Spectroscopic Study on the Polymer Condensates Formed via Pyrolysis of Levitated Droplets of Dicyanamide-Containing Ionic Liquids

To investigate the suitability of ionic-liquid-based hypergolic fuels as replacements for traditional hydrazine-based propellants, a 1-methyl-4-amino-1,2,4-triazolium dicyanamide ([MAT] [DCA]) droplet, with and without hydrogen-capped boron nanoparticles, was acoustically levitated in argon and heated to successively higher temperatures by a carbon dioxide laser. At each temperature, in situ Fourier-transform infrared and near-infrared, Raman, and UV-visible spectra were recorded. The droplet became increasingly yellow before exploding at 400 K to produce a brown foam-like substance and dense smoke. The foam was subsequently studied ex situ by X-ray photoelectron spectroscopy, infrared diffuse-reflectance spectroscopy, and elemental analysis. The combined spectroscopic analyses suggest that the foam is formed by linking two or more melamine molecules to yield a combination of melem, melon, and possibly graphitic carbon nitride. At least 37 +/- 3% of the [MAT] [DCA] liquid was transformed into the stable, solid-state foam, which would be problematic for the use of such an IL-based hypergolic fuel in rocket engines. 1-Butyl-3-methylimidazolium dicyanamide ([BMIM][DCA]) did not explode and form the foam even at temperatures of up to 430 K. Elimination of the amino group (-NH2) during the decomposition of [MAT](+) to volatile products or the increased energy density provided by the additional nitrogen atom in the triazolium ring therefore seem to be required to produce the foam. The present results, provided by an original and accurate experimental technique, elucidate how the nitrogen content affects the stability of an ionic liquid and reveal potential hazards when implementing ionic liquids in bipropellant systems.