## 12 Porous Polyimide Nanocomposites for Electromagnetic Interference Shielding  
Polyimides (PI) are a class of high-performance polymer materials which contain the imide group (-CO NHCO-) in the chain backbone. They are well-known for their excellent mechanical strength, creep resistance, thermo-oxidative stability, chemical resistance, superior electrical insulation and remarkable dielectric properties. Owing to these exceptional properties, polymides find application in microelectronic devices, flexible electronics, aircraft composite structures, cryogenic insulation and radiation shielding films. The synthesis of polyimides generally proceeds via a two-step condensation reaction mechanism, preferably in melt or solution polymerization using aromatic diamines and aromatic tetracarboxylic dianhydrides. In the first step, a soluble polymer precursor, poly(amic acid) (PAA), is formed from the ring-opening reaction of the dianhydride with diamine, followed by ring closure (imidazol) of PAA to form aromatic polyimides. Recently, there is a growing need for high-performance polymer foams, especially in the aerospace industry for applications such as fireproof panels, cryogenic tank insulation and energy absorbers, which has led to the use of polyimide foams to meet these requirements. Henceforth, researchers are also making efforts to develop robust PI nanocomposite foams for potential use as high-performance microwave absorbers in aerospace, satellite communication and military sectors. In this context, [PERSON] et al. [116] fabricated an array of robust and lightweight polyimide (PI)/carbon nanotube (CNT) foams with superior microwave absorption and thermal resistance. Initially, aqueous CNT suspension was  
Figure 13: Schematic demonstrating the fabrication process of PLA/CNT foams. Reproduced with permission from [115]. Copyright 2017, Elsevier Ltd.  
stabilized with poly(vinyl pyrrolone), and later ultrasonicated in different wt.% with PAA to form a homogeneous CNT/PAA solution, which was then freeze-dried to form CNT/PAA foam. These foams were heated at gradient temperatures of 150-300 \({}^{\circ}\)C to induce thermal imidization to form PI/CNT foam, as shown in Figure 14a. The foam exhibited an average EMI SE of 41.1 dB, an 82.3% absorption coefficient (A) and a foam density as low as 32.1 mg-cm\({}^{-3}\)[116]. It is noteworthy that, even after subjecting these foams at 300 \({}^{\circ}\)C for 48 h, PI/CNT foams showed a 35 dB EMI SE value. The performance of PI nanocomposite foams portrays their tremendous application potential for use in the harsh condition requirements of aerospace, defense and military sectors. Figure 14: (**a**) Schematic illustrating the preparation of PI/CNT foam. Reproduced with permission from [116]. Copyright 2020, American Chemical Society. (**b**) Schematic demonstration of the fabrication process of carbon foams from PI foam. Reproduced with permission from [117]. Copyright 2020, Elsevier Ltd.  
Carbonization of polymeric foam/aerogel often results in superior electrical conductivity and an enhancement in the specific surface area, which has a tremendous impact on the overall microwave absorption performance. In order to elucidate the effect of carbonization, PI foams were converted to carbon foams by subjecting them to extremely high temperatures in the range of 1000-1200 \({}^{\circ}\)C. This strategy was employed by [PERSON] and coworkers, wherein they fabricated rigid PI foams from the starting monomers, as show in Figure 14b. The PI foams were then carbonized at different temperatures (600 to 1500 \({}^{\circ}\)C) for 1 h under an N\({}_{2}\) environment to yield carbon foams with a maximum electrical conductivity of 0.215 S.cm\({}^{-1}\). Interestingly, foams carbonized at 1500 \({}^{\circ}\)C demonstrated high volume shrinkage of 46%, which can be ascribed to the highly crosslinked structure of the precursor PI foams.