Pyrolysis behavior of silicone aerogels with different side groups through experimental and ReaxFF MD

Abstract

Silicone materials, widely used as ablation thermal protection materials, have complex degradation mechanisms at ultra-high temperatures that remain poorly understood. In this work, we investigate the pyrolysis behavior of silicone aerogels through experiments and ReaxFF molecular dynamics (ReaxFF MD) simulations, revealing the impact of silicone side groups on their high-temperature stability. The introduction of methyl, vinyl, and phenyl groups through modifying the crosslinker side chains in aerogels, due to their steric hindrance effects and higher bond energies, inhibits the occurrence of cleavage reactions, thereby improving the thermal stability of the material and providing a basis for material design. We obtained kinetic parameters of the pyrolysis process, including activation energy, pre-exponential factor, and reaction mechanism functions, through thermogravimetric analysis, thereby establishing an accurate and reliable decomposition kinetics model. Fast pyrolysis experiments, alongside ReaxFF MD simulations, systematically elucidated the pathways for forming of gaseous, liquid, and solid products during thermal decomposition. Pyrolysis is primarily triggered by the cleavage of Si-C bonds, leading to the cyclization of the Si-O-Si main chain to form cyclic siloxanes. The cleavage of small molecules undergoes a rearrangement reaction, ultimately resulting in the formation of amorphous silica. This study enhances our understanding of the pyrolysis mechanisms of silicone aerogels and provides theoretical insights for improving their thermal stability.