Screening of polymer-plasticizer systems for propellant binder applications: an experimental and simulation approach

Composite propellants of advanced propulsion systems demand high energy, sufficient structural integrity, and low temperature capability. Polymers and plasticizers contribute to all these three important characteristics of propellants and they need to be screened rigorously to achieve the most suitable combination. In this investigation, experimental and simulation studies were performed to predict the miscibility of polymers and plasticizers. Solubility parameter is the measure of miscibility of any two material of interest, evaluated by equilibrium swelling and intrinsic viscosity experiments. Further, molecular dynamic simulations were also used to evaluate the solubility parameter for a few important polymers and plasticizers that are used in rocket propellants. Flory-Huggins interaction parameter (χ) was calculated for identifying the suitable pair. Simulated and experimental data proved that energetic plasticizers are not miscible with hydroxyl terminated polybutadiene (HTPB), while n-Butyl-nitratoethylnitramine (Bu-NENA) was found to be the most suitable plasticizer for polymers containing nitrogen atoms. Therefore, this study finds its importance in identifying an appropriate plasticizer for any specific polymer, which is the foremost critical step for achieving successful propellant formulations with desired characteristics.