Study of High Explosives by Optical Thermal Analysis Billy D. Faubion Mason & Hunger-Silus Mason Co., Inc., Puntex Plant, P.O. Box 647, Amarillo, Texas 79105 An automatic thermomicroscopy method was used to study the polymorphic transitions of several high explosives. The method was first tested using three compounds whose polymorphic states had previously been characterized-ammonium nitrate, 2,4,6-trinitroresorcinol, and pentaerythritol tetranitrate. The temperatures for the polymorphic transitions of dipentaerythritolhexanitrate (diPEHN), tripentaerythritol-octanitrate (triPEON), and tetrapentaerythritoldecanitrate (tetraPEDN) were then measured. DiPEHN had two polymorphic forms with melting points Four polymorphic forms of 75 O C (I) and 72.5 O C (11). were found for triPEON with melting points of 83 O C (I), 76-77 O C (II), 68-70 O C (Ill), and 64-65 O C (IV). Only two polymorphs of tetraPEDN were found with melting points of 90 O C (I) and 80 O C (11).
THESTUDY OF HIGH explosives by microscopy fusion methods is well established (1-3). This area of thermal analysis is more specifically called thermal microscopy or thermomicroscopy. The latter designation will be used in this report. The complete area of thermomicroscopy includes all observations made during the heating of a compound, on the melt itself, during solidification of the melt, and as the sample cools. The observed variables may be color, shape, optical rotation, or index of refraction of the sample. An extensive treatment of fusion methods can be found in McCrone’s book ( 4 ) . The usual procedure followed in thermomicroscopy involves the visual observation of changes in the sample. Photographs may be used for a permanent record. Because the eye is not particularly sensitive to subtle changes taking place slowly over a period of several minutes, several workers have favored automatic methods of detecting changes (5-8). These methods utilize a photodetector to measure changes in intensity of birefringence of the sample. Because only one observed variable is used, the number of phenomena which can be detected is decreased. The automatic methods are primarily used in the accurate determination of melting points, the study of polymorphic transitions: and the measurement of crystallization rates. In this work, the automatic microscope method has been used to study the polymorphic transitions of several high explosives. EXPERIMENTAL
Apparatus. The experimental arrangement used in this work is similar to that described by Kolb (5). A schematic diagram of the instrument assembly is shown in Figure 1. ( I ) W. C. McCrone, Microchem. J . , 3, 479 (1959). (2) A. A. Teetsov and W. C. McCrone, Microsc. Cryst. Front, 15, 13 (1965). ( 3 ) A. T. Blomquist, “Microscopic Examination of High Explosives
and Booster,” OSRD 3014 (1944). (4) W. C. McCrone, “Fusion Methods in Chemical Microscopy,” Interscience Publishers, Inc., New York, N. Y., 1957. (5) A. F. Kolb, C. L. Lee, and R. M. Trail, ANAL. CHEM., 39, 1206
(1967). (6) C. W. Hock and J. F. Arbogast, ibid., 33,462 (1961). (7) H. P. Vaughn, Microscope, 17, 71 (1969). (8) D. R . Reese, P. N. Nordberg, S. P. Ericksen, and J. V. Swintosky, J . Pliurm. Sci., 50, 177 (1961).
~ O TSTAGE
Figure 1. Schematic diagram of instrument assembly The components consist of a Polarstar polarizing microscope (American Optical Company) with the trinocular body. A Polaroid camera was attached to the microscope to enable photomicrographs to be made. The microscope was mounted on an Ortho-Illuminator Model 600 (American Optical Company). A Photovolt Corporation Photometer, Model 501A, was used to measure the light intensity. A sleeve was machined to adapt the photomultiplier housing for attachment in place of one of the eyepieces. The photometer was modified to allow convenient connections to an X-Y recorder (Moseley Autograph Model 2D-2). A Kofler hot stage was mounted in place of the standard microscope stage, A Chromel-Alumel thermocouple was inserted in the thermometer well of the hot stage. The output of the thermocouple was fed directly into the X-Y recorder. The natural heating rate of the hot stage was used. Plots of temperature us. time were made for several settings of the hot stage Variac. The heating rate ranged from about 20 “C per minute initially to about 3 O C per minute near the maximum temperature. The Variac setting was chosen to give a maximum temperature slightly above the melting point of the sample. The temperature of the hot stage was calibrated by recording the melting points of several standard components. A calibration curve was generated using the least-squares method. The standard deviation between the calibration curve and the experimental melting points was h0.75 “C. Preparation of Microscope Slides. Two procedures were used in the preparation of the microscope slides. All the samples which did not decompose on melting were melted between the cover slip and the microscope slide. Samples which decompose on melting were mounted on the microscope slide by crystallization from solution. Run Procedure. A set procedure was used for each run. The apparatus was turned on and allowed to warm up for 30 minutes. The Xaxis of the recorder was set to correspond to the temperature of the hot stage and the sensitivity chosen to cover the desired temperature range (0.5 mVjinch or 1 mV/inch). The Y axis of the recorder was adjusted to zero
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I 203
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I
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Figure 2. Heating curve for AN
Figure 4. Heating curve for TNR Table I. Polymorphs of Ammonium Nitrate Stability range Polymorphs Crystal form temp., "C. I Cubic 125 to 175 I1 Tetragonal 84 to 125 32 to 84 I11 Monoclinic or orthorhombic IV Orthorhombic -16 to 32 V Tetragonal below -16
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Figure 3. Cooling curve for AN with the light beam blocked and the sensitivity chosen to keep the pen on scale. Crossed polars were used; however, the Polaroids allowed some light to be transmitted. A small amount of unpolarized light is desirable, as it enables isotropic phases to be seen. Draftz (9) has recommended that the polars be slightly uncrossed for this reason. The hot stage was then turned on and the Variac adjusted to give the desired maximum temperature. A plot of light intensity us. temperature was then obtained. After the sample was completely melted, the recorder paper was changed and a cooling curve for the sample obtained. The natural cooling rate of the hot stage was used. RESULTS AND DISCUSSION Since the behavior of each sample was unique, the results and discussion for each sample will be presented in separate sub-sections. Ammonium Nitrate. The crystal properties of ammonium nitrate (AN) have been determined by microscopic techniques (3). For this reason A N was chosen as an ideal compound for checking out the experimental procedure. AN has five different crystal forms. Table I summarizes the informa(9) R. G. Draftz, "The Particle Analyst (compiled)," Ann Arbor Science Publishers, Inc., Ann Arbor, Mich., 1968. 242
tion on these polymorphs. Only those polymorphic transitions occurring above room temperature were observed, as the hot stage was not equipped for cooling. The heating curve for AN (reagent grade) is shown in Figure 2. The four transitions between 25 and 200 "C are clearly visible. The transition of IV to I11 at 42 "C is observed as a n increase in intensity. The transition of 111 to I1 occurs at 78 "C and is accompanied by a drop in intensity. A sharp drop in intensity indicates the transition of I1 to I at 124 "C. Form I melts at 169 "C. Since both AN I and the melt are isotropic, a change in intensity was not expected. The small amount of light transmitted by the crossed polars is probably scattered more by the solid than by the melt. This would account for the increase in intensity a t the melting point. Figure 3 is the curve obtained as the melt is allowed to cool at the natural cooling rate of the hot stage. The melt crystallizes as AN I at 175 "C. The discrepancy between the melting temperature (169 "C) and the crystallization temperature (175 "C) is probably due to the differences in heating and cooling rates. For a Variac setting of 200 "C (maximum temperature) the heating rate a t 170 "C is 4 "C per minute. The cooling rate at 175 "C after the hot stage has been heated to 200 "C is 15 "C per minute. The transition from I to I1 occurs at 124 "C. Form I1 does not transform to 111 on cooling, but goes directly to IV (3). The transtition of I1 to IV is indicated as a sharp drop in intensity a t 44 "C. 2,4,6-Trinitroresorcinol. The polymorphic states of 2,4,6trinitroresorcinol (TNR) have been well characterized (1, 3). It was, therefore, chosen as another compound for checking out the experimental procedure. Only two polymorphs of T N R have been detected. The low temperature hexagonal form (11) is stable below 111 "C and the monoclinic form (I) is stable from 111 to 177 O C where it melts. Figure 4 shows the plot obtained for the
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\
7
.c
Figure 7. Transformation of PETN I1 to PETN I
Figure 5. Cooling curve for TNR
a.
IO Se:ondr
--I ,
25
10
75
T
100
,*5
,...
I 150
125 Seconds
Y
Figure 6. Heating curve for PETN
heating of TNR. The exact temperature at which T N R I1 begins to transform to TNR I is difficult to determine from this plot. A gradual increase in intensity from 25 to 130 "C indicates some change in the sample. This could be due to a change in the birefringence of the sample caused by thermal expansion. The increase in intensity beginning at 130 "C shows the transformation of II to I. Visual observation ofthe sample indicates that the transformation is incomplete. The unchanged TNR II melts at 170 "C, accompanied by a sharp decrease in intensity. The melt then instantaneously solidifies to TNR I. The solid TNR I then melts at 177 "C. On cooling, the T N R will solidify as either I or 11, depending on the degree of super-cooling which occurs. Figure 5 shows one of the curves obtained for the cooling of TNR. The melt super-cools to 110 "C where it crystallizes as TNR 11. The crystallization is accompanied by a drop in intensity. This drop in intensity is probably due to increased scattering ofthe light by the solid phase. Pentaerythritol Tetranitrate. Pentaerythritol tetranitrate (PETN) has two polymorphic forms ( I , 3). PETN I is stable below the melting point which is reported as 141.5 OC. The second polymorph (11) crystallizes from the melt, but is unstable and immediately transforms to PETN I. Figure 6 shows a typical curve for the melting of PETN. A sharp change in intensity was not obtained at the melting
C.
600 Seconds
Figure 8. Crystals of PETN after crystallization Magnification: -125X
point; consequently, the melting point ranged from 130 to 141 "C. This is lower than the reported melting point, which could indicate an impure sample. This does not seem likely since the sample of PETN used had been carefully purified by recrystallization. It is possible that some decomposition occurred during preparation of the microscope slide, thus lowering the melting point.
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I
L . - - ~ J 100
li T
7 ~C
Figure 9. Heating curve for diPEHN I1
100
./
Figure 11. Heating curve for diPEHN 1 (diPEHN). The melting point has been reported as 75 "C (IO). Two polymorphs were found in this investigation. These results are in agreement with results obtained by Cady (11).
C.
7U'C
d.
74-C
Figure 10. Crystals of diPEHN Magnification: -125X
On cooling, the melt super-cools to about 90 "C before rystallizing as PETN 11. The transformation of 11 to I is hown in Figure 7. In this curve, the intensity of light DS. ime is plotted. The transformation occurs with no apparent hange except for a decrease in birefringence. The transfornation is further illustrated in Figure 8. The photographs in 'igure 8 show the crystals of PETN (a) 10 seconds, (b) 125 econds, and (c) 600 seconds after crystallization. The three Bhotographs were taken with the same camera setting. Dipentaerythritol Hexanitrate. There is little reported iork on the polymorphs of dipentaerythritol hexanitrate 144
A sample of diPEHN prepared at Pantex and purified by column chromatography was used in this investigation. In the preparation of the slides, it was noted that diPEHN does not readily crystallize. In order to stimulate crystallization, the melt was seeded with crystals of the original sample. It was found that two polymorphs could be produced in essentially pure form, depending on the temperature at which the melt was seeded. When the melt was seeded after it had cooled to room temperature, fine grain crystals were formed. These have now been designated diPEHN 11. Figure 9 shows the curve obtained as these crystals are heated. At about 53 "C, a new crystal form appears (diPEHN I). The intensity begins to drop as the new crystals grow. At 72.5 "C the remaining crystals of diPEHN I1 melt, indicated by a sharp drop in intensity. The diPEHN I crystals melt at 75 "C. The transformation is also illustrated in Figure IO with photographs. If the melt is seeded at 70 "C it is possible to produce pure diPEHN I. Figure 11 shows the curve obtained for the heating of diPEHN I. There is essentially no change in intensity until the rapid increase followed by the sharp drop at 75 "C indicating the melting. At the peak intensity, the sample is a bright yellow white color. Similar results have been observed for other compounds (5, 7,s). Reese (8) suggests the following reasons: disappearance of crystal dislocations prior to liquefying, formation of a new crystal form, .or premelting which might reduce light scattering. The phenomenon could also be due to the formation of an anisotropic liquid (liquid crystalline) state. DiPEHN crystallizes without seeding if the slide is allowed to sit for several days. When treated in this manner, a mixture of the two polymorphs could be obtained. There appeared to be no transformation of diPEHN I to I1 at room temperature. Tripentaerythritol-Octanitrate. The melting point of tripentaerythritol-octanitrate (triPEON) has been reported as 82.5 "C (IO). No work on the polymorphs of triPEON has been reported in the literature. Unpublished results by Cady (IO) T. Urbanski, "Chemistry and Technology of Explosives," Vol. 11, Pergamon Press, New York, N. Y., 1965. (11) H. H. Cady, Unpublished Report, Los Alamos Scientific Lab., Los Alamos, N. M.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 2, FEBRUARY 1971
I* r
a.
I 75
T
b.
t r i P E O N I V [red)
2
75
IO
t r i P E O N I1
100
.r
Figure 12. Heating curve for triPEON I1
Table 11. Melting Points of Polymorphs of TriPEON This Work Cady's Work Polymorph T,"C T, "C I 83 83.3 I1 16-17 14.6 I11 68-70 72.1 IV 64-65 69.0
indicate that there are four or possibly five polymorphs ( I I ) . Cady gives melting points of 83.3, 74.6, 72.1, and 69 "C for four of the polymorphs. The results of this investigation were difficult to interpret because of the complexity of the system. At least four polymorphs were found, but the melting points differed slightly from those found by Cddy. Table I1 summarizes the results on triPEON. The polymorphs have been labeled according to their melting points, starting with the highest melting form. The triPEON used in this study was prepared at Pantex and was purified by recrystallization. TriPEON behaves similarly to diPEHN in that it does not readily crystallize from the melt. Crystallization would occur if the melt were allowed to remain overnight at room temperature. Crystallization was induced by seeding the melt at room temperature and at elevated temperature. A variety of he$!?iiig curves were obtained for triPEON. The appearance of the curves depended on the number and types of polymorphs present. Figure 12 shows the heating curve for triPEON 11, which appears light gray when viewed between crossed polars. Figure 13a is a photograph of the crystals at 27 "C. The intensity remains essentially constant up to 73 "C where it begins to drop. The melting point was taken as the mid point of the intensity drop. The slight increase in intensity at 80 OC is due to the formation of low birefringent triPEON I which immediately melts. The heating curve for triPEON IV depends on the initial color of crystals. The variations in the curves are probably due to differences in the sensitivity of the photomultiplier to different wavelengths of visible light. TriPEON IV appears to be spherulites ranging in color from red to green when viewed between crossed polars. Figure 136 shows the red crystals and Figure 13c shows the green crystals. The differences in color are possibly due to variations in sample thickness. The heating curve for the red crystals of triPEON IV is shown in Figure 14. The increase in intensity beginning
Figure 13. Polymorphs of triPEON Magnification: -125X
' i 30
T
100
.
