Multistep pyrolysis behavior of core-shell type hyperbranched azide copolymer: Kinetics and reaction mechanism via experiment and simulation

As an attractive new energetic fuel, core–shell type hyperbranched azide copolymer (POG) have been investigated with regard to its pyrolysis kinetics and mechanism. Through Asymmetric Double Sigmoidal (Asym2sig) function deconvolution, POG pyrolysis profiles could be separated into two reactions well (the first and second step). Based on reasonable kinetics analysis methods, mechanism functions of the first and second step reactions were constructed to n-th order reaction model of f₁(α₁) = (1 − α₁)^1.05 and 3D diffusion model of f₂(α₂) = 1.95(1 − α₂)^2/3[1 − (1 − α₂)^1/3]^−0.54. The gaseous products and residual morphology in different pyrolytic steps are identified by TG-FTIR-MS and SEM. Through molecular simulation, the intermediate reactions between the first and second step were successfully captured, and then the detailed pyrolysis mechanism of POG was established. In the initial stage of POG pyrolysis, thermal cracking of –N₃ generates nitrenes and releases N₂ (fitting n-th order model). Next, the intermolecular crosslinking and intramolecular cyclization reactions of nitrenes form a new cross-linked layer on the outside of hyperbranched polyether core of POG (PEHO-c), delaying the decomposition of inner PEHO-c. Furthermore, pyrolysis of cross-linked layer and PEHO-c is gradually performed by 3D diffusion model from outside to inside, which is very suitable for 3D core-shell structure of POG.