Due to a demand by the aerospace industry, NASA has begun developing the next generation of polyimide foams which could be utilized to reduce vehicle weight for the X-33 and Reusable Launch Vehicle (RLV) programmes. The activity at NASA Langley Research Center focuses on developing polyimide foam and foam structures which are made using monomeric solutions or salt solutions formed from the reaction of a dianhydride and diamine dissolved in a mixture of foaming agents and alkyl alcohols. This process can produce polyimide foams with varying properties from a large number of monomers and monomer blends. The specific densities of these foams can range from 0.008 g cc−1 to 0.32 g cc−1. Polyimide foams at densities of 0.032 g cc−1 and 0.08 g cc−1 were tested for a wide range of physical properties. The foams demonstrated excellent thermal stability at 321°C, a good thermal conductivity at 25°C of 0.03 W m−1 K−1, compressive strengths as high as 0.84 MPa at 10% deflection and a limiting oxygen index of 51%. Thermomechanical cyclic testing was also performed on these materials for 50 cycles at temperatures from −253°C to 204°C. The foams survived the cyclic testing without debonding or cracking. Thermal forming of the 0.032 g cc−1 foam was performed and a minimum radius curvature of 0.0711 m was achieved. The foams exhibited excellent properties overall and are shown to be viable for use as cryogenic insulation on the next generation RLV.

Activity at the NASA Langley Research Center (LaRC) has focused on developing low density polyimide foam and foam structures which are made using monomeric solutions or salt solutions formed from the reaction of a dianhydride and diamine dissolved in a mixture of foaming agents and alkyl alcohol at room temperature. Monomer blends may be used to make a variety of polyimide foams with varying properties. The first foaming process developed consisted of thermal cycling the polymer precursor residuum and allowing the inflation of the particles to interact to create the foam. This process has resulted in foam structures with higher percentages of open cell content. Another innovative foaming process has been developed that begins with partially inflated microspheres, “friable balloons”, with incomplete polymer molecular weight gain, which when fully cured into a foam results in more closed cell structures. In a research study performed by NASA Kennedy Space Center (KSC) and LaRC, two closely related polyimide foams, TEEK‐H series and TEEK‐L series, (4,4′‐oxydiphthalic anhydride/3,4′‐oxydianiline and 3,3′,4,4′‐benzophenonetetracarboxylic acid dianhydride/4,4′‐oxydianiline) were investigated for density effects and closed versus open cell effects on the thermal, mechanical, and flammability properties. Thermal conductivity data under the full range of vacuum pressures indicate that these materials are effective insulators under cryogenic conditions. Contributing factors such as cell content, density, and surface area were studied to determine the effects on thermal conductivity. Cone calorimetry data indicated decreased peak heat release rates for the closed cell system, TEEK‐H friable balloons, compared to the TEEK foams with higher open cell content. Mechanical properties including tensile strength and compressive strength indicated that the materials have good structural integrity. Foams with more open cell content resulted in greater tensile and compressive strengths than the closed cell foams. The maximum closed cell content achieved in the “friable balloon” system was 78% at a foam density of 0.048 gm/cm3. Published in 2005 by John Wiley & Sons, Ltd.

In this paper the synthesis and characterization of two series wholly aromatic, main-chain, reactive liquid crystalline oligomers is reported. Phenylethynyl end-capped oligomers, based on 4-hydroxybenzoic acid (HBA) and 6-hydroxy-2-naphthoic acid (HNA), were successfully synthesized using standard high-temperature melt-condensation techniques. Melt processable oligomers, with M n ) 1000, 5000, 9000, and 13 000 g mol -1 , and oligomers with HBA or HNA concentrations as high as 95 mol % were prepared. All oligomers showed well-defined, homogeneous, nematic melt behavior over a broad temperature range. The phenylethynyl endcapped oligomers could be cured at 370 °C and formed films with excellent mechanical and thermal properties, and without losing the nematic order. Low molecular weight oligomers, i.e. M n < 5000 g mol -1 , tend to form nematic thermosets, while oligomers with M n > 5000 g mol -1 , polymerize predominantly via chain extension chemistry. The fully cured nematic polymers exhibit glass-transition temperatures (T g ) up to 203 °C, as determined by DMTA measurements, and thermal stabilities (5 wt % loss) up to 500 °C in both air and nitrogen atmosphere. The cured 1000 and 5000 g mol -1 oligomers, with 95 mol % HBA or HNA compositions, show significant improvements in storage modulus at elevated temperatures as compared to their high-molecular weight counterparts. Rheology experiments showed that these reactive nematic oligomers are melt-stable for at least 30 min at 300 °C and exhibit complex melt viscosities |η*| as low as 1 Pa‚s at 100 rad‚s -1 . In all examples no loss of mesophase could be observed during chain extension or cross-linking, which indicates that phenylethynyl cross-link chemistry is completely compatible with mesophase formation.

We have synthesized homologous series of para-, meta-, and ortho-substituted aryl ether diamine monomers, with either 2, 3, or 4 ether linkages per monomer unit, and prepared their corresponding poly(ether imide)s with 3,3′,4,4′-biphenyl dianhydride (BPDA) and 3,3′,4,4′-oxydiphthalic dianhydride (ODPA). All polymers were obtained in high molecular weights and gave good quality films with expected mechanical and thermal properties. The ortho-and meta-substituted diamines gave fully amorphous polymers, whereas the para-based diamines resulted in semicrystalline polymers. The glass-transition temperatures (T g ) drop in the order of para > ortho > meta, and the T g drops considerably as a function of the aryl ether content and appears to level off at four ether linkages. The T g values of our polymers were contrasted with a simple quantitative model and found to be in good agreement with the experimental results ((10 °C). BPDA in combination with an all para-substituted, aryl ether-based diamine (BPDA-P3) forms a thermotropic liquid crystalline phase. Optical microscopy experiments confirm the presence of a nematic melt. Highly aligned films could easily be obtained by stretching the films in the liquid crystal phase, and XRD analysis of quenched films confirmed the presence of a highly aligned smectic A phase (SmA) with an order parameter 〈P 2 〉 ) 0.87, indicating a high degree of molecular alignment. To the best of our knowledge, this is the first example of an all-aromatic liquid crystalline poly(ether imide). Dynamic mechanical thermal analysis (DMTA) showed that the para-aryl ethers display broad β-transitions (25-160 °C), whereas the meta-and ortho-series do not show β-transitions. All PEIs with 3 or 4 aryl ether linkages become thermoplastic in nature. The purpose of this systematic study is to provided a basic set of design rules toward the design and synthesis of all-aromatic poly(ether imide) architectures.