Light-Weight and Hydrophobic PI Composite Aerogel and Its Application in Electromagnetic Interference Shielding

Hydrophobicity is essential for the stability of aerogels, especially when the aerogels are utilized as electromagnetic interference (EMI) shielding materials in the aerospace and electronic communication fields. We used 4,4′-diaminodiphenyl ether (ODA) and 2,2-bis-trifluoromethoxy-biphenylenediamine (TFMOB) as diamine monomers and used 1,2,4,5-benzenetetracarboxylic anhydride (PMDA) as a dianhydride monomer to prepare polyamide acid ammonium salt wet gels; then the hydrophobic polyimide (PI) aerogels were obtained via the fast-freezing and slow-freezing methods, respectively. Compared with the hydrophilic homo-ODA/PMDA aerogel, both fPI-5 and sPI-5 (with 50% TFMOB) exhibited high hydrophobicity with a contact angle of about 126°. To endow fPI-5 and sPI-5 with EMI shielding properties, a series of PI composite aerogels have been prepared by loading carboxyl-functionalized multiwalled carbon nanotubes (MWCNTs-COOH) as conductive fillers. The lightweight PI/MWCNTs-COOH composite aerogels (PMCAs) showed high porosity, excellent hydrophobicity (surface contact angle >137°), and prominent thermal stability (with T5% up to 555 °C in nitrogen). In particular, the PMCAs exhibited a superior EMI shielding performance. The EMI shielding effectiveness of the slow-frozen aerogel CsPI-5 (18 phr) could reach 35.7–40.3 dB in the X-band. Meanwhile, the fast-frozen aerogel CfPI-5 (6 phr) possessed outstanding electromagnetic wave-absorbing performance; its minimum reflection loss could reach −59.1 dB at only 3.9 mm thickness in the horizontal direction. This work provides feasible guidance for the preparation of electromagnetic shielding materials.