Theoretical Kinetic Modeling of Preceramic Polymer Pyrolysis to Silicon Carbide

Silicon carbide (SiC) is a ceramic material that has gained attention in aerospace engineering as a structural material due to its high strength and hardness at elevated temperatures. SMP-10 is a preceramic polymer that transforms into SiC upon pyrolysis, offering significant flexibility in fabricating thermal-resistant materials with complex geometries. Despite its wide range of applications, there is limited understanding of the mechanistic details of the polymer-to-ceramic conversion process of SMP-10. The present study aims to provide a theoretical simulation of SMP-10 pyrolysis based on molecular dynamics (MD) simulation and density functional theory (DFT). First, we conducted an MD simulation on the high-temperature (2000 – 2500 K) pyrolysis of SMP-10. The simulation at 2500 K demonstrated the rapid formation of char with a yield of 83 %wt. by 500 ps, showing excellent agreement with the experimental char yield of 82 %wt. Furthermore, MD results indicated that SMP-10 quickly loses the substituted allylic group right at the start of pyrolysis and undergoes a series of H2 elimination reactions, resulting in an unsaturated structure. The reaction pathway inferred from the MD simulation was further analyzed using the theoretical potential energy surface calculated from DFT at the B3LYP/6-31G(d) level of theory. We found that the allylic groups in SMP-10 can be eliminated via a 6-membered transition state with a low energy barrier (50 kcal/mol), creating a double bond in the backbone of SMP-10. This double bond further promotes a series of H2 elimination reactions, resulting in a diene and triene structure, with the energy barrier gradually decreasing from 75 kcal/mol to 20 kcal/mol as the reaction progresses. Eventually, SMP-10 forms a 1,3,5-trisilabenzene structure, which can serve as a precursor for β-SiC. Further analysis of the bond dissociation energy (BDE) of SMP-10, conducted at M06-2X/def2tzvp, revealed that the Si-H bond in the backbone of SMP-10 has approximately 10 kcal/mol lower BDE than the C-H bond, facilitating the H2 elimination reaction. This study provides the first mechanistic understanding of the polymer-to-ceramic conversion of SMP-10 preceramic polymer, contributing to the development of a detailed kinetic model for the quantitative prediction of β-SiC yield from preceramic polymer pyrolysis.