Study on Energetic Reactive Plasticizers Utilizing Ring Strain Energy and Solid-State Copper-Free Azide-Alkyne Click Reactivity in GAP-Based Polyurethane Binders

Plastic-bonded explosives (PBXs) are energetic composites composed of highly energetic molecules bound together by polymers which can create void-free and protective matrix. As a processing aid in the manufacture of PBXs, plasticizers are usually added to ease the processibility of composite formulations, and tune the mechanical properties of cured polymer composites. Inert plasticizers like dioctyl adipate (DOA) find application in hydroxy-terminated polybutadiene (HTPB)-based cast-cured PBXs formulations. However, both DOA and HTPB are non-energetic to decrease the energetic performance of PBXs.<br/>Energetic polymers and plasticizers functionalized with explosophore groups such as nitrate ester (-ONO₂), azide (-N₃), or nitro (-NO₂) group have emerged as a replacement to non-energetic ones. For instance, an energetic polymer, glycidyl azido polymer (GAP), is a hydroxy-terminated polyether which can form the polyurethane (PU) matrix with a suitable isocyanate. Plasticizers may readily migrate out of polymer matrix after certain periods of time even under the relatively mild storage condition, giving rise to the vulnerability of polymeric matrix leading to the energetic composites sensitive to impact, friction and heat. Series of energetic reactive plasticizers have been reported to prevent the migration problem of plasticizers within GAP-based PU binders by utilizing in-situ Cu(I)-free azide-alkyne Click reaction. Although explosophore groups of energetic polymers and plasticizers provide an improved energetic performance, disadvantages of the explosophore groups such as high impact sensitivity, instability at high temperatures and easy oxidation in air have been apparent.<br/>In this study, ring strain energy of cyclic compounds is scrutinized to be an alternative, safe and reliable energy source for energetic reactive plasticizers instead of explosophore groups. Notably, the efficacy of ring strain energy is deemed to provide a relatively inactive energy bearing some immunity to impact and shock because the release of which is commonly initiated by ignition. The energetic reactive plasticizers are designed to have both “energetic” ring-strained cyclic moiety as a new energy source and “reactive” alkyne group for solid-state copper-free azide-alkyne Click reaction with azide groups of GAP-based PU binders. The energetic reactive plasticizers are synthesized via an esterification of ring-strained cyclic carboxylic acids and alkynols. The homodesmotic schemes designed for computing the intrinsic ring strain energy of the energetic reactive plasticizers are developed and those ring strain energies are predicted at B3LYP/6-31G* level of theory. The plasticization effect of the energetic reactive plasticizers on the GAP prepolymer in terms of miscibility and rheological behavior are evaluated respectively. The energetic reactive plasticizers are found to exhibit a distinct copper-free azide-alkyne Click reactivity toward the GAP prepolymer. The experimental determination for the Click reactivity is in good agreement with the theoretical prediction of frontier molecular orbital. The GAP-based PU binders chemically linked to the energetic reactive plasticizers impart an enhanced tensile properties and impact insensitivity due to the transformation from azide to triazole group. Additionally, heat of formation calculated from heat of combustion of energetic reactive plasticizer/GAP-based PU binders is also analyzed.