Thermal decomposition of energetic materials 71: Structure-decomposition and kinetic relationships in flash pyrolysis of Glycidyl Azide Polymer (GAP)

Abstract

Well-characterized, purified samples of glycidyl azide polymer ( MW ― ≃ 700 ) having one, two, or three terminal -OH groups were flash pyrolyzed (dT/dt = 800 K/s) to 540–600 K under 2 atm Ar by T-jump/FTIR spectroscopy. This technique emphasizes condensed-phase pyrolysis chemistry as opposed to gas-phase flame chemistry. The volatile products identified from the condensed phase were CH4, HCN, CO, C2H4, NH3, CH2O, CH2CO, H2O, and GAP oligomers. IR-inactive N2 is, of course, also present. The low MW products result from homolysis of the heavy atom bonds and H-atom migration, as opposed to heavy atom recombination reactions. After N2 release, the relations between the mole fractions of the products and the parent GAP structure were determined. The NH3 content increases with the -OH content, which suggests that NH3 is mostly formed by the end-chain azide groups. The hydrocarbon mole fractions correlate with the structure of the parent sample of GAP. CO appears to form from both the parent polymer and secondary sources, such as CH2O and CH2CO, at higher temperature. The HCN/NH3 ratio increases with increasing temperature. By using the product mole fractions and heat of formation of GAP, the calculated heat of decomposition (⋍−1.4 kcal/g) is found to be three times larger than that measured by DSC (⋍−0.43 kcal/g). The large ΔHd helps explain the reported high surface temperature. The relatively low reported flame temperature during combustion reflects the limited number of secondary exothermic reactions that are possible among the products. If it occurs during combustion, the apparent release of some GAP oligomers observed during flash pyrolysis would raise the flame temperature by allowing the decomposition of some of the GAP in the gas phase. The kinetics of formation of NH3 [Ea = 49 kcal/mol, In(A/s) = 42] were determined below the autothermal stage. Above the autothermal stage, the products form at too rapid a rate to be measured by the methods used.