High-performance polyimide foams belong to a class of polymers characterized by strong thermal and chemical stability and resistance to degradation. At high densities they have strong mechanical properties as well. Recent advancements in high-temperature polymeric materials at NASA Langley Research Center have led to the development of new polyimide foam systems with attractive properties that allow for applications such as thermal and acoustic insulation and flame-retardant panels. They also have applications as insulation for the cryogenic fuel tanks for the next generation of reusable launch vehicles. It is therefore critical that we possess a thorough understanding of the behavior of these materials in extreme conditions and of their degradation processes. These studies can predict utilization parameters and could potentially enable early warning of impending material failure and greatly help failure analysis studies. In this study, select polyimide foams were thermally degraded to different extents under controlled conditions and then examined with both infrared (IR) and Raman spectroscopy in an effort to qualitatively and potentially to quantitatively describe the degradation processes.

Figure 1. Molecular Structures of the Monomers for the Foam
(All are aromatic, cyclic imide structures.)

The various foam samples were present in both degraded and virgin, or nondegraded, states. Thermal degradation of the foams was carried out by exposure to a radiant panel flame at a set distance. Multiple samples, each with a different flame exposure distance, were examined in an effort to observe changes in the foam throughout the degradation process. Samples of the foam materials were analyzed using a Bio-Rad 575C FT-IR spectrometer in combination with an FT-Raman accessory and a UMA 500 microscope.

Analyses of the samples’ spectra were done both qualitatively in Bio-Rad Win-IR and semi-quantitatively in Microsoft Excel. Prior to studying changes in the spectra with degradation, it was necessary to correlate major functional groups with the IR and Raman spectra of the foams. Initially, the TEEK-HH structure was correlated, and then it was used as a guide in examining the other spectra. All of the samples were easily differentiated in the IR and Raman. The IR and Raman spectra of the TEEK-HH series are correlated in figure 2.

A definite increase in the OH-NH/CH ratio in the IR was observed with degradation between the TEEK polymers, with the degradation mechanisms similar. The OH-NH band peak at 3,631 cm-1 corresponds to O-H and N-H bonds in the molecular structure of degraded polyimide material; there are no such bonds in the original molecular structure. The IR of the degraded samples has primarily only shown changes in peak intensity and no major structural changes with degradation. The ratios, though, do seem to have great potential in gauging the degradation of the foam. The OH-NH/CH increase with degradation indicates the formation of a polyamic acid. It had proved more difficult to monitor degradation in the Raman. It is believed that the response in Raman is decreased significantly because of the weak response of Raman overall and the actual polymer per area in the charred samples. Also, Raman is a surface scattering technique and, in the case of charring, the surface is coated with char after exposure. This appears to be represented in the Raman for direct exposure. For exposure removed directly from the flame, Raman spectra show a decrease in overall intensities as was observed in the IR. The ratio increased by a factor 2.52 in the TEEK-HH foam from virgin to degraded, 1.58 for the TEEK-L8, and 10.70 for the TEEK-CL polymer. Note in the last case there was an unusual marked difference between the thick and thin samples examined, resulting in a high (approximately 28 percent) standard deviation in the ratio values. Most standard deviations were below or around 10 percent.

Key accomplishments:

• Ability to correlate IR and Raman spectroscopic data to degradation of polyimide foam samples.
• Sample handling and preparation for Raman analysis.

• Better understanding of correlation between IR and Raman spectral analysis to degradation mechanisms of polyimide foams.
• Several more publications and presentations on related research were made in 2001, including an American Chemical Society book chapter.

Contacts: J.J. Palou (Jaime.Palou-1@ksc.nasa.gov), YA-F2-C, (321) 867-9404; and M.K. Williams, YA-C2, (321) 867-4554
Participating Organizations: Langley Research Center (E.S. Weiser) and Albion College (D. Holland)