Polyimide (PI) foam with excellent microwave absorption
(MA) performance
and desirable compressive strength is highly critical and in demand
in the structural MA components. Although the satisfactory MA performance
of the present PI-based MA foams has been achieved by employing diverse
methods, the relatively low compressive strength (∼KPa) restricted
them from use as structural MA foams in practical application. Herein,
isocyanate acid was introduced to the backbone of PI resin, which
not only increased the PI backbone polarity and strength as rigid
chain segment, but also served as a self-foaming component. The porous
structure of PI foams was readily regulated by adjusting the water
and carbon nanotube (CNT) filler contents of precursor dispersion.
As a result of the improved polarity of the PI backbone resulted from
the isocyanate group and high dielectric loss of CNT, the high compressive
strength of 7.04 MPa and impressive MA property of the resultant PI
foam with a low CNT loading ratio of 1.5 wt % were achieved, which
were much higher than those reported previously. Especially, the effective
absorption bandwidth (EAB) (RL < −10 dB) was up to 10.7
GHz (at the thickness of 3 mm), covering the C, X, and Ku bands simultaneously.
Meanwhile, the EAB of the as-prepared PI foam retained 9.3 and 9.7
GHz even after being subjected to liquid nitrogen (−196 °C)
and high temperature (300 °C) treatments due to the desirable
stability of PI. In addition, the excellent thermal insulation resulted
from the pores structure and low filler content was achieved, where
the top surface only presented 60 °C after placing on 300 °C
platform for 30 min. The high compressive strength, impressive MA
property, and thermal insulation endowed the resultant CNT/PI foam
with great potential application as structural MA foam in a harsh
service environment.