In Situ FTIR Analysis of Bond-Scale Degradation Behaviors in HTPB Elastomers Under Multi-Field Coupled Aging

Changes in chemical bonds driven by combined thermo-oxidative aging and mechanical loading directly influence the structural integrity of hydroxyl-terminated polybutadiene (HTPB) propellants. Due to uncertainties surrounding the chain scission mechanism of HTPB subjected to thermo-oxidative aging, and the confounding effects of concomitant cross-linking and chain scission on experimental measurements and data interpretation, the present work identifies urethane cross-links as characteristic recognition sites. In situ infrared spectroscopy is utilized to systematically probe chemical bond evolution of HTPB elastomers under static thermo-oxidative aging as well as thermo-oxidative aging coupled with mechanical stress. At the early stage of thermo-oxidative aging, the hydrogen-bond network of urethane groups breaks down first. Bond scission takes place simultaneously across the soft and hard segments of HTPB polyurethane, producing large quantities of amide groups (  CONH  ) via oxidative rearrangement. However, secondary cross-linked structures possess limited stability and fully degrade via hydrolysis, oxidation and chain cleavage at the late aging stage. Scission of long molecular chains into shorter segments alters the modulus contributions of chemical cross-linking and physical entanglement. Chemical cross-linking modulus peaks at week six, while physical entanglement modulus maximizes at week eight. When subjected to coupled thermal, humid, oxidative and mechanical fields, stress changes the degradation pathways of HTPB polyurethane. In HTPB soft segments, it mainly causes conformational rearrangement, mechanochemical chain scission and formation of new unsaturated groups. For polyurethane hard segments, stress mainly promotes fatigue degradation and bond scission. This work reveals the microscopic causes behind mechanical property evolution and provides theoretical basis for predicting the service life and optimizing anti-aging formulations of HTPB propellants. Highlights Hydrogen-bond networks and dual-segment chain of HTPB preferentially fracture. Oxidation and chain scission remove secondary cross-links of aged HTPB. Shortened molecular chains alter the modulus of aged HTPB. Mechanical loading alters the thermo-oxidative degradation pathways of HTPB. External loading accelerates bond rupture within hard-segment domains of HTPB.