Core–shell type multi-arm azide polymers based on hyperbranched copolyether as potential energetic materials in solid propellants

A series of novel multi-arm azide copolymers (POGs) with the same hyperbranched poly[3-ethyl-3-(hydroxymethyl)oxetane] core (PEHO-c) and different content of linear glycidyl azide polymer shell (GAP-s) have been synthesized by sequential cationic ring-opening polymerization and azidation. Detailed structural information of these copolyethers was deduced from Fourier transform infrared, ¹H NMR and inverse gated decoupled ¹³C NMR spectroscopies, matrix-assisted laser desorption ionization time-of-flight mass spectrometry, gel permeation chromatography and elemental analysis. The molecular weight of POG having GAP-s and PEHO-c with a molar ratio 14.95:1 (R_s/c) was around 31 000 g mol⁻¹, far above that of linear GAP (around 4000 g mol⁻¹). The apparent viscosity and glass transition temperature (−51 to −23 °C) decreased first and then slightly increased with increasing molecular weight. Thermal analysis revealed that all the obtained POGs exhibited excellent resistance to thermal decomposition up to 220 °C. Moreover, the energetic properties, investigated using oxygen bomb calorimetric measurements, indicated that the enthalpy of formation of the POGs was higher than that of general linear GAP, but similar to that of branched GAP under reasonable R_s/c. The compatibilities of the POGs with common materials used in solid propellants were studied using differential scanning calorimetry and the results indicated that the POGs had good compatibility with these materials.