Multifunctional foams with oriented bimodal cellular structure and barbule-like surface fabricated by Bi-thermoplastic expanding microsphere mold-opening foaming

Abstract

The growing demand for multifunctional lightweight materials integrating electromagnetic (EM) wave absorption, impact resistance, thermal insulation, and self-cleaning poses significant challenges due to structural and processing trade-offs. This study proposes a bi-thermoplastic expanding microsphere (Bi-TEM) mold-opening foaming (BTMOF) strategy to fabricate polypropylene/carbon nanotube/Fe3O4 (PP/CNT/Fe3O4) composite foams with oriented bimodal cells and barbule-like surface topology in a single step. The synergistic foaming of high- and low-temperature TEMs under mold-opening stress creates an oriented bimodal structure, while in-mold micro-template imprinting spontaneously constructs superhydrophobic surface microstructures. The oriented bimodal cells extend EM wave propagation paths, achieving a reflection loss (RL) of −47.82 dB and an effective absorption bandwidth (EAB) of 5.04 GHz using enhanced interfacial polarization and multiple reflections. The structure also enables 92.06 % impact energy absorption efficiency through progressive folding and reduces thermal conductivity to 0.0336 W/(m K) by phonon scattering. Meanwhile, the barbule-like surface ensures super-hydrophobicity (contact angle of 161.6°; sliding angle of 3°), rendering the foam self-cleaning attributes. This BTMOF approach overcomes traditional scalability limitations, offering a facile route to fabricate multifunctional foams for aerospace, defense, and wearable electronics sectors.