The growing interest in replacing traditional metal foams with polymeric foams generated a great drive in the development of high-performance polymeric foams which are rigid, thermally stable polymeric foams for high-temperature (HT) applications. This new material takes the advantage of the thermal stability and chemical resistance of polymer while maintaining the low weight. A foam material with such combination of properties has potential applications in space technology; as the resulting nanocomposite foam would have ranges of stiffness and resilience that are outside the limits of pure polymer foams, be flame resistant, demonstrate electrical and thermal conductivity and yet be both light weight and cost-effective space stable materials.

Polyimide Foams

Polyimide demonstrates outstanding advantages of being high-temperature resistant and oxidation resistant and is one of the most widely chosen candidates for high-performance polymer foam fabrication to be used in areas requiring flame-retardant materials and fire protection, thermal and cryogenic insulation, gaskets and seals, vibration damping pads, spacers in adhesives and sealants, extenders, and flow/leveling aids.
NASA Langley Research Center (LARC) developed polyimide foam from a large number of monomers and monomer blends. The specific densities of these foams can range from 0.008 to 0.32 g/cm3. Figure 14 shows a simple illustration of the foaming concept. The foam precursor powder was placed in the foaming mold and subject to 140 C for 60 min using heat plates on the top and bottom. The mold was then rapidly transferred to a set at 300 C and held for 60 min before cooled to room temperature. Table 2 summarized the thermal stability of the as-fabricated PI foam in nitrogen at 10, 50, and 100 wt%, respectively. The Tg obtained by differential scanning calorimeters (DSC) was one of the highest among polymer foams known till today.

Phthalonitrile Foams

Phthalonitrile (PN)-based resin was proven to be an extremely thermally resistant polymer with service temperature more than 300oC after proper post-curing treatments. Keller from the United States Naval Research Laboratory has intensively synthesized and patented a series of phthalonitrile-based monomers over the past few decades, with bisphenol (Laskoski et al. 2005), aromatic ether (Keller 1994), and phosphine oxide (Laskoski et al. 2007) as spacer linkages incorporated between the terminal phthalonitrile units. Hu and Liu developed foams based on PN polymer targeted for high-temperature applications. The one-step foaming process as shown in Fig. 15 involved proper synchronization of gas decomposition from BA and polymerization. The resulting foam density can be precisely controlled through proper processing control with the lowest obtainable density of 0.04 g/cm3. A typical PN foam produced by Hu and Liu is shown in Fig. 16.

Table 3 summarizes the thermal oxidative parameters of PN foam. Char yields of the 0.2 g/cm3 foams were found to be 82.3 and 68.5 wt% in N2 and air, respectively. The values were comparable to its void-free samples and unattainable by most thermoset foams. Even at a density as low as 0.08 g/cm3, the char yield at 800 C in N2 was maintained at 63.5 %. The high thermal stability was attributed by the high cross-linking density and the thermally stable macrocyclic structures formed during cross-linking. 
Figure 17 shows the percentage of compression stress retention of PN-based foams after thermal aging at 280oC for 100 h. Foams with density larger than 0.12 g/cm3 achieved 95 % property retention. The results proved the potential of such foams to be used for HT applications for extended period of time.