Structure and energetics of hydroxyl‐terminated polybutadiene via density functional theory

Hydroxyl‐terminated polybutadiene (HTPB) is a commonly used binding polymer in industry to produce polyurethanes. HTPB synthesis via allylic radical propagation leads to varied compositions and conformations that govern the bulk physical and mechanical properties of this polymer. While HTPB is incredibly well studied, its enthalpy of formation is still an open question and is critical to understanding the energetic contributions of the binding polymer in propellant formulations. This study investigates a series of HTPB polymers of experimentally relevant size and composition to understand the effects of conformations and monomer compositions on enthalpies of formation. The distribution of component monomer species comprising HTPB polymers is shown to have a significant effect on the condensed phase enthalpy of formation. The present study provides a best estimate of 331.2±50.3 kJ/mol or 150.7±22.9 kJ/kg for the condensed phase enthalpy of formation for commercially available HTPB formulations. The effects of the polymer folding motifs are smaller but distinct, with more highly folded structures exhibiting lower enthalpies of formation.