Ultrasound-Assisted Solvent-Induced Morphological Alteration: A Straightforward Strategy for Broadband Sound Absorption in Polyimide Foam

Conventional polyimide (PI) foam suffers from insufficient sound absorption in the low- to midfrequency range (160–1000 Hz). Addressing this deficiency without altering its chemical composition remains a challenge. This work presents a physical postfoaming strategy based on an ultrasound-assisted solvent-induced process to tailor the cellular morphology. In contrast to chemical modification routes, this method offers a straightforward pathway for performance enhancement. With anhydrous ethanol as a low-boiling-point solvent under ultrasonic assistance, a synergistic active-passive control was achieved over the cellular morphology. The cavitation actively perforates closed cells, while solvent exchange and evaporation induce polymer network swelling-shrinkage, passively generating wrinkled cell walls and reducing the average cell size to 283 μm. This unique microstructure enhances sound absorption via increased tortuosity of the sound propagation path, thereby effectively dissipating sound energy into heat through intense frictional and viscous effects. The optimized foam exhibits an average absorption coefficient of 0.78 over 160–6400 Hz, with a 100% increase in the average coefficient for the low- to midfrequency range (reaching 0.32), while maintaining a low thermal conductivity of 0.0343 W/(m·K). This work provides a scalable, physical approach for designing high-performance PI foam with broadband sound absorption performance for applications in marine and transport noise control.