(632e) Investigation of the Processability of Tetrazole Polyelectrolytes As Binders for Nitrogen-Rich Composite Propellants

Poly(5-vinyltetrazole) is a brittle energetic polymer of little interest to industrial or military applications due to its poor mechanical properties and low processability. However, a weakly acidic tetrazole moiety makes it an interesting starting point to formulate energetic polyelectrolytes with very different physical properties from their neutral precursor: the thermal, physical and rheological properties of each polyelectrolyte vary depending on the nature of the electrolyte present with the tetrazole ion and must be studied accordingly. In this work, several concentrated 5-vinyltetrazole polyelectrolyte solutions were studied to compare their processability at different temperatures; the most promising polyelectrolytes were extruded, foamed and compounded with various energetic compounds to study their thermal stability, mechanical properties, and compatibility for use as energetic binders in water-processable composite propellants.

Commercially available high molecular weight polyacrylonitrile (PAN, >100kDa) was used as the preferred starting point for the synthesis of poly(5-vinyltetrazole) (PVT) to increase the molecular weight of the resulting polymer, improving its mechanical properties. The process used to synthesize PVT is described in the literature and takes place in either N,N’-dimethylformamide or water in the presence of a catalyst (ammonium chloride or zinc chloride, respectively) and sodium azide. The resulting neutral poly(5-vinyltetrazole) is ground into a coarse powder and converted to a variety of energetic polyelectrolytes in water: it can be neutralized by alkali metals and organic bases to yield 5-tetrazolate polymers or protonated by strong acids to yield 5-tetrazolium polymers.

Through this method, previously known basic polyelectrolytes (e.g. triaminoguanidinium, ammonium polyvinyltetrazolates) as well as mostly undocumented polyelectrolytes (e.g. polyvinyltetrazolium perchlorate and nitrate) were prepared and their properties reported. The thermal stability of these samples was studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Glass transition temperatures were evaluated through Dynamic Mechanical Analysis (DMA). Rotational rheometry tests were also conducted at different temperatures to observe the thermoplastic behavior of concentrated polyelectrolyte solutions. Sample molding was completed using a heated press, foamed samples were obtained using small amounts of azobisisobutyronitrile (AIBN) as a low-temperature foaming agent. Chemical compatibility with energetic compounds, oxidizers and possible plasticizers was assessed by comparing thermal stability (DSC) of mixtures to each of their respective components.

Overall results indicate that compounds using various tetrazole polyelectrolytes as binders can be used to tailor materials with desired properties over a wide temperature range. A limited number of these compounds in concentrated solutions also show thermoplastic behaviors deemed interesting for automated manufacturing, offering new possibilities for water-processed propellants.