Internal Plasticized Glycidyl Azide Copolymers for Energetic Solid Propellant Binders

Energetic polymers are crucial components of next-generation composite propellants with enhanced performance. A solid rocket motor requires propellants that retain their mechanical properties over a wide range of temperatures. Compared to the state-of-the-art binder systems based on HTPB, the literature shows that energetic polymers, including the well-known glycidyl azide polymer (GAP), often exhibit bad low- temperature properties. The aim of this thesis is the synthesis of novel energetic glycidyl azide copolymers with improved low-temperature properties, and their characterization with special emphasis on their use as prepolymeric binders for cast-cured composite propellants. Comonomers with nonpolar n-alkyl side chains were incorporated in the molecular structure of GAP. They act as internal plasticizers that are not able to migrate in the cured propellant formulation, providing improved low-temperature properties and better processability of the copolymer. The first part of the thesis describes the small-scale synthesis of various glycidyl azide copolymers with a systematic variation of the n-alkyl side chain lengths and molecular compositions via cationic ring-opening polymerization. The molecular structure of the copolymers was characterized with spectroscopic and chromatographic techniques (IR, NMR, GPC). Their energetic and thermal properties were investigated using thermal analysis methods (DSC, TGA, bomb calorimetry). Further properties that are essential for binder development such as density, viscosity and equivalent weight were also determined. The results of the experiments were discussed and compared to the commercially available GAP homopolymer, which was selected as a reference compound. The second part of the thesis covers the scale- up of one selected copolymer with the most interesting properties in order to obtain enough material for further evaluation concerning its use as a suitable binder system for composite propellants. The third part of the thesis presents formulation studies of the copolymer, starting with a theoretical evaluation using the ICT thermodynamic code, followed by the investigation of binary polymer-plasticizer mixtures. Finally, model propellants with ammonium dinitramide (ADN) as an oxidizer were prepared in a cast-cure process in order to study the influence of the novel copolymer on the processability and mechanical properties of cured composite propellant specimen.