340
The Journal of Physical Chemistry, Vol. 83,No. 3, 1979
F. Goetz
and T. B. Brill
Laser Raman Spectra of CY-,p-, y, and 6-Octahydro-I ,3,5,7-tetranitro-I ,3,5,7-tetrazocine and Their Temperature Dependence F. Goetz and T.
B. Brill”
Depadment of ChemMry, Universify of Delaware, Newark, Delaware 197 I 1 (Received February 23, 1978; Revised Manuscript Received July 25, 1978) Publication costs assisted by the Air Force Office of Scientific Research
The laser Raman spectra of the four known polymorphs of octahydro-l,3,5,7-tetranitro-l,3,5,7-tetrazocine (HMX) have been recorded. Comparison of the spectra with the predictions of factor group analyses of a-, P-, and 6-HMX are presented. The spectra reveal that P-HMX contains a different ring conformation than do a-, y-, and 6-HMX. a-, y-, and 6-HMX have ring conformations which are similar to one another. y-HMX appears to contain more anharmonicity in its vibrational motions and this may contribute to the fact that 7-HMX does not form thermally by solid-solid transformations from the other polymorphs, but readily converts to 6-HMX upon heating. The spectral regions which proved to be the most useful for identification of each polymorph are 300-550 and 700-1050 cm-l. The lattice regions were not as useful because of the difficulty of recording some of the spectra near the exciting line. The solid-solid phase-transition behavior between the polymorph forms was studied at two heating rates and the Raman spectra recorded at isothermal conditions. The transitions observed were generally similar to those reported previously, but the transition temperatures differ somewhat. The gas-solid heat transfer method used in these studies appears not to yield thermodynamic transition temperatures characteristic of equilibrium conditions.
Introduction Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine is commonly referred to as HMX and is an important monopropellant material. Figure 1 shows the skeletal structure of the molecule. HMX is known to exist in four solid-state polymorphs labeled a-, P-, y-, and 6-HMX. Evidence for these polymorphs is based on results from melting point diagrams in the presence of solvent1 and hot stage microscope studies,2 optical measurement^,^ and X-ray diffraction studies., The ambient thermal stabilities of the four forms are P > a > y > 6.’ X-ray crystal structure determinations on the a , P, and 6 polymorphs have been carried o ~ t . ~ -a-HMX I and 6-HMX are quite similar to one another as far as the conformation of the HMX molecule is concerned, but differ in the positioning of the molecules relative to one another in the lattice. The conformation of the eight-membered ring is a “boat” form in the sense that the NOz groups are positioned on one side of the m ~ l e c u l e .On ~ ~the ~ other hand, the ring in P-HMX is found to exist in more of a “chair” form giving the entire molecule a center of symmetry.G P-HMX is the commonly encountered €orm but the other polymorphs have relevance to the thermal decomposition of HMX. Our interest in HMX is derived from two aspects of the area of thermal decomposition studies. The first involves the development of general methodology to examine thermal behavior of solid state materials on a fundamental level.* Laser Raman spectroscopy has been employed in this research to directly investigate the molecular parameters of a solid material whose thermal behavior is of practical importance. Raman spectroscopy provides the flexibility in sample manipulation needed for these kinds of studies. The second more specific goal stems from the hope of obtaining information about the solid phase of HMX. Laser Raman spectroscopy was used here to extract vibrational frequency information from the four HMX polymorphs. The spectra of the HMX polymorphs were obtained a t room temperature. The complete Raman spectra of the polymorphs allows a comparison to be made between the three forms for which X-ray crystal structure 0022-3654f79/2083-0340$01.OOfO
data are available, and the y-polymorph, whose structure is not known. Qualitative conclusions about the structure of y H M X can be extracted. By heating a t various rates and following the spectral details, crystal transformations occurring below the melting point can be seen. Of most practical interest were the faster heating rates, but we have looked a t slow heating rates because these are more amenable to detailed analysis. The slow heating rate data are not in complete accordance with previous ~tudies,l-~ probably because of the difficulty in achieving chemical equilibrium in the phase changes of HMX. The gas-solid heat transfer method we employed to imitate a heating mechanism of HMX in propellant combustion allows us to produce phase transitions, but not necessarily a t the thermodynamic transition temperature. Actual identification of decomposition products in solid HMX has eluded us to this point.
Experimental Section Instrumentation. All spectra were recorded using a Spex Model 1401 double monochrometer spectrometer with a Spectra-Physics Model 164 4-W argon ion laser source. The laser was tuned to 488.0 nm with a power output of 0.6-1.35 W. Monochrometer slit widths were set at 150 pm. The spectrometer was calibrated using the 218-, 314-, and 459-cm-l bands of CCl,. The vibrational frequencies are accurate to within fl cm-’ in a relative sense and f 2 cm-l in absolute sense. The spectra of a-, p-, and y-HMX and the slow heating studies were done using photon counting methods. Each spectrum took a maximum of 15 min to record. Spectra obtained in the rapid heating experiments and the spectrum of the 6-HMX polymorph were recorded using the above system interfaced to a Nicolet 1180 data acquisition system. The average time for a multiple scan spectrum was 30 min. These spectra have been computer treated to minimize the effects of noise and fluorescence. The spectra of the polymorphs were not all of equal quality. Fluorescent decomposition products and surface deterioration lead to poorer spectra a t elevated temperatures. Computer 0 1979 American Chemical Soclety
Laser Raman Spectra of a-, @-, y,and 6-HMX
I N 02
Figure 1. Structural formula of HMX. treatment greatly improved the appearance, however. Heating Experiments. The equipment and techniques employed during the slow heating experiment were those developed during our study of ammonium perchlorate8 and involve passing heated nitrogen gas over the sample. The temperature of the sample was controlled by adjusting the nitrogen flow rate through a Pyrex heating coil in a tube furnace set at 500 "C. The temperature was monitored by a thermocouple placed within 5 mm of the sample between the sample and the nitrogen source. Calibration of the temperature of a sample in the Harney-Miller cell was performed by measuring the known phase transition temperature or melting temperature of several compounds in the temperature range covered by this study. The compounds used and their literature melting temperature followed parenthetically by the melting temperature measured in our system were: 4-methoxy-2-nitroaniline 123-126 "C (124.5-127.4 "C); malonic acid 135.6 "C (134.5 "C); ammonium nitrate 169.6 "C (170.4 "C); hexachlorobenzene 230 "C (230.2 "C). The orthorhombic to tetragonal phase transition at 84.5 "C in ammonium nitrateg was observed at 85.5 "C in our system. The laser beam has no noticeable effect on these temperatures. The overall heating rate during controlled heating experiments (slow heating) was limited to less than 10 OC/min due to the restrictions on heat transfer of the nitrogen flow system. The temperature of the sample was allowed to equilibrate at a particular temperature for a minirnum of 5 min before the Raman spectrum was recorded. Spectra were recorded in temperature intervals of 25 "C, except in the regions where polymorph conversions were found. Near the transition temperature, intervals of less than 10 "C and heating rates of 1"C/min were used. Rapid heating experiments were performed by taking a 1.6 X 90 mm thin wall capillary tube filled to a depth of 25 mm with sample and immersing it in a silicon oil bath having the desired temperature. The samples were immersed for several minutes to ensure uniform temperature throughout the sample, and then removed, cooled, and the spectrum immediately recorded. Heating rates for the rapid heating experiments were approximately 50 "C/s in the temperature range of 25-200 "C. The heating rate was obtained by measuring the average time required for a Fluke Model 2100 A digital thermometer probe starting a t 25 "C t o reach 180 "C after immersion of the thermocouple in the oil bath set at 185 "C. Polymorph Preparation. Samples of large crystals of P-HMX and granular P-HMX were obtained from T. L. Boggs of the Naval Weapons Center. The large crystals were used without further purification while the granular sample was purified by two different methods in an attempt to remove any residual RDX (hexahydro-1,3,5trinitro-s-triazine). One method involved extracting HMX for 24 h with 1,2-dichloroethane, filtering, and air drying the sample.2 A second method involved heating HMX in a vacuum oven at 140 "C for 24 h, extracting the product material with acetone, and evaporating the acetone.1° Both procedures led to the same results upon heating and were found to contain less than 0.1% RDX by mass spectral
The Journal of Physical Chemistry, Vol. 83, No. 3, 1979 341
analysis.1° Samples of a-HMX and y-HMX were prepared by the method of Cady.2 a-HMX crystals used for spectroscopic studies existed as fine crystalline needles. y-HMX was also prepared by pouring a hot acetone solution of HMX into excess n-heptane. Our attempts at the synthesis of 6-HMX by previous methods2J1 failed to yield a pure 6-HMX polymorph.12 The method used to produce small samples of 6-HMX involved immersing a partially filled capillary tube of granular O.HM[X in an oil bath set at 185 "C. An immersion time of 3-5 min gave complete conversion to 6-HMX and the least amount of sample degrad at'ion. X-ray powder patterns of the above samples of a-,0-, y-, and 6-HMX were compared to those reported by Cady2 to ensure their identity.
Results and Iliscussion It should be noted immediately that single crystals of a- and P-HMX show orientational dependence in their spectra. A difference is found in the relative intensities in the internal mode region but no significant change in the frequency or number of bands is seen. In the external mode region the number and intensity of bands change with a change in orientation. In the spectral regions used to identify the polymorphs, intensity differences occur as a function of orientation but the number and position of the bands does not change. Hence these ranges are suitable for polymorph identification without producing any ambiguity. Spectra a t 26 "C. The vibrational spectrum of 0-I-IMX has been examined in several reports.13J4 The infrared spectra of the cy-, P-, y-, and 6-HMX polymorphs irc KBr pellets have been recorded by Cady.2 Cavagnat et al.13 carried out a polarized IR and Raman spectral study of a single crystal of 0-HMX in the 10-200-cm-l region and assigned the vibrations. A complete IR and Raman analysis of p-HMX has been reported by Iqbal et al.,I4 who carried out an isotopic substitution study (15N, 13C, and 2H) and therehly determined the band assignments over the entire spectrum. To our knowledge the Raman spectra and assignments for the other three polymorphs have not been reported before. We have recently reported the pressure dependence of the Raman and infrared spectra of the polymorphs of HMX.I5 The spectra of all four polymorphs were recorded and qualitatively compared to the predictions of factor group analyse@ where possible. The room teimperature-stable form of HMX has been labeled the &polymorph. It is the most dense of the four forms, and both X-ray5 and neutron diffraction6 studies reveal that 0-HMX contains two molecules in a unit cell having a space group of P2Jc (C;,J. The molecule has a center of symmetry and the eight-membered ring exists in what can be thought of as a chair conformation. 6-HMX has 28 atoms and therefore 78 possible internal vibrations. The molecule exists on a centrosymmetric site (C,) in the crystal lattice so that these modes are distributed among the symmetry species of the C, site group as 39Ag + 39A,,. The A, modes are Raman active while the A, modes are IR active. There are two formula units in the cell, and the resulting splittings introduced by the C 2 h factor group could cause a doubling in the number of internal modes to 156. These would be distributed among the symmetry species: 39A, + 39Bg + 39A, + 39B,. The g and u modes continue to remain mutually exclusive in the IR and Raman. The lattice modes are distributed among the irreducible representations of the CZhfactor group as 3A,(R) + 3Bg(R)+ 2A,(T) + lB,(T), where R and T represent rotational (librational) and translational modes, respectively.
342
The Journal of Physical Chemistry, Vol. 83, No. 3, 1979
F.
Goetz and T. B. Brill
Chart I point group C,,, 22A'> 18A,
site group
factor group
C,
CN,
40A,
40A
Chart I1 site group cm-1
factor group
c*,
c 2
Raman spectrum of a single crystal of recorded with reduced photomultiplier sensitivity. Flgure 2.
6-HMX.
Inserts
A,(T -I- R) A-A,(T t R) B,(2T + 2R) B-=====B2(2T t 2R) I
I _ " " ' " I ''
35 3
ECCZ
MG
IOCO
3 3
icm-11
Figure 4. Raman a-HMX
-L___
L A 1
Figure 3. Low
0
frequency Raman spectra of a-, &, and
in the unit cell but only two in the Bravais cell. Each molecule has CZLpoint group symmetry. The ring conformation in a-HMX is a puckered boat form in which all of the NOz groups are located on one side of the molecule and all of the CH2 groups reside on the other. The CZu point group of a-HMX can produce up to 78 internal modes having the symmetries 22A1 MAz 19B1f 19B2. All internal vibrations have Raman activity. The correlation field for site and factor group splitting of the internal modes is shown in Chart I. Hence in full factor group splitting 156 modes might be observed. The Raman spectrum of a-HMX shows only about 40 modes and is reproduced in Figure 4. The spectrum is characteristic of the point group symmetry in which coupling of vibrations is small. The spectra in the 200-3200-~m-~ range of a- and P-HMX contain notable similarities as they should, but because the ring conformations differ, the ring stretch and torsion regions (300-550 and 700-1050 crn-l) show significant differences. Both the number and position of the ring mode frequencies are different. The modes assigned to NO2motions are similar in number and frequency as might be anticipated. The lattice modes in a-HMX have the correlation field shown in Chart 11. After removing the three acoustic modes which do not appear in the Raman, up to nine lattice modes having the symmetries A,(R) A2(R + T) R,(T f 2R) f B2(T+ 2R) might appear. Figure 3 shows the lattice region spectrum and several low energy internal modes in a-HMX. A useful feature of the lattice spectrum is that the polymorphs could be easily distinguished from one another using this spectral region. However, no attempt was made to assign the origin of these modes and the lattice region was not used to identify polymorphs. This is partly because the lattice region spectrum for y H M X could not be recorded, but more importantly because the lattice region spectrum of 6-HMX could not be routinely obtained after prolonged heating of the sample. The poor resolution in the 10-200-~m-~ region of heated HMX probably stems from poor crystallinity due to decomposition.
+
p-HMX '
oc cm-1
spectrum of polycrystalline a-HMX.
6-HMX.
The Raman spectrum of P-HMX in Figure 2 shows only about 40 modes rather than the 80 or so predicted by the factor group analysis. As is frequently the case, the factor group splittings are not apparent in the molecular spectrum. In other words the molecular vibrational modes behave as nearly independent uncoupled units and only the site group symmetry is needed to understand the vibrational spectrum. Table I summarizes the vibrational frequencies in P-HMX. The frequencies are found to be in good agreement with those reported by Iqbal et aL1* Their general assignments of these frequencies are also shown in Table I. These assignments serve as basis for analyzing the spectra of the other polymorphs. None of the assignments should be thought of as indicating pure uncoupled vibrational motions, however. They merely indicate what are probably the largest component motions in the normal coordinate. The low frequency region of P-HMX is shown in Figure 3, and Table I1 contains the specific frequencies. The spectrum agrees with the spectrum reported by Cavagnat et in the aa direction of the polarizability tensor. No attempt was made to analyze the bands further and the spectrum is shown primarily for purposes of comparison with the other polymorphs. a-HMX is found to crystallize in an orthorhombic lattice with the space group Fdd2.j There are eight formula units
+
+
+
Laser Raman Spectra of
a-, /3-, y,and 6-HMX
The Journal of Physical Chemistry, Vol. 83, No. 3, 1970 343
Chart I11 point group
site group
factor group
c2
c,
C6
I_-
3KC
"
'
203G
1
5GC
"
"
1
"
1
"
GOO
500
c m-11
Figure 6. Raman spectrum water.
3000
2CYX
E X
coo
500
icm"
Figure 5. Raman spectrum of polycrystalline 6-HMX. Five scans were added and base line corrected for fluorescence.
6-HMX is the high temperature-stable polymorph and therefore may be the form of greatest interest in kinetic modeling of thermally decomposing HMX. The crystal structure of 6-HMX shows7 the molecule to possess C2 symmetry and to differ from a-HMX primarily through small differences in the relative positioning of the NO2 groups. The ring conformation as a puckered boat is the same as a-HMX. The space group of 6-HMX is P66 (C:) which requires all the molecules to reside in C1 lattice sites. The factor group is C6. The 78 internal modes of 6-HMX have symmetry species of 40A + 38B and are IR and Raman active. The internal mode correlation splittings are shown in Chart 111. Because there are six molecules in the Bravais cell, a total of 312 internal modes might exist in full factor group splitting. None of the B symmetry modes are vibrationally active, however, so a maximum of 234 modes could exist.16 Figure 5 shows the Raman spectrum of 6-HMX. Nowhere near as many modes are present as is predicted by the factor group or even the site group splittings. The point group symmetry without mode coupling appears to be enough to permit an analysis of the spectrum. In fact all three polymorphs for which a factor group analysis can be carried out (a-,@-,6-HMX) show about 40 modes rather than the number which would result from site or factor group splittings. Moreover, the A and B branches do not appear to be distinct from one another in practice in either a- or 6-HMX. From these two observations, we can conclude that HMX molecules behave as independent species in the solid state so far as the internal modes are concerned, and that vibrations on one side of the molecule do not couple to any great extent to vibrations on the opposite side. This lack of coupling gives rise to a considerable amount of accidental degeneracy in the internal modes such that only about half of the point-group-predicted modes appear in a- and 6-HMX. A factor group analysis of the lattice modes in 6-HMX results in seven translatory modes (2A 2E1 -t 3E2) and nine rotatory modes (3A + 3E1 + 3EJ in the Raman spectrum. The low energy spectrum of 6-HMX is reproduced in Figure 3. Its quality is poorer than those of a- and @-HMXand this is particularly evident by the fact that the 6-HMX spectrum is a summation of five scans whereas the a- and 0-HMX spectra are single pass spectra. y-HMX is a metastable polymorph which is not observed as a component in the heating scheme of a-, @-, or 6-HMX. The molecular structure of y-HMX is not known
+
of polycrystalline y-HMX prepared from
although the lattice is known to exist in a monoclinic crystal system with the space group P, or P2/ca514 satisfactory Raman spectrum of y-HMX was obtained and this is shown in Figure 6. The spectrum has the same general appear,ance as the a-, 0-, and 6-HMX spectra but it corresponds more with a- and 6-HMX than with pHMX. This is particularly true in the ring-motion region of the spect,rum. From this fact it is reasonable to conclude that y-HMX contains a puckered boat ring conformation such as that found in a- and 6-HMX, although probably not as distorted a boat configuration as proposed by Bedard et al.17 The polycrystalline nature of the sample disperses the light near the exciting line to the point that no lattice region spectrum was obtained. 7-HMX contains a conspicuous feature not observed in the other three polymorphs. Bands in the 1950-2060- and 3600-3750-~m-~ regions appear in y-HMX but not in any of the other three polymorphs. The modes in the 36003750-cm-' range may be combination bands involving CH2 and ring motions. The lower frequency modes may be N-N02 and ring mode combinations. They do not appear to be first overtone modes because no bands appear at half the frequency in either case. The possibility that these modes might arise from lattice intercalated H 2 0 was considered since y-HMX is made by rapid crystallization from H20. The 3600-3750-~m-~modes are not Linreasonable for 0--H stretching frequencies but the 19502060-cm-l modes are about 400 cm-' higher than the bending modeti of H 2 0 would normally appear. These bands continue to show up when y-HMX is heated to 140 "C, but it is known that HMX can tenaciously hold solvents. Acetone in the HMX lattice is not driven off below 190 The Raman spectrum of acetone contains no bands at these frequencies. y-HMX was therefore prepared in an anhLydrous environment by pouring an acetone solution into n-heptane. The bands in the 3600-3750- and 1950-2060-cm-' regions continue to appear a t similar intensities whilch verifies them as not being due to H20. Apparently, there are subtle differences in the conformation of y-HIMX which increase the importance of anharmonic terms in the potential energy and thereby give rise to combination bands. If this is the case it is not unreasonable to expect that y-HMX should be difficult to form thermally from the other polymorphs. On the other hand, coinversion of y-HMX to other polymorphs should easily occur. This is indeed what is observed2 and y-HMX has been referred to as a metastable form. All of the spectra were scrutinized to identify frequency regions which are unique in the four polymorphs. These ranges can be utilized to diagnose polymorph transformations in HMX. The ring stretch-torsion frequencies at 300-550 and 700-1050 cm-l were finally settled upon because of the unique appearance of the 300-550-~m-~ range in each polymorph and the intensity of the 700-
344
The Journal of Physical Chemistry, Vol. 83, No. 3, 1979
F. Goetz and T. B. Brill
TABLE I: Raman Active Vibrational Frequencies (cm-*)and Probable Assignments of Modes in Crystalline HMX Polymorphs P a Y 6 assignment l 4 P a! Y 6 assignmentt4 ring motion 1168 200 1190 1190 205 1215 224 1225 1225 us (NO,) and v s (N-N) 230 233 1248 233 1256 235 1258 281 1259 312 1268 328 1271 358 1275 365 1280 375 1291 392 1294 400 1312 402 1317 412 1318 432 1319 446 1330 450 1332 458 473 1350 1368 1368 485 6 (CH,) 1371 590 ring motion and y (NO,) 1375 592 1382 594 1385 1 3 8 5 597 1391 601 1392 603 1393 618 1411 620 1412 622 1415 636 1418 638 1419 646 1422 1422 648 1438 654 1440 662 1442 710 1448 71 2 1452 1452 713 1460 721 1502 135 735 1508 6 and y (NO,) 740 1524 7 50 1525 751 1528 753 1532 1532 758 1536 759 1539 763 1557 ring stretch 834 1558 840 1561 846 846 1562 84 7 1563 870 1568 878 878 1573 881 combination bands 1958 882 1975 893 893 1982 928 928 2030 930 2050 940 2068 943 2865 945 2910 950 2918 965 2928 995 99 5 2967 1030 2974 1063 2975 1080 2982 1085 2989 1090 1090 2991 1092 1111 2992 2997 1112 1166
Laser Raman Spectra of a-,0-, y, and 6-HMX
The Journal
of Physical Chemistry, Vol, 83, No. 3, 1979 345
TABLE I (Continued) P
CY
6
7
assignment '
P
Y
CY
3595 3672 3684 3700 3722 3728 3736
3037 3045 3047 3053 3058
assignment"
6
-
combination bands
3060 TABLE I1 : Raman Active Vibrational Modes in the Lattice Frequency Region of a-,o-, and 6 -HMX
P
a
w
s
870 882
3% 37 sh 42 sh 44
9m
39: 44f
943
47
I
47 53 sh 58 sh 60 67 sh 70
%c
83 sh
846
90 sh
693
841 878 928
72
%c
78
878
84
928 94 5
92
995
103 110 115 127 134 sha 142 1 5 1 sh
834 881 950
160
--'
166
Icxx) 903 800 CM'
178
sh = shoulder.
1050-cm-' modes. Expansion of these spectral ranges is shown in Figure 7. 0-HMX is clearly very different from a-, y-, and 6-HMX in these spectra. The a-, y-, and 6HMX [spectra in the 700-1050-~m-~ range are somewhat different but not as much so as in the 30CF550-crn-' range. The most confidence was placed in the appearance of the 300-550-crn-' modes to distinguish the polymorphs during heating experiments. The higher frequency region was used as a back-up check. Spectra at 25-205 "C. Having obtained the room temperature spectra of the four known HMX polymorphs, a great deal of effort was directed at the use of Raman spectroscopy to map the solid-solid polymorph conversions of HMX. The 700-1050- and 300-550-cm-' ring motion regions were used to diagnose the changes. The slowheating method used to produce the phase transitions involved passing hot N2 gas over the solid sample. This approximates the convective heat transfer from a gas to the solid during combustion. Our phase transition results are broadly consistent with past results on the thermodynamic phase transitions of HMXlU3in terms of the polymorph interconversions observed. However, they differ in the temperatures at which these transitions occur. Figure 8 shows the typical spectral changes which occur upon slow heating of a crystal of P-HMX to 142 "C and maintaining that temperature for 60 min. Under these conditions gradual broadening in the 950-cm-' vibration occurs, and eventual splitting into two poorly resolved modes takes place after about 30 min. Similarly, there was a pronounced shift and broadening in the 834-cm-l vibration during this time. In the low frequency region the
700
500
400
C M"
300
Figure 7. The ring motion regions of a-, p-, y-, and 6-HMX. Frequencies of main bands are listed to the right of the spectra.
1000
900
800
'00
CM
'
500
400
300
Flgure 8. Spectral changes upon slowly heating 0-HMX to 142 OC,
maintaining that ternperature for 1 h , and then cooling to room temperature. A 0-l o 6-HMX transition occurs.
disappearance of the @-polymorphvibrations at 312, 358, 412, and 432 cm-l with the concomitant growth of inew vibrations at, 392, 446, and 473 cm-I indicated the formation of 6-HMX. Upon cooling to 25 " C for 12 h the sample remained as the &polymorph.
346
F. Goetz and T. B. Brill
The Journal of Physical Chemistry, Vol. 83, No. 3, 1979
r-n-
r+/----m
TABLE I11 : Phase T r a n s i t i o n Temperatures (" C) of HMX
this work
transitions I?
+-+ CY
a-6 a-7 cy +-+
a-7
7-6
lY!xLfL++
;J
&', \r
1000 300 e x 700 cvl 530 4m 30 Flgure 9. Spectral changes upon slowly heating a-HMX to 170 "C and then cooling to room temperature. An a- to 6-HMX transition occurs.
Figure 9 illustrates the slow heating spectral change in a-HMX. The polymorph was stable through 147 "C. Above 147 "C a transition from a- to 6-HMX was observed as evidenced by the appearance of the characteristic 392-cm-' mode of 6-HMX. The sample was cooled to 25 "C and the spectrum recorded 12 h later. The 6-polymorph still predominated but some a-HMX was beginning to appear. In general, a very large hysteresis exists in the conversions of these polymorphs back to P-HMX at room temperature. It can be concluded that when 8-HMX is heated to a high enough temperature and then cooled, it does not return to the P-polymorph for a long period of time. a- and 6-HMX are routinely present after such heating. Hence, HMX which is thermally cycled several times from 25 "C through the 150 "C temperature range may give different experimental results in experiments that are sensitive to polymorphic form because different polymorphs may be present after each cycle. Table 111 summarizes the conversion temperatures of the HMX polymorphs reported previou~lyl-~ and those obtained in slow and rapid heating in this work. The phase transition work of Teetsov and McCronelB and Cady2 employed a solid-liquid interface involving solid HMX and an organic solvent. The solvent served as a catalyst in an effort to achieve chemical equilibrium during a phase transition. In our experiments we attempted to use gas heat transfer for slow heating of HMX. The discrepancy in the transition temperatures between our work and those reported before can probably be traced to nonequilibrium conditions that may exist in our system in spite of the slow heating rates and the maintenance of isothermal conditions.ls Several observations are nevertheless noteworthy. The phase transitions and the temperatures we obtained are fully reproducible. Single crystals and powder samples of P-HMX were tested at the /3 6 transition and pro-
-
6
re€ 1
ref 2
slow heating
103.8-115 158 173 165.5 183 125
102-104.5 149-151
142
175 175
160-164
147
185
205
170
rapid heat-
ing
duced the same transition temperature. However, the fact that the transition temperatures are different at the two heating rates indicates that kinetic factors are mixed with thermodynamic effects in the phase transitions we observe. An additional factor which may modify the solid state phase transitions of HMX is decomposition during heating. Decomposition leads to impurities and strain in the crystal lattice. We have been unsuccessful a t detecting decomposition products in the HMX crystal a t elevated temperatures, but foreign materials such as decomposition products can alter the temperature of phase transitions. The present results show that Raman spectroscopy can be used to clearly detect phase transitions in HMX. The gas-solid heat transfer method probably does not yield accurate thermodynamic transition temperatures because of the difficulty in achieving solid-phase equilibrium conditions (at least in HMX).
Acknowledgment. We are grateful to the Air Force Office of Scientific Research for support of this work through AFOSR-76-3055. References and Notes A. S.Teetsov and W. C. McCrone, Microsc. Cryst. Front, 15, 13 (1965). H. H. Cady and L. C. Smith, LAMS-2652, Los Alamos Scientific Lab., May 3, 1962. W. C. McCrone, Anal. Chem., 22, 1225 (1950). P. F. Eiland and R. Pepinsky, Z.Kristallogr., 106, 273 (1955). H. H. Cady, A. C. Larson, and D. T. Cromer, Acta Crystallogr., 16, 617 (1963). C. S.Choi and H. Boutin, Acta Crystallogr., Sect. 5, 26, 1235 (1970). R. E. Cobbledick and R. W. H. Small, Acfa Crysfa//ogr., Sect. 5, 30, 1918 (1974). T. B. Brill and F. Goetz, J. Chem. Phys., 65, 1217 (1976); T. B. Brill and F. Goetz, AIAA Progress in Astronautics and Aeronautics Volume on Measurementsin Combustion Research, 14th Aerospace Sciences Meeting, Washington, D.C., 1976 (to be published). A. ThBor6t and C. Sandorfy, Can. J . Chem., 42, 57 (1964). B. B. Goshgarian, Air Force Rocket Propulsion Laboratory, personal communication, 1977. W. Selig, Explosivstoffe, 9, 201 (1969). 6-HMX prepared by the method of Selig (ref 11) by refluxing HMX in N,N-dimethyl-p-toluidine gave a Raman spectrum different from the 6-HMX prepared by heating P-HMX. In the 300-550-cm-' region, for example, bands at 400, 425, and 486 cm-' appear. The entire spectrum was not of very high quality and showed marked fluorescence. 8-HMX prepared in this way gives a nonstoichiometrlc solvate of HMX and N,N,dimethyCp-toludine. (H. H. Cady, personal communication.) R. Cavagnat, M. T. Forei, and M. Rey-Lafon, C. R . Acad. Sci., Paris, 273, 658 (1971). Z. Iqbal, S. Bulusu, and J. R. Autera, J. Chem. Phys., 60, 221 (1974). F. Goetz, T. B. Brill, and J. R. Ferraro, J . Phys. Chem., 82, 1912 (1978). W. G. Fateiey, F. R. Dollish, N. T. McDevitt, and F. F. Bentley, "Infrared and Raman Selection Rules of Molecules and Lattice Vibrations: The Correlation Method", Wiley-Intersciance, New York, 1972. M. Bedard, H. Huber, J. L. Myers, and G. F. Wright, Can. J . Chem., 40, 2278 (1962). We thank a reviewer for pointing this out.
