First-principles based modeling of the degradation of the polyhedral oligomeric silsesquioxane (POSS) materials caused by the collision with energetic atomic oxygen.

摘要

The aggressive conditions found in low earth orbit (LEO) present considerable challenges to the development of resilient coatings that can protect space vehicles from damaging higher energy collisions with atomic oxygen species, solar ultra-violet (UV) radiation, thermal cycling, and irradiation by protons and electrons.; Organic functionalized polyhedral oligomeric silsesquioxane (POSS) polymers have been suggested as novel potential coating materials. POSS has the chemical formula of R8(SiO1.5)8. Its eight corners can be functionalized with various organic tethers which can ultimately copolymerize with conventional organic polymers and thus reinforce thermal and mechanical properties. Experimental results suggest that the hybridized POSS polymers degrade and ultimately form CO2, H2O, and silica overlayer that maintains a hybrid polymer structure underneath. Little, however, is known about the elementary paths and mechanisms.; Density functional theory and ab-initio molecular dynamics simulations were carried out to systematically investigate dynamics of atomic oxidative degradation of various organic-tethered POSS systems. The results demonstrate that the ability to quantitatively follow multiple threshold-kinetic-energy surfaces and projected threshold forces allows for a detailed understanding of the relationship among the incident angle, kinetic energy, and product distributions. Steric hindrance and strong covalent bondings between the tethers and cage are essential to protect the structural integrity of POSS. The subsequent oxygen collision simulations show that alkyl-incorporated POSS can undergo POSS cage-opening reactions which likely lead to silica surface layer formation.; A more in depth analysis necessitates the use of coarse-grained atomistic and mesoscale modeling. These simulations, however, are reliant on the development of high-fidelity forcefields. A comprehensive set of ab initio calculations were carried out to determine the intramolecular and intermolecular interactions for alkyl-substituted POSS molecules. The resulting potentials suggest that standard alkane forcefields may be used to describe alkyl substituents on the POSS cage. These potentials can be improved by noting the more repulsive interactions due to increased steric interactions by POSS. The POSS cage may be treated as a rigid cube, and the weak attraction between nonreactive tethers can be modeled by a common van der Waals potential. These potentials were subsequently used by collaborators to simulate the thermophysical behavior of tethered-POSS systems.