Thermogravimetric analysis, kinetic study, and pyrolysis-GC/MS analysis of 1,1'-azobis-1,2,3-triazole and 4,4'-azobis-1,2,4-triazole.

Background: In general, the greater the number of directly linked nitrogen atoms in a molecule, the better its energetic performance, while the stability will be accordingly lower. But 1,1'-azobis-1,2,3-triazole (1) and 4,4'-azobis-1,2,4-triazole (2) show remarkable properties, such as high enthalpies of formation, high melting points, and relatively high stabilities. In order to rationalize this unexpected behavior of the two compounds, it is necessary to study their thermal decompositions and pyrolyses. Although a great deal of research has been focused on the synthesis and characterization of energetic materials with 1 and 2 as the backbone, a complete report on their fundamental thermodynamic parameters and thermal decomposition properties has not been published.

Methods: Thermogravimetric-differential scanning calorimetry were used to obtain the thermal decomposition data of the title compounds. Kissinger and Ozawa-Doyle methods, the two selected non-isothermal methods, are presented for analysis of the solid-state kinetic data. Pyrolysis-gas chromatography/mass spectrometry was used to study the pyrolysis process of the title compounds.

Results: The DSC curves show that the thermal decompositions of 1 and 2 are at different heating rates involved a single exothermic process. The TG curves provide insight into the total weight losses from the compounds associated with this process. At different pyrolysis temperatures, the compositions and types of the pyrolysis products differ greatly and the pyrolysis reaction at 500 °C is more thorough than 400 °C.

Conclusions: Apparent activation energies (E) and pre-exponential factors (lnA/s^-1) are 291.4 kJ mol^-1 and 75.53 for 1; 396.2 kJ mol^-1 and 80.98 for 2 (Kissinger). The values of E are 284.5 kJ mol^-1 for 1 and 386.1 kJ mol^-1 for 2 (Ozawa-Doyle). The critical temperature of thermal explosion (T _b ) is evaluated as 187.01 °C for 1 and 282.78 °C for 2. The title compounds were broken into small fragment ions under the pyrolysis conditions, which then might undergo a multitude of collisions and numerous other reactions, resulting in the formation of C₂N₂ (m/z 52), etc., before being analyzed by the GC/MS system.