Wetting of oxidizer particles by binder and plasticizer molecules - microcalorimetry experiments and computer simulations

Composite rocket propellants (CRP) contain oxidizer particles and fuel particles in an elastomeric binder matrix: The oxidizing agent is mostly ammonium perchlorate (AP) and the elastomer is hydroxyl terminated polybutadiene (HTPB) based polyurethane. The components differ widely in terms of crystallinity, thermal expansion behaviour and polarity. Especially, under broad temperature variations insufficient resilience leads to detachment or de-wetting of filler material from the binder. The ageing of such material was successfully characterized by DMA (dynamic mechanical analysis) measurements. The loss factor shows characteristic changes in shape and intensity. The shape of the loss factor is also determined by the intermolecular interactions between binder elastomer and filler materials [1-3]. Also the transitions between non-glassy and glassy states of polymer binders and plasticizers are strongly determined by intermolecular interaction forces [4]. This work intends to contribute to elucidating this aspect: Molecular dynamics simulations (MD) procedures are employed for a determination of binder-oxidizing agent interaction. MD is based on the approximation of atoms as point masses and point charges. Iterative solution of classical equations of motion with interatomic forces yields correct structure and energies within simulated volume elements. The procedure uses the crystal structure of the oxidant and the molecular topology of the elastomer as input parameters, at given temperature and pressure: First, sets of representative crystal surfaces are identified, which are then loaded with a bulk of adsorbate molecules, means binder and plasticizer molecules. From simulation runs at experimental conditions, the energy of interaction is extracted. Results are compared to those from solution microcalorimetry: This methods provides with the thermal response upon mixing AP powders with uncured HTPB, or plasticising agents as dioctyladipate (DOA), and azido-terminated glycidylazide polymer (GAP-A). Although originally designed for measurement of heats of solution, the setup is used to measure the heat of immersion within saturated solutions.