PAH Induction upon Pyrolysis of Hydroxyl-Terminated Polybutadiene-Based Solid Rocket Fuels

Using a molecular beam (MB)-based approach with time-of-flight mass spectra (TOFMS) product detection, we have effected pyrolysis of hydroxyl-terminated polybutadiene/ammonium perchlorate (HTPB/AP) mixtures under conditions relevant to solid fuel rocket motor operation - pyrolysis temperatures >1000 °C with chemistry occurring on μs time scales. In earlier studies, it was implicitly assumed that AP did not significantly affect HTPB pyrolysis chemistry, that it was a featureless oxidant that also raised pyrolysis temperatures. This appears not to be the case. For HTPB pyrolysis in the presence of AP, a series of polycyclic aromatic hydrocarbon (PAH) products were detected up to m/z 240-260. No such PAHs were detected for pyrolysis of HTPB alone, with traces of them occurring when HTPB was cross-linked with methylene diphenyl diisocyanate. The presence of AP also resulted in the concomitant induction of cyclic C₆ products with m/z 76 and 78, benzyne and benzene, at the expense of acyclic C₆ product peaks at m/z 83-84 (presumably, an inhomogeneous distribution of hexenes and the corresponding hexenyl radicals) that were predominant in the absence of AP. The occurrence of PAH peaks was confirmed by evolved gas analysis with mass spectrometric detection (EGA-MS). The observation of similar m/z values, up to 240-260, when using short time scale MB and much longer time scale EGA-MS and pyrolysis-GC-MS (Pyr-GC-MS), indicates that the observed PAH growth occurs on a short time scale of tens of microseconds yielding the aromatic products with up to four and five rings. The use of Pyr-GC-MS enabled the identification of the exact PAH products and tentative pathways toward their formation.