Reactive Molecular Simulation of the Damage Mitigation Efficacy of POSS-, Graphene-, and Carbon Nanotube-Loaded Polyimide Coatings Exposed to Atomic Oxygen Bombardment.

Reactive molecular dynamics simulation was employed to compare the damage mitigation efficacy of pristine and polyimide (PI)-grafted polyoctahedral silsesquioxane (POSS), graphene (Gr), and carbon nanotubes (CNTs) in a PI matrix exposed to atomic oxygen (AO) bombardment. The concentration of POSS and the orientation of Gr and CNT nanoparticles were further investigated. Overall, the mass loss, erosion yield, surface damage, AO penetration depth, and temperature evolution are lower for the PI systems with randomly oriented CNTs and Gr or PI-grafted POSS compared to those of the pristine POSS or aligned CNT and Gr systems at the same nanoparticle concentration. On the basis of experimental early degradation data (before the onset of nanoparticle damage), the amount of exposed PI, which has the highest erosion yield of all material components, on the material surface is the most important parameter affecting the erosion yield of the hybrid material. Our data indicate that the PI systems with randomly oriented Gr and CNT nanoparticles have the lowest amount of exposed PI on the material surface; therefore, a lower erosion yield is obtained for these systems compared to that of the PI systems with aligned Gr and CNT nanoparticles. However, the PI/grafted-POSS system has a significantly lower erosion yield than that of the PI systems with aligned Gr and CNT nanoparticles, again due to a lower amount of exposed PI on the surface. When comparing the PI systems loaded with PI-grafted POSS versus pristine POSS at low and high nanoparticle concentrations, our data indicate that grafting the POSS and increasing the POSS concentration lower the erosion yield by a factor of about 4 and 1.5, respectively. The former is attributed to a better dispersion of PI-grafted POSS versus that of the pristine POSS in the PI matrix, as determined by the radial distribution function.