Determination of CHN Content in Energetic Binder by MIR Analysis

Azide polymers samples were analyzed by FT-MIR in order to develop a quantitative methodology to determine the C, H and N content in energetic polymers used in propellants. The elemental analysis data were used as reference. The FT-MIR results show a good agreement with CHN analysis. A good linear relationship was obtained suggesting that the methodology developed in the CTA laboratory can be used for quality control of these energetic polymers.


Introduction
The elastomeric binder of a composite system like propellants plays an important role in dispersing and immobilizing the fuel material and oxidizer.The materials used in the binder normally burn with lower energy than does the fuel material itself.Therefore the binder imposes limits on the energy content that can be recovered from the fuel material.In order to minimize this limitation some energetic binders can be used since they can release much more energy when they are burned together with the fuel material [1] .The energetic binders are the ones that have in their structure chemicals groups with high energy content such as azide, nitrate, difluroamine, nitro and nitroamine, etc.
A formulation using this energetic binder system can result in a propellant with better performance [2] .They are the so called modern solid propellants [3] and are expected to give higher burning rate and specific impulse when used for rockets.
To establish the relation between structure and properties of a composite solid propellants is important to know the structure of the elastomeric binder that has been used in the system.Therefore this study [7] has concentrated in the characterization in a wide spectral band of infrared, NIR/ MIR/FIR, of all thermoplastic elastomers (TPE's) that has been synthesized at ICT, which will therefore, allows the identification of analytical bands for future quantitative and kinetic studies of these compounds.
A quantitative analytical method using MIR has been developed at CTA/IAE (Chemistry Division-analytical department) for CHN determination of the TPE's that has been synthesized at ICT (poly AMMO, poly BAMO and copolymers of poly AMMO/BAMO).The CHN elemental analysis has been used as reference.

Experimental
The TPE's samples used in this study, as mentioned before have been synthesized at ICT (AK 81 Poly BAMO; AK 97 Poly BAMO; AK 96 Poly AMMO; AK 101 Poly AMMO; AK 98 Poly AMMO/BAMO; AK 120 Poly AMMO/BAMO; AK 109 Poly AMMO/BAMO) following a method that has already been described in the literature [4] .
The MIR spectra have been acquired at CTA using FT-IR spectrometer SPECTRUM 2000 PERKIN ELMER, in the region of 4000 to 400 cm -1 , gain 1, resolution of 4 cm -1 and 40 scans.The samples were analyzed by the KBr pellet technique (0.8:400 mg).Due to the uncertainty inherent to this technique, each sample has been analysed ten times in order to reach higher accuracy in the results.
For the quantitative analysis of the compounds using MIR, the values of the stretching frequencies at 2100 cm -1 ( a N 3 ) have been related to the ones corresponding to C-O group at 1100 cm -1 .The baseline points at 2358 cm -1 and 1779 cm -1 were established for the calculations of the absorbance values of the N 3 groups at 2100 cm -1 and the baseline points at 1779 cm -1 and 841 cm -1 established for calculations of the absorbance values of C-O absorption at 1100 cm -1 .The values of the relative absorbance A 2100 /A 1100 represent an average value ( ) of 10 measurements of each sample.
For the calibration curve, the CHN values measured at ICT [8] for the compounds, have been used as reference.

Results and Discussion
The chemical structure of the compounds and their starting materials that have been used in this work are presented in Table 1.
The frequencies and assignments of AMMO; BAMO; PolyAMMO and PolyBAMO are compiled in Table 2 and the spectra shown in Figures 1 to 3.

MIR analysis of AMMO and BAMO
As it has been mentioned in previous paper [7] , the main difference between AMMO and BAMO chemical structures is the methyl group at AMMO and one additional azide group at BAMO, which can be visualized at the IR spectra (Figure 1).The assignments of the bands are in Table 3.The peak at 1382 cm -1 ( s CH 3 ) is attributed to CH 3 group and the peaks at 2100 cm -1 ( a N 3 ), 2500 cm -1 ( 2 s N 3 ), 1250 cm -1

Monomer
Polymer  2. Mid IR frequencies (4000-400 cm -1 ) of the energetic binder [3][4][5] .-stretching; a -asymmetrical stretching; s -symmetrical stretching; a -in plane asymmetric bending; s -in plane symmetric bending; -rocking vibration; and w -wagging.( s N 3 , or C-N) are attributed to azide group.It can also be observed that for BAMO the vibrational mode for N 3 broadens and/or increase in the intensity, which can be attributed to the presence of two azide groups in the compound.

MIR analysis of AMMO and PolyAMMO
The main change observed in IR spectroscopy due to the polymerization of AMMO is the opening of the oxetane ring with the formation of the C-O bond.In the spectra of the polymer one can notice the absence of the band related to the ring at 980 cm -1 (ring stretching) and the appearance of the new bands typical of C-O (1000-1100 cm -1 -C-O), and OH (3340 cm -1 -OH) attributed to the end groups of the polymer (Figure 2).
The broadening of the bands at the C-O group region (aliphatic ether) is due to the repetition of the units in the polymer (Figure 2).

MIR analysis of BAMO and PolyBAMO
The main changes observed in IR spectrum due to the polymerization of BAMO are basically the ones that have been observed for AMMO (ring opening and C-O formation) and shown in Figure 3.

Measurement of CHNO of the PolyAMMO and polyBAMO throught MIR technique
The relative absorbance A 2100 /A 1100 and the CHN content (measured at the analytical department of ICT) [8] for all sev-en samples of polyAMMO and polyBAMO, are presented in Table 3.The standard deviation , of the average value of the absorbance is calculated according to the equation [9] :
is represented by: K R is the coeficient for the calculation of the standard deviation in a range of values and R is the difference between the higher and lower value The relative error for each sample has been determined according to the following equation: Relative Error (%) = ( / ) x 100 (4) Figure 4 shows the analytical or calibration curve A 2100 / A 1100 vs. %N of the energetic binders.A good linear relationship was obtained (R = 0.990) and is represented by the following equation: Where: y is the average value of the absorbance A 2100 /A 1100 and x is the %N value.The curves of A 2100 /A 1100 vs. %N/C and A 2100 /A 1100 vs. %N/O have been also built to establish correlation with the content of C and O.
Figure 5 shows the calibration curve A 2100 /A 1100 vs. %N/C for the energetic binders.A good linear relation was obtained (R = 0.993) and is represented by the following equation: Figure 6 shows A 2100 /A 1100 vs. %N/O.A good linear relation was also obtained (R = 0.954).The following equation represents the curve: The average relative error inherent to the MIR method is equal to 1.48%, which is inside the precision of IR methods in general ( 2%), mentioned in the Literature [9] .
The 2,4-Toluene diisocyanate (TDI) was used to link the block PolyBAMO to block PolyAMMO in the copolymer synthesis Poly-AMMO/BAMO.Their structure is shown in Table 1.The contribution of TDI to the nitrogen content of the copolymer was also investigated.Therefore, it is lower than 1.0 wt.(%) weight of total nitrogen in the copolymer, then it does not play any important role in this method.

Measurement of CHNO for sample AK 113 using MIR methodology
Sample AK 113 (Poly AMMO) was used as test sample for the method as it had an unknown CHNO content.It was used to verify the accuracy of the method.
The A 2100 /A 1100 measured value was 2.564 ( = 0.015 and relative deviation = 0.59%).The %C, %N and %O in the sample were measured using the three (Equations 5, 6 and 7) calibration curves.From the equations the following values were determined: %N = 29.08,%C = 46.60%O = 17.84 and %H = 6.48.These values are in a good agreement with the CHNO obtained from other similar samples of PolyAMMO as can be seen in Table 3.
Considering that CHN analysis has the same precision of IR, that IR is a technique available in the majority of analytical laboratories and the accuracy of the results obtained, the methodology developed can be a simple and quick alternative for the quality control of energetic binders.

Conclusion
The use of IR spectroscopy in the MIR region for characterization and quantification of the amount of CHN in energetic binders, showed to be a good alternative to elemental analysis.The new method is simple, quick and precise enough to be used in the quality control of those compounds in industry.

Table 1 .
Monomers and polymers studied in this work.

Table 3 .
MIR data of the energetic binders with different CHN contents.