Chemical Reaction Mechanism of Proton Irradiation to Polyimide Materials under Low Earth Orbit Using ReaxFF MD Methods.

Proton irradiation in the inner radiation belt significantly degrades the thermal stability and mechanical properties of aerospace polymer materials, but the involved reaction mechanism remains elusive. In this work, we systematically investigate the chemical reactions and performance degradation mechanisms of polyimide materials under proton irradiation using ReaxFF MD methods. The reactions between proton H and polyimide can be classified into substitution, hydrogenolysis, and hydrogenation reactions, while the latter two can create unsaturated reaction sites that trigger additional reactions with proton H. The formation pathways of products including CO, HO, OH, and H are disclosed, offering valuable insights into comprehending the microscale reaction processes. The C═O site on the imide ring is found to be most susceptible to proton irradiation, due to the elevated electron density and low electron binding as calculated by DFT. Based on these findings, we propose a strategy to improve the proton irradiation resistance of polyimide by lowering the electron density of the imide ring, which can be fulfilled through the conjugation effect of introduced benzene ring. Correspondingly, a substantial reduction of 92.93% in erosion yield could be achieved according to the ReaxFF MD calculations. The present study provides valuable insights into the reaction mechanisms of proton irradiation and opens a fresh route to enhance the proton irradiation resistance of polyimides.