Preparation and mechanical and microwave absorption property characterization of polymethacrylimide foam reinforced with a multiwalled carbon nanotube/carbon black hybrid

To meet the growing demand for materials integrating structural load-bearing capability with radar stealth functionality, a lightweight microwave-absorbing polymethacrylimide foam reinforced with a hybrid system comprising multiwalled carbon nanotubes (MWCNTs) and carbon black (CB) was developed in this study. By optimizing the composition of the hybrid absorber and employing a bulk polymerization–foaming strategy, a continuous and stable conductive network was constructed within the foam cell walls while maintaining favorable processability and low density. The dispersion behavior of the hybrid absorber; effects of initiator composition on polymerization and foamingas well as the microstructure, electromagnetic properties, and mechanical performance of the composite foam were systematically investigated. Among the three formulations investigated, the composite foam with 1.5 wt% MWCNTs and 1 wt% CB exhibited the best broadband microwave absorption, showing a minimum reflection loss of −33 dB at ∼7 GHz (thickness of 20 mm) and an effective absorption bandwidth of 14 GHz covering 4–18 GHz (at a thickness of 50 mm).At the same time, the foam maintained a low density (approximately 80 kg·m −3 ), together with outstanding compressive specific strength (2.18 × 10 4 m 2 /s 2 ) and specific modulus (1.42 × 10 6 m 2 /s 2 ) (i.e., compressive strength and modulus normalized by density, respectively), outperforming most previously reported carbon-based absorber foams. This high performance was attributed to the synergy between MWCNTs and CB, which enabled balanced impedance matching and electromagnetic attenuation while reinforcing the cellular structure. Thus, this work provides practical insights into the design of multifunctional structural absorbers combining mechanical robustness and broadband microwave absorption for advanced engineering applications.