Investigation of the pyrolysis mechanism and structural evolution of polyimide under space environment based on molecular dynamics simulations

Polyimide (PI) is widely used in aerospace applications due to its excellent electrical insulation and thermal stability. However, prolonged exposure to high-temperature vacuum conditions in space can significantly compromise its structural integrity and reliability. To gain deeper insight into the pyrolysis behaviour and structural evolution of PI, this study employs molecular dynamics (MD) simulations to conduct full-scale dynamic tracking of the heating process from 300 K to 3800 K. The evolution of bond breakage, gas-phase product formation, and system free volume is monitored in real time. The results indicate that PI pyrolysis proceeds in three distinct stages: an initial induction phase, a rapid decomposition phase, and a prolonged evolution phase. Product analysis reveals that CO accounts for 47.3% of gaseous species, primarily originating from the cleavage of carbonyl groups in the imide ring; $H_2$O makes up 18.4%, closely related to hydroxyl radical recombination; and $C_5$-$C_{10}$ hydrocarbon fragments constitute 15.6%, reflecting deep fragmentation and molecular rearrangement. As temperature increases from 300 K to 800 K, the free volume fraction rises from 16.9% to 27.7%, indicating significant structural relaxation and diffusion pathway expansion. This work elucidates the multi-stage cooperative mechanism of PI pyrolysis at the atomic level and provides a theoretical basis for improving thermal stability and evaluating service life in aerospace environments.