The objective of this SERDP Exploratory Development (SEED) project was to establish the proof of concept for an entirely new class of binders for cross-linking polymeric formulations. The new binder system rests on the Diels-Alder cycloaddition reaction between previously undisclosed components. The identification of a novel mode of reactivity offered by cyclooctynes in biological contexts paves the way for the development of a neutral, non-toxic binder system that could be implemented using existing equipment.
Besides its neutrality vs. living organisms, the binder system must operate under specifically defined thermal conditions in a reasonable time. While reactions occurring rapidly have been challenging for casting operations, reaction times too long are impractical for a variety of operations. The gum stock produced by the reaction of the binder system with the formulation’s polymer also requires specific physical and mechanical properties. Most importantly, chemical compatibility with common formulation ingredients is fundamental for the establishment of a new binder system.
The project first prepared the new components of the binder system. The cyclooctyne component was dimerized using different linker units, following the hypothesis that the nature of the linker could modulate the physical and mechanical properties of the final polymer. The second reaction partner, a functionalized diene, was integrated to hydroxyl-terminated polybutadiene with the objective to again preserve the desirable physical properties of current formulations and impose only a minimal change to the system. The stability of each component was evaluated by differential scanning calorimetry and thermogravimetric analyses and the key binding reaction was evaluated under thermal conditions. Finally, the new binder system was assessed in an actual formulation featuring ammonium perchlorate as energetic material and dioctyl adipate as plasticizer.
After surmounting significant synthetic challenges, the cyclooctyne partner was produced by considerable modification of existing chemistry. Three dimeric cyclooctyne units were produced using different linkers. Following existing precedents for isocyanate binders, the project team elected to maintain a similar molecular arrangement of the linker. As such, the linkers chosen were the amine version of corresponding established isocyanates. The reactivity of the cyclooctyne towards Diels-Alder cycloaddition was assessed with a variety of dienes. This study led to the identification of isobenzofuran, which provided the desired reactivity at temperatures and timelines ideal for integration into current formulation workflows.
A modified diene was produced using a new method developed at Nalas, and integrated onto commercial hydroxyl-terminated polybutadiene using mild and non-toxic conditions. A proof of concept for the binder system was obtained by the reaction of the cyclooctyne units with the functionalized polymer. This experiment led to the down selection of the linkers and identification of the optimal combination that provides a gum stock with ideal properties for use in formulations.
The new binder components showed good stability towards energetic ingredients and good solubility with dioctyl adipate. An energetic formulation was tested and gave promising results.
The individual components of the new binder system are well-known for their compatibility with living organisms. Various functionalized cyclooctynes have been used as probes in living cells and animals without disrupting biological functions.
The binder showed compatibility with energetic materials and common formulation ingredients while offering promising physical properties upon reaction. Moreover, upon further testing, the new system could be directly integrated into current formulations with no or minimal change from the formulator’s perspective.