Interaction energies between binder polymers and crystalline surfaces in composite materials determined by solution microcalorimetry and computer simulation

Composite rocket propellants (CRP) contain oxidizer particles and fuel particles in an elastomeric binder matrix: The oxidizing agent is mostly ammonium perchlorate and the elastomer is hydroxyl terminated polybutadiene (HTPB) based polyurethane. Whilst being processed easily, mechanical and thermal stability of CRPs may suffer compared to pure elastomer. The components vastly differ in terms of crystallinity, thermal expansion behaviour and polarity. Especially, under wide temperature variations insufficient resilience leads to detachment or dewetting of filler material from the binder and reduced performance. The ageing of such material was successfully characterized by DMA (dynamic mechanical analysis) measurements. The loss factor tanshows characteristic changes in shape and intensity. Furtheron the shape of the loss factor is also determined by the intermolecular interactions between binder elastomer and filler materials [1]. This work intends to contribute to elucidating this aspect. Adhesion strength is a crucial measure within the optimization process of CRP mechanical stability: Solution micro calorimetry is employed to investigate the thermal response upon mixing AP with uncured HTPB, or plasticising agents as dioctyl adipate, DOA. Although originally designed for measurement of heats of solution, the setup is readily used to measure the heat of adsorption within saturated solutions. For normalization to heat per surface area, particle size and surface area distributions are determined by laser scattering, scanning electron and optical microscopy, as well as gas adsorption measurements (BET method). Heats of adsorption scale linearly with the total surface of the crystalline sample, including an offset from blank measurements. The results are further used for validation of molecular dynamics computer simulations. Sets of representative crystal surfaces are identified, and loaded with adsorbate. From simulation runs at experimental boundary conditions (pressure, temperature), heats of interaction are straightforwardly extracted, alongside the contribution of each functional group. As results show good quantitative agreement with the corresponding experiments, the simulation procedure qualifies as a screening method in CRP development or generally for composite materials.