has little effect on these results. However, some scatter in the data points in Figure 3 can be a result of low-level impurities, particle size effects, and the lack of perfect first-order kinetics. Table I summarizes the experimental nucleation activation energy, E,,and the frequency factor, A , for the thermally induced phase transitions of HMX. The a 6 transition is the most difficult of these solid phase changes to measure because ho mode of a-HMX in the 4000-700-cm-' region is well separated from those of 6HMX. Although the rate of the a 6 transition can be
-
-
-
(15) A commonly used equation for fitting solid-state reaction kinetic data is the Avrami-Erofeev equation:
log [ l / ( l - x ) ] = (kt)" x is the extent of the reaction. The value of n can vary with temperature, but n = 1 for a first-order reaction. In the temperature range used for these studies, n = 1.0 f 0.1, indicating that the solid phase transformations of HMX are essentially first-order processes.
polymorph
density," gm c m - 3
0-HMX a-HMX yHMX 6-HMX
1.903
1.87 1.82 1.78
-+
crystal system
space group
monoclinic orthorhombic monoclinic hexagonal
P 2 iCb Fdd2c Pc or P 2 / c d P6,22'
a McCrone, W. D. Anal. Chem. 1 9 5 0 , 2 2 , 1 2 2 5 . Choi, C. S.;Boutin, H. P. A c t a Crystallogr., S e c t . B 1 9 7 0 , 2 6 , Cady, H. H . ; Larson, A. C . ; Cromer, D. T. A c t a 1235. Reference 8 . Crystallogr., Sect. B 1 9 6 3 , 1 6 , 6 1 7 . e Cobbledick, R. E . ; Small, R . W. H. A c t a Crystallogr., Sect. B 1 9 7 4 , 3 0 , 1 9 1 8 .
measured early in the reaction, the interference from 6HMX bands precludes accurate intensity measurements well into the reaction.
Results and Discussion Kinetic Basis of the HMX Polymorph Conversions. The HMX molecule is composed of alternating relatively positive and negative charged atoms in a flexible eightmembered ring. The cohesive forces in the crystal lattice are very large as a result of the numerous intermolecular attractions of the type C*..O,N. C...N, and N...N.16 Intermolecular hydrogen bonding is not particularly important in the crystal structures.16J7 Consistent with the high lattice energy is the fact that HMX is a solid until about 270 "C, where rapid decomposition becomes important. The many intermolecular interactions and the flexibility of the eight-membered ring could be expected to generate a number of potential energy minima each representing a different crystal structure. Indeed, HMX exhibits four crystal phases labeled a, 0, y , and Some of the distinguishing features of these phases are given in Table 11. In addition, the ring conformation of P-HMX is chair while that in a-, y-, and 6-HMX is chair-chair. The stability of these polymorphs at room temperature is /3 > e . 0 ,
~
a
3
.
~
3
~
> y > 6.697
Several studies have been conducted of the heat-induced conversions among the HMX polymorphs.6ps Because (16) Brill, T. B.; Reese, C. 0. J . Phys. Chem. 1980,84, 1376. (17) Stals, J. Aust. J. Chem. 1969, 22, 2505.
SolM Phase Transition Kinetics
The Journal of Physical Chemistry, Vol. 86, No. 27, 1982 4283
a (orthorhombic)
T > 188' C AH= 3 kJlmoie
(monociinic)
T > 165' C
p AH
=
/
8
(hexagonal)
9 kJlmoie
A H = 6 kJlmoie
1
/
T > 170'C
Flgure 8. Summary of the enthalpy change and activation energy of the 0-HMX 6-HMX phase transition in the 165-205 "C range.
(monociinic)
Flgure 5. Summary of the thermally induced reconstructive phase transkions of HMX.
@-HMXis stable at room temperature and is the form sought in manufacturing, its conversion to the other less stable polymorphs is an important concern. Neat @-HMX converts to the 6 polymorph above about 160 "C. Some a-HMX occasionally forms along with the 6-HMX,8but the @ a conversion is rare and unpredictable in our e x p e r i e n ~ e . ~ J ~ J After ~ J ~ their preparation by other methods, a-HMX converts to 6-HMX above 188 "C while y-HMX converts to 6-HMX above 170 "C. These thermal conversions are summarized in Figure 5. No a y or @ y transitions have been detected with neat crystals. The enthalpy changes during solidaolid transformations of HMX are known at atmospheric and with high and variable p r e ~ s u r e . ' ~ JAH ~ is small and endothermic at atmospheric pressure (Figure 5). This finding is not particularly surprising in view of the fact that the intermolecular interactions which reorganize during the phase transition involve the same types of atoms separated by roughly the same distances.16 The enthalpy change cannot be looked to for an explanation of the experimentally observed transformations among the HMX polymorphs. Kinetic data for the polymorph transformations have not been available in spite of their potential value in understanding the properties of solid phase HMX. This information complements the thermodynamic aspects of polymorph transitions but, more importantly, could be the basis for the transformation scheme of Figure 5. Vibrational modes that are characteristic of each polymorph offer an observable for measuring the rate of the solidaolid phase transitions in HMX. The loss of intensity in the 958-cm-' mode due to C-N ring torsion in @-HMX20reflects the rate of the @ 6 conversion at 160-200 "C. Concurrent growth of bands due to 6-HMX occurs in the spectra shown in Figure 2. Although the fundamental modes of a-,y-, and 6-HMX are similar to one another owing to the similar molecular structure in these polymorphs, the NO2or N-N stretching mode9v20at 1311 cm-' in y-HMX and 1313 cm-l in a-HMX is sufficiently well separated from the 6-HMX modes to make rate measurements possible. The Arrhenius parameters for the CY 6 and y 6 transformations are summarized with error limits in Table I. The frequency factors in Table I are rather large. Very high values of A have been measured before in other solid phase transitions.2l The frequency factor for a solid phase
-
-
-
-
REACTION COORDHATE
-
- -
(18) Landers, A. G.;Brill, T. B. J.Phys. Chem. 1980, 84,3573. (19)Karpowicz, R. J.; Brill, T. B. AIAA. J.,in press. (20) Iqbal, Z.; Bulusu, S.; Autera, J. R. J. Chem. Phys. 1974,60,221.
reaction should not be thought of in the same context as it is for a gas phase reaction because solid phase processes usually involve multiple events including nucleation, propagation, and the movement of many atoms relative to their neighbors. The meaning of A in the solid phase is not well understood.21 As noted above, the @-HMX 6-HMX transformation is the predominant thermal polymorph conversion of HMX. The activation energy of the forward reaction is 204 kJ mol-' while that for the reverse reaction is 195 kJ mol-' due to the fact that the enthalpy of the 6 phase is 9 kJ mol-l above the @ phase in the temperature range of the conversion (Figure 6). AH for the a 6 conversion is 3 kJ mol-' and so E, for the a 6 and 6 a conversions is about 208 and 205 kJ mol-', respectively. AH for the y 6 is 6 kJ mol-'. E , for the 6 y transformation is, therefore, about 211 kJ mol-'. The activation energies above are very similar considering the precision of the measurement. However, after repeated experiments we believe that the general trend may be real. It would be hazardous to rely on these values to predict the competitive phase transition behavior, but, when coupled with the experimental facts, the data provide a basis for understanding the observed phase transformation scheme. The @ 6 transition has the lowest activation energy of those in Table I. This result may explain why heating 0-HMX always produces 6-HMX and cooling 6-HMX always produces @-HMX(Figure 5). As long as the heating rate permits kinetic control of the phase transition, /3- and 6-HMX will interconvert with the exclusion of the other polymorphs. Once formed by other means, a- and y-HMX convert to 6-HMX because the 6 polymorph is thermodynamically stable at high temperature. Phase transition sequences such as y a 6 and a y 6 are not observed on heating. This suggests that the values of E, for y 6 and a 6 transformations are probably very similar. The problems in measuring the a 6 transformation rate preclude our verifying this point with more precision than is done in Table I. 0 a and @ y transformations are not observed in these studies, but approximate values of E, can be estimated from the data in Table I and Figure 5. E, values in the 217-226 kJ mol-' range are expected. These transitions will not occur in the face of those shown in Figure 5 provided kinetic considerations control the system. Being kinetically isolated by high potential energy barriers, pure @-, a-,and y H M X are indefinitely stable at room temperature with respect to conversion to other polymorphs. The same might be expected of 6-HMX.
-
- --
-
-
--
-
+
-
-
--
(21) Hill, R.A. W. "Reactivity of Solids"; DeBoer, J. H., Ed.; Elsevier: New York, 1961; pp 294-300.
4264
The Journal of Physical Chemistry, Vol. 86, No. 21, 1982
TABLE 111: Selected Arrhenius Data for the Thermal Decomposition of HMX in t h e Condensed Phase"
E,, kJ mol-'
log A
temp, "C
isothermal DSC' manometric4 flow reactor mass spectrometry3
214 220 209
18.8 19.7 17.8
271-285 271-315 261-276
DTA
220
method
On the basis of the rates of the phase transitions, it appears possible to identify more clearly the initial ratedetermining process in HMX decomposition. The Arrhenius equation for the rate of the b 6 phase transition when extrapolated to the temperature range of interest in combustion (300-450 "C) suggests that the crystalline transformation is fast enough to occur in the propellant. Because the D-HMX 6-HMX transition will occur in the thermal wave below the burning propellant surface, it may actually be the first important molecular event in the thermal decompositi~n.~~ The accompanying density change will make this first-order transformation an important feature in the mechanical properties of a propellant. With this point aside, however, the kinetic data here have very important implications about the thermal decomposition process in the solid and liquid phase of HMX. The conclusions presented below apply to the heating conditions used here and in the thermal decomposition work. The Arrhenius data for the condensed-phase decomposition of nitramines have been acquired largely by DSC, manometric, and mass-spectrometry methods.'-4 The gaseous decomposition products are measured from pyrolyzed nitramines by mass spectrometry, although some work has been done on identifying nonvolatileE and radical species.z7 A problem arises in connecting the Arrhenius data for decomposition to specific reactions in the condensed phase because these two facets are not examined in the same experiment. Their linkage is made still more speculative by the fact that the rates of the specific bond-breaking steps receiving the most attention have only been measured in the gas phase. In this vein, Fifer28recognized a phase dependence in the Arrhenius parameters for nitramine decomposition. Both the frequency factor and the activation energy follow the trend gas < liquid < solid. This trend was attributed to a "cage effect" wherein a decomposition reaction, such as N-NOZ bond cleavage with release of NOz, would be less often successful when the escape of the NOz radical is hindered by a cage of liquid or solid phase molecules. For instance, it is known that larger particles of HMX retain decomposition products to a much greater extent than do smaller parti~1es.l~ The barrier to chemical reactions in the condensed phase necessarily involves both intra- and intermolecular energetics. If the intermolecular forces are very strong, as is the case with many solids including the nitramines, then these forces may provide a large (and in some cases the primary) potential energy barrier to chemical changes.z9 The same can be said of a strongly associated "melt" of nitramine molecules. The experimental Arrhenius data for the phase transitions of HMX pertain to this point. Notice that E, for the phase transitions (Table I) and E , for the condensed-phase thermal decomposition of HMX (Table 111) are the same within the error of the measurements. Also, the values of A , the frequency factor, are very similar. As was mentioned above, the rate of the solid-solid phase transitions of nitramines involves the rupture and reorganization of the intermolecular electrostatic interactions in the crystal lattice.29J" The similarity between the
-
-
243-276
a Error ranges are n o t given in all studies, but are probably in the range of 5-10%.
However, 6-HMX gradually converts at room temperature to P-HMX in the time of hours to weeks depending on its purity and crystallinity. The relative instability of 8-HMX is most likely a manifestation of the difficulty of isolating the polymorph in the pure crystalline form. Very small amounts of 0-HMX in the 6-HMX are known to catalyze the 6 @ conversion? On this basis, the ease of nucleation of the new phase appears to be a critical factor in these solid phase transformations. The sluggishness of the 6 /3 conversion at room temperature might be attributed to genuine hysteresis because of the large crystal volume change.zz However, the fact that 0-, CY-, and y-HMX are indefinitely stable at room temperature and also have significantly different densities suggests that hysteresis is not entirely responsible for the behavior of the HMX polymorph transitions. The ring conformation changes from the chair to the chair-chair form during the P-HMX 6-HMX transition. Buchi models of HMX reveal that a ring torsion mode much like a pseudorotation vibration23converts the chair of P-HMX to the chabchair of the other polymorphs. The ring torsion is essentially a normal mode of the molecule and requires comparatively little energy (about 4 kJ mol-'). Therefore, for all practical purposes the phase transition activation energy relates to the barrier to disruption of the intermolecular interactions in the crystal lattice. Rocking motions of the HMX molecule that strain the intermolecular cohesive interactions are known to increase as the temperature rises to the phase t r a n s i t i ~ n . ~ ~ Relationship between the Phase Transition and Decomposition Kinetics. The products of gas, liquid, and solid phase nitramine decomposition are extensively disc ~ s s e d .Kinetic ~ measurements of the condensed-phase decomposition have been made by a variety of methods including DSC, DTA, and mass ~pectrometry.'-~The agreement among all kinetic data available is not very good5bprobably because HMX decomposition can depend on the temperature, pressure, heating rate, sample purity, thermal contact between the sample and heat source, and other experimental variables. The most widely used Arrhenius values are summarized in Table 111. The best values5b of E, for condensed-phase decomposition are around 209 kJ mol-' or slightly higher and the best value of log A is about 20. While the mechanism of nitramine decomposition is believed to include primary and autocatalytic follow-up steps, its complexity precludes drawing firm conclusions. However, the reaction reflected in the kinetic constants given in Table I11 is always assumed to involve covalent bond rupture in the nitramine molecule per ~ e N-N02 . ~ bond cleavage, C-N bond cleavage, and HONO elimination are widely presumed primary reaction steps.
-
Brill and Karpowicz
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-
(22) Ubbelohde, A. R. Q.Reu., Chem. SOC.1957, 11, 246. (23) Lame, J. Q.Rev., Chem. SOC.1971, 25, 533. (24) Landers, A. G.; Brill, T. B.; Marino, R. A. J. Phys. Chem. 1981, 85,2618.
(25) Karpowicz, R. J.; Gelfand, L. S.; Brill, T. B. AIAA J.,in press. (26) Kimura, J.; Kubota, N. Propellants Explos. 1980, 5 , 1. (27) Morgan, C. U.; Beyer, R. A. Combust. Flame 1979, 36,99. (28) Fifer, R. A. "JointONR, AFOSR, ARO Workshop, Berkeley, CA, Jan 1981", pp 153-68. (29) Bawn, C. E. H. In "Chemistryof the Solid State";Garner, W. E., Ed.; Butterworths: London, 1955; pp 254-67. (30) Rao, C.N. R. "Modern Aspects of Solid State Chemistry";Plenum Press: New York, 1970; p 589.
The Journal of Physical Chemistry, Vol. 86,
Solid Phase Transttlon Kinetics
Arrhenircs values for the phase transition and the thermal decomposition strongly suggests that both involve the same process-namely, the breakdown of the intermolecular forces. This conclusion contrasts with all previous interpretations of the condensed-phase thermal decomposition kinetics which assume that intramolecular covalent bond breaking controls the reaction rate. The footing upon which the present interpretation rests is firmer than what has been previously available. This is the first measurement of solid phase process in which the rate data and the event being studied are clearly identifiable. The Arrhenius data for the phase transition and those for decomposition would probably not be so similar were they not controlled by essentially the same molecular phenomenon. Consistent with this interpretation is the fact that E, for decomposition and AH for the /3 6 transition approach the same value at high pressure.lg The reactions leading to the observed gaseous products could occur to some extent before and during the breakdown of the intermolecular interactions, but it seems most reasonable that the largest percentage occur after the matrix has become severely destabilized. The bond enthalpies have been measured in the gas phase for several molecules similar to HMX and RDX. Although these values are undoubtedly not the same in the solid and liquid phases, they have often been relied upon for insight into the decomposition process. For example, the N-N bond e n t h a l p p for gaseous (CH3)2NN02is 193 kJ mol-'. Estimates of the C-N bond enthalpy5c produce a value of about 251 kJ mol-l. To the extent that these enthalpy values can be applied to the activation energies for the cleavage of these bonds, it is reasonable that such reactions could occur in varying amounts in conjunction with the breakdown of the intermolecular forces. However, the unrecognized importance of the intermolecular interactions in the decomposition process is at least partly responsible for the difficulties experienced in formulating the pathways of molecular decomposition of HMX. The present interpretation of the decomposition of nitramines relates to the mobile liquid layer that is observed
-
No. 21, 1982 4265
during the rapid thermal decomposition of HMX and RDX.31 This layer, sometimes called a melt layer, is probably not an authentic change of state of HMX. Rather it is better thought of as the dynamic breakup of the nitramine lattice accompanied by chemical decomposition. The liquefaction of the nitramines results from the mixture of decomposition products and HMX molecules, which is too heterogeneous to remain solid above 270 OC. The rate of decomposition of this liquid, which contains highly polar species, can be controlled by the energy of the intermolecular interactions. It should be noted that Robertson4 found smaller values of A and E, for the thermal decomposition of HMX in dilute solutions where the intermolecular forces are much less important. Finally, a similarity exists in the destabilization of the HMX lattice during decomposition and the behavior of another important oxidizer, NH4C104,at its decomposition point. The temperature at which the C104- ions join the NH4+ions in essentially unhindered tumbling in the crystal NH4C104crystal lattice is also found to be the temperature at which significant decomposition of the solid is also In other words, the initial chemical decomposition of both HMX and NH4C104appears to be controlled by the breakdown of the intermolecular or ionic forces in the condensed phase.
Acknowledgment. We are grateful to the Air Force Office of Scientific Research for support of this work through AFOSR-80-0258. (31) See references in ref 5a. (32) A possible avenue to increasing the rate of nitramine decompo-
sition is suggested by these conclusions. If the rate-determining step involves mostly breakdown of intermolecular forces, then it follows that a material which relaxes the intermolecular forces could increase the decomposition rate. Appropriate compounds might be cocrystallized with HMX or be used in such a way as to alter the electrostatics of the molecule and in turn modify the decomposition rate. However, it may be difficult to find materials that alter the intermolecular forces but do not adversely affect the chemical reactions or properties of HMX. We are examining the role of dopants in the HMX crystal lattice at the moment. (33) Brill, T. B.; Goetz, F. J. Chen. Phys. 1976, 65, 1217. (34) Brill, T. B.; Goetz, F. Prog. Astronaut. Aeronaut. 1978, 63, 1.
