J. Phys. Chem. 1995, 99, 6010-6018
6010
Laser Initiation of the Decomposition of Energetic Polymers: Shock Wave Formation Yeshayahu Ben-Eliahu and Yehuda Haas* Department of Physical Chemistry and the Farkas Center for Light Induced Processes, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
Shmuel Welner A m m e n t Development Authority, POB 2250 Haifa, Israel Received: November 17, 1994; In Final Form: January 30, 1995@
Irradiation of the energetic polymer GAP (glycidyl azido polymer) by a high power pulsed IR laser leads to its rapid decomposition. A large amount of solid and gaseous material is released, and, in the presence of an inert gas, a shock wave develops. Comparison with an inert polymer indicates that the energy released by the exothermicity of the decomposition reaction contributes significantly to the shock’s formative energy. The energy released in the micro-explosion can be estimated from the analysis of the shock’s front propagation velocity.
I. Introduction Laser ablation of polymers’ is a well-established method for accurate etching of polymers and fabrication of precision parts. In this process solid material is rapidly converted to gaseous products upon absorption of laser radiation; since the solidgas transition is endoergic, the radiant energy is the sole source of energy in this process. In the last few years, a new class of polymers has been developed in which chemically reactive groups have been incorporated.2 These so-called active polymers contain, for instance, azido (-N3) groups and release large amounts of heat upon decomposition. They may be used as propellants or as a substitute to fillers or binders in composite propellant mixtures, increasing the specific thrust and the burning rate.3 Furthermore, they are considered as cleaner, environmental friendly replacements for older, perchlorate based
The average molecular weight of the liquid polymer is 1900, but it is often used after cross-linking and curing, as a rubbery solid. Its thermal decomposition is exothermic, releasing 3100 J/g.5 The burning rate is relatively high (10.7 “/s at 293 K, as compared to 6.9 “/s for a nitrocellulosehitroglycerine based pr~pellant).~The decomposition mechanism of this compound is generally b e l i e ~ e d ~to- ~begin by the scission of the N-N2 bond of the azide group, to form molecular nitrogen and a nitrene radical, 11 (eq 1). This is supported by the measured value of the activation energy, - 174 kJ/mol, that is typical for azide group dissociation, and by the fact that N2 is the main gaseous product. In the next step the nitrene isomerizes to an imine, III (eq 2), which in turn dissociates by two main routes shown in eq 3. In both routes the main products
propellant^.^ Characterization of the burning process of these new polymers and estimation of their relative efficiency as propellants are not easy tasks, since the burning process is complex. In particular, the rate of conversion of the solid material to gaseous products and the specific thrust that it generates depend to a large extent on the experimental conditions such as packing and ignition mechanism and are not readily measurable in a small, laboratory scale apparatus. In calorimetric experiments? one can determine the overall heat of decomposition of the reaction-a thermodynamic property of the material. However, the thrust depends also on the efficiency of in situ heating of the gaseous products, before they escape from the reaction zone, and therefore on the kinetics of the reaction. A method of rapid heating on a small scale is required, and, as we show in this paper, it can be realized by pulsed laser irradiation, in a setup similar to that used in photo-ablation experiments. One of the most commonly used active polymers is GAP (glycidyl azido polymer, I), whose properties were more CHz-N-N=N H+LH--CHz--O+
I
extensively studied than those of any other similar compounds. @
Abstract published in Advance ACS Abstracts, April 1, 1995.
are Nz, Hz, CO, ethylene, and acetylene. The third nitrogen atom in the azide group ends up primarily in ammonia and HNCO at relatively low temperatures (eq 3a) and as HCN when the temperature exceeds 700 K (eq 3b). At still higher temperatures (> 1000 K), mostly N2 is formed. We have recently used a pulsed COz laser to induce the decomposition of GAP: a polymer containing azido groups, and found that the distribution of the end products was similar to, but not identical with, that observed by thermal dissociation. By varying the laser fluence (energy incident on the sample per unit area), it was possible to control the decomposition rate continuously from no observable reaction to complete dissociation of the entire sample in a few shots. In the course of these
0022-365419512099-6010$09.00/0 0 1995 American Chemical Society
Shock Wave Formation in Energetic Polymers experiments it was found that, under intermediate fluence levels, dissociation led to the formation of both gaseous and solid products, the latter appearing as a fine powder or "smoke". In an attempt to learn more about the products' formation mechanism, we have used time resolved light scattering to follow the formation kinetics of the powder. The method has been extensively used in the study of laser ablation of inert polymer^,'^^-'^ whose decomposition does not involve a net energy release. When carried out in the presence of external pressure, ablation can induce a shock wave, allowing the measurement of system properties such as the initial speed of the gaseous products. Such studies have been carried out for polymer~,'.~-'~ dielectrics,14 and metals.15 Lasers have been used to initiate the detonation of explosives,16but we are not aware of the application of the method to the study of energetic polymers. The results of this study show that, under appropriate conditions (fluence levels, foreign gas pressure), the ablation of GAP leads to shock wave formation. Analysis of the shock's propagation characteristics allows the estimation of the energy released in its laser initiated decomposition.
J. Phys. Chem., Vol. 99, No. 16, 1995 6011
PUMP
Figure 1. A scheme of the experimental setup. The sample S is located in an evacuated chamber and can be rotated by the motor (MOT) which is supported by the movable platform (h4P) that is mounted on a vertical beam is mildly focused off translation stage. The initiation laser (E) center on the sample. The monitor laser (ML) beam traverses the cell at a vertical displacement h from the sample's surface (see inset) on its way to a photodiode narrowband optical filter: P = pulse generator; COMP = personal computer. 1.05
11. Experimental Section
Cross-linked solid samples of GAP were prepared as described before.6 Briefly, uncured GAP prepolymer was prepared by azidation of polyepichlorohydrindiol with sodium azide. The resulting prepolymer was cross linked using a mixture of isophorone isocyanate and the polyfunctional isocyanate (Hills Chemicals). A 1 mm thick disk of the rubbery material was placed on a rotatable holder inside a vacuum chamber. The chamber was evacuated to about 0.1 mbar and then filled with an inert gas to the desired pressure. Dissociation was initiated by a home-made TEA C02 lase+ whose pulse consisted of a 100 ns leading part (80% of the energy), followed by a weaker "tail" that lasts -1 ps. The mildly focused initiating laser beam was admitted into the cell through a NaCl window, fitted on the top face of the cell. A He-Ne laser beam (the monitoring beam) was passed at a distance h above the upper surface of the sample, so that the two laser beams crossed at right angles. A similar arrangement has been used for the study of laser ablation by several worker^.^-^^'^ The sample was slowly rotated at a rate ensuring that a fresh surface was exposed to the initiation laser at each shot. The monitoring beam was passed horizontally at a constant height with respect to the sample's upper face, and its vertical displacement from the sample was varied by lowering the sample holder. A schematic representation of the experimental setup is shown in Figure 1. The He-Ne laser beam intensity was measured by a fast photodiode, whose output was recorded by a digital oscilloscope. In control experiments, we have replaced the GAP slab by one made of an inert polymer, in which each azido group is substituted by a chlorine atom. 111. Results
Figure 2 shows a typical output signal of the diode as a function of time, following the irradiation of the sample under vacuum by the COz laser. A very rapid rise of an attenuation signal, followed by an equally rapid decrease, is observed. Irradiation of an inert polymer leads to very different characteristics, as shown in the lower panel of the figure. In this case both rise and decay times are significantly slower; at a distance of 1 mm from the polymer's surface, the attenuation reaches a maximum amplitude after about 28 ps. Under vacuum, the attenuation signal decreases rapidly upon increasing the obser-
.-8
I
I
0.95
GAP Vacuum h - lmm
0.80
0.75 0.70
~
53 mJ '""'"""''"''
0
1
2
"
~
'
3 4 5 Time (CIS)
~
6
~
~
7
"
'
8
~
~
~
I
1.05 1 .oo
0.95 0.90 0.85 0.80
Inert h-lmm Vacuum
0.75
53 mJ 0
10
20
30
40
50
60
70
80
Time (p) Figure 2. A typical recording of the transmission of the He-Ne laser under vacuum ( h = 1 mm; initiating laser pulse energy, 53 d): upper panel, GAP; lower panel, inert polymer. vation distance for both the active and inert polymers, becoming indistinguishable from the noise at a distance of 2 mm. When some inert gas such as N2 is added to the cell, a rapidly rising, narrow attenuation peak was seen to be superimposed on the broad attenuation signal. This signal is seen to decrease upon increasing h, but remains fairly narrow (-500 ns). Preliminary experiments showed that the intensity of these narrow attenuation peaks depended strongly on the gas used to fill the cell. When nitrogen was used, the signal obtained was typically 4 to 5 times larger than with helium under the same experimental conditions. Occasionally, but mostly with N2 pressures exceeding 1 atm and under high fluence levels, strong visible light flashes were observed. When argon was used, this happened even at much lower pressures, precluding the possibility of obtaining quantitative He-Ne laser deflection data. These phenomena are due to plasma formation, the study of which was not taken up at this stage. Therefore, further data were taken with nitrogen as the bath gas. Figure 3 shows typical
'
~
"
"
~
'
6012 J. Phys. Chem., Vol. 99, No. 16, 1995
Ben-Eliahu et al. GAP, Eusm-16 ml
h d . 5 mm
1
1 0.90
0.80 -
0.70
-
0.60
-
0.50
!
\ i V
-
'V
Inert " 1
1
I
'
' 1
'
I '
'
-
f
t
1 . 1
700 tm
500 m4
700 torr h = 4mm 1
1
1
'
' ' ' 1 ' '
' ' 1
',
1
'
c
I
-
"
200 torr
,i .-
5
GAP, 300 tom Nz,ELAsER -21 d
0.4
boo200
1
100 toll
I
2
3
4
5
6
7
8
9
Time (p.)
Figure 5. The effect of inert gas (N2) pressure on the attenuation signal for GAP irradiation: laser pulse energy, 16 d;h = 3.5 mm.
t
v
3 4.0 mm
.pE 0.91 . 1I .s a08
B
typical results selected from the 20 repetitions carried out for each h value and are close to the average; the standard deviation was about 5%. As explained in the next section, the narow, sharp attenuation peak is assumed to be due to scattering of the monitoring beam by a shock wave created in the cell. It was noted that the attenuation amplitude does not attain its largest value close to the polymer's surface; it was found to increases initially as h (the vertical distance of the monitoring laser from the sample's surface) increases, and reached its maximum value at about h = 2-4 mm, depending on the experimental conditions. Thereafter, the amplitude was found to decrease monotonically. Figure 5 shows an example of the effect of changing the nitrogen gas pressure while keeping other experimental parameters constant. T,, defined as the time required to reach maximum attenuation, is seen to increase as the pressure of the foreign gas increases. Furthermore, it is seen that, as the pressure increases, the attenuation signal becomes more pronounced. If a shock is indeed formed, the geometry of the system requires it to have a demispherical form-expanding radially from the point on the polymer's surface irradiated by the initiating laser. Thus, T, should depend only on the radial distance and not on the direction. This prediction was tested and found to be bome out by experiment. Most data were taken with a laser pulse energy of less than 30 rnJ (leading to an estimated fluence at the sample's surface of about 7.5 J/cmZ), since a larger energy content led easily to saturation of the signal and/or plasma formation. The weight loss per shot was found to be 0.005 mglpulse with a 16 mJ pulse, and did not vary with the pressure of nitrogen. Several tests were run in order to verify that the signal was obtained only when the energetic polymer absorbed the laser energy. Irradiation of the inert samples (whose IR absorption cross section at the laser frequency was very similar) under the
H 0:7 -
3.5 mm
examples of the signal observed in the presence of nitrogen, for the active and the inert polymers; it is clear that irradiation of the active polymer results in a much more intense attenuation signal than irradiation of the inert one. Figure 4 shows the distance-time characteristics of this signal for a given laser pulse energy and a constant N2 pressure. The data shown are
Shock Wave Formation in Energetic Polymers same conditions was found to lead to a negligible signal, as compared to that obtained with GAP (cf. Figure 3). Earlier experiments showed that the dissociation yield upon irradiation in the 10.6 pm region was much smaller than when 9.3 p m radiation was used (see Figure 2 of ref 6). In line with this observation, irradiation at 10.6 pm at a given fluence level was found to lead to a negligibly small signal by comparison to 9.3 ,um irradiation.
IV. Analysis
a. Preliminary Considerations. The laser radiation is totally absorbed in our experiments by a thin layer of the material. In a typical experiment, 5 x g of GAP polymer is consumed per laser pulse, when the laser pulse energy reaching the sample was 16 mJ. A crude estimate of the maximum pressure rise that results under these conditions may be made by assuming that the solid material transforms instantaneously to gaseous products. Each monomer unit (formula weight = 99 g) produces on average one gaseous nitrogen molecules,6 and the yield of nitrogen gas is about onethird of the total gas produced? Recalling that ethylene, ethane, acetylene, HCN, and hydrogen are major products, we assumed an average molecular weight of 28 g for the gaseous products, arriving at a total gas production of 1.5 x lo-’ mol per laser shot. Assuming further that the gas is formed in the volume occupied by the solid (specific density 1.3),3 we find that the pressure of nitrogen, if produced at ambient temperature, is about 970 atm. Taking into account the heat released by the reaction (3100 J/g),5 and assuming that it is used for instantaneous heating of the emanating gas, a temperature increase of AT = 3550 K is calculated, leading in turn to an initial pressure of about 1.25 x lo4 atm. These crude calculations indicate the very large pressure increase that can be produced by the laser initiated decomposition of GAP, for an essentially instantaneous gas formation. The emanating gases will immediately begin to expand, compressing the surrounding gas, and a strong spherical shock wave can be formed. A more detailed analysis (see below) shows that the pressure calculated from the shock’s propagation velocity is much smaller, and that in practice shock formation is not instantaneous. This finding is in line with the gradual buildup of the monitoring laser’s attenuation with h, mentioned in the previous section. A more realistic description is that a relatively slow ambient gas compression takes place for a few (up to 3) microseconds, a period during which the volume occupied by the emanating gas is much larger than that of the heated solid. Thus, the maximum pressure realized in practice is considerably smaller (by 2-3 orders of magnitude) than that calculated in the previous paragraph for instantaneous complete decomposition. Furthermore, the actual heat and pressure released will depend on many other factors, such as the heat dissipation into the sample, the production of other gases, shock wave formation in the solid, etc. Nonetheless, compared to other initiating methods, pulsed laser heating leads to a rapid and significant pressure rise. The situation can be approximately treated as a point explosion (ref 17, pp 93-106). If the surrounding gas pressure is significantly lower than the pressure jump created by the explosion, one expects the development of a shock wave. The shock, in turn, may develop into a blast wave that will propagate radially from the point of impact. Both the shock and the blast waves are characterized by an advancing front formed by the sudden change in temperature, pressure, and gas density. Since the refractive index n varies with density, the shock front’s location can be easily monitored by observing
J. Phys. Chem., Vol. 99, No. 16, 1995 6013
the sudden change of n. The sharp attenuation peak observed upon irradiating the samples in the presence of a foreign gas is thus attributed to light scattering at the shock’s front. We proceed (section 1V.c) with a more detailed analysis of the shock’s characteristics, which can be used to calculate the energy released by the laser induced decomposition of the polymer. For completeness sake, we briefly review in section 1V.b the theory of spherical shocks and introduce the equations and parameters that will be used later. b. Shock Wave Formation. The theory of strong shocks (blast waves) has been worked out in detai1,17-20and the conditions for a fully developed blast wave are well understood. The variables that characterize the gas flow are the particle velocity u, the speed of sound c, the density e, and the pressure p , all of which vary as a function of the coordinates and of the time. In the ideal case, one considers a perfect inviscid gas with density eo and pressure PO,in which a large amount of energy EOis liberated in a small volume in a short time. It is found that a spherical shock wave with a radius D propagates from the energy release point, the explosion site. The analysis is simplest for the case where D is large compared to the dimension of the region where the explosion took place, and at a time long compared with the explosion duration. The explosion’s energy is assumed to be large enough to neglect the initial speed of sound co with respect to the velocities of the gas set in motion and of the wavefront. In the absence of dissipative forces, the gas motion in this case will be self-similar (ref 19, pp 146-155); that is, the flow variables can be expressed as a function of a new variable ( which is a combination of the space and time variables. When expressed as a function of (, the distributions of the flow variables are “frozen”; that is, they do not change with time. In the case of a strong explosion, the dimensionless quantity E = D ( ~ d E ~ r can ~ ) serve l / ~ as the similarity ~ a r i a b l e . ’ ~Under J~ these conditions the shock radius D is given by (4) or in a logarithmic form log D = 0.2 log(Ed@,)
+ 0.4 log t + log to
(4a)
where (0 is a constant depending on y , the ratio of the heat capacity at constant pressure to the heat capacity at constant volume. 60 is of the order of unity; for instance, for y = 1.4 its value is 1.033 (ref 17, p 99). The velocity of the shock wave propagation, c, is given by
Under these conditions a plot of log D versus log t should yield a straight line with a slope of 0.4, and from the intercept one can obtain EO,the initial energy released. The derivation of these expressions is based on the “strong shock” a s s ~ m p t i o n s . ~ ~These ~ ’ ~ J require ~ that the mass of the gas encompassed by the shock wave’s radius (Le., 2/3z~&3) must be much larger than the mass of the initial explosion Mo, so that D >> (3M&z@0)”~.This condition implies that the blast will be measured at a large distance from the source causing it. Secondly, the pressure driving the shock’s front must always be much larger than the pressure ahead of it. This leads to the condition D
