(161as) A Combined Quantum and Classical Atomistic Modeling Approach to Study the Aging of AP-HTPB Solid Propellants

The aging process in conventional solid propellants has been a topic of interest for decades due to prohibitively long timescales that have hindered experimental investigations into the relationships between particular degradation pathways and the resulting material properties of the aged material. Experimental approaches typically rely on accelerated aging techniques using elevated temperatures which could skew the relative occurrence of different degradation products relative to ambient conditions. Additionally, these accelerated aging studies lack the ability to discern the degradation mechanisms that are occurring and the weighted contribution of a particular degradation pathway to the resulting material properties of the propellant.

This work is focused on developing an integrated quantum and classical framework for the atomistic modeling of traditional composite propellants consisting of an ammonium perchlorate (AP) oxidizer and a hydroxyl-terminated polybutadiene (HTPB)-based polymeric binder. Quantum mechanical methods are utilized to ascertain thermodynamic and kinetic quantities associated with a particular decomposition pathway in the polymeric binder. Classical atomistic modeling is used to evaluate the effect of different thermo-oxidative degradation products on the resulting material properties of the propellant. The use of a kinetic Monte Carlo scheme is proposed for introducing degradations to the polymeric binder to effectively model the aging of the propellant over the course of years or decades, at ambient conditions. Investigating the behavior of the propellant while under strain is critical to understanding how the polymeric binder adheres to the particles of crystalline oxidizer. One goal is to observe how particular degradation pathways affect the bonding of the binder to the oxidizer and the formation of microvoids or cracks while the propellant is under strain. Future endeavors consist of coarse-graining and parameterizing this atomistic model to a finite-element based model to extend the feasible length scale of the simulations to be able to model the debonding of the binder from oxidizer particles on the length scale of tens of microns.