Polybutadiene crosslinked by 1,3-dipolar cycloaddition: Pyrolysis mechanism, DFT studies and propellant burning rate characteristics

In a composite solid propellant, it is desirable to have energetic groups in a binder that contribute to the combustion characteristics. For this the conventional hydrocarbon binder hydroxyl terminated polybutadiene (HTPB) was chemically modified via a urethane route to derive azide and propargyl end capped polybutadiene namely azidoethoxy oxy carbonyl amine terminated polybutadiene (AzTPB) and propargyl oxy carbonyl amine terminated polybutadiene (PrTPB). The blend of the polymers underwent curing to form triazole-triazoline networks by 1,3-dipolar addition reaction. Thermal decomposition of the cured triazoline-triazole crosslinked network was investigated. The decomposition occurs in two stages. The first stage is attributed to urethane cleavage along with decomposition of triazoline-triazole. The second is due to polybutadiene backbone decomposition. Pyrolysis gas chromatography-mass spectrometry (GC-MS) studies at 300 degrees C indicated that, the major products of decomposition for the first stage are nitrogen, N-2, CO2, 3-isocyanato-4-methylbenzenamine, tolylenediisocyanate (TDI), propargyl alcohol and 2-(vinyloxy)ethanimine). The nature of the products reveals that urethane cleavage follows two different pathways. In the first case, the thermal dissociation of polyurethanes yielding alcohol and isocyanate occurs. The theoretical activation barrier for this reaction computed by density functional theory (DFT) is 197 kJ/mol. The second pathway is through an internal elimination mechanism wherein an amine and alkene are formed with the liberation of carbon dioxide. The computed activation barrier for this step is 217 kJ/mol. During cleavage of urethane, triazole-triazoline groups also undergo ring rupture. The triazole ring cleavage takes place through a two step process initiating with N-N cleavage with an activation energy of 144 kJ/mol. The burn rate of the propellant was evaluated and a lower pressure exponent is exhibited. This could be directly correlated to the thermal decomposition pattern of the cured binder and propellant. This study provides insight for processing energetic solid propellants wherein the basic decomposition mechanism of the new triazole-triazoline crosslinked polybutadiene based propellant binder and ballistics of the solid propellant processed using the new binder. (C) 2016 Published by Elsevier Inc. on behalf of The Combustion Institute.