Terry L. St. Clair

Dispersion of single wall carbon nanotubes by in situ polymerization under sonication

Single wall nanotube reinforced polyimide nanocomposites were synthesized by in situ polymerization of monomers of interest in the presence of sonication. This process enabled uniform dispersion of single wall carbon nanotube (SWNT) bundles in the polymer matrix. The resultant SWNT-polyimide nanocomposite films were electrically conductive (antistatic) and optically transparent with significant conductivity enhancement (10 orders of magnitude) at a very low loading (0.1 vol%). Mechanical properties as well as thermal stability were also improved with the incorporation of the SWNT.

Polyimide Foams for Aerospace Vehicles

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.

A new flexible backbone polyimide adhesive

A new linear, aromatic, thermoplastic polyimide, identified as LARC-IA, has been synthesized and evaluated, primarily as an adhesive, which is physiologically safe and relatively inexpensive. The polymer was prepared from oxydiphthalic anhydride (ODPA) and 3,4-oxydianiline (ODA) in diglyme. In order to obtain optimal flow properties, which improves wetting, the molecular weight of the polymer was controlled by use of a monofunctional anhydride, phthalic anhydride (PA). Adhesively bonded lap shear specimens, using Ti-6AI-4V adherends, were prepared and tested to assess its adhesive potential. Specimens were exposed to water boil and thermal aging to determine the adhesive systems durability. Flatwise tensile strength and critical fracture energy were also determined. Results were compared to data for LARC-TPI. Preliminary flexural strength and modulus results were also obtained for composites fabricated from LARC-IA (5% PA) and graphite fibers (AS4, 12K). Initial results of this study indicate considerabl...

Spiral Tunneling Cracks Induced by Environmental Stress Cracking in LaRC™-TPI Adhesives

Abstract Some currently-available formulations of LaRC™-TPI, a thermoplastic polyimide originally developed at NASA-Langley, were found to be highly susceptible to environmental stress cracking when exposed to solvents such as acetone, toluene, diglyme and methyl ethyl ketone. The combination of stress and solvent led to rapid cracking in films and adhesive layers of this material system. Residual cool-down stresses induced when the LaRC-TPI is used as an adhesive or coating led, in the presence of a solvent, to dense “mud crack” patterns which relieve a portion of the stored energy. Because these through-the-thickness cracks are not able to relieve the stored energy in the vicinity of the adherends, additional fractures in the form of curious spiral tunnel cracks initiated and grew inward within each adhesive fragment. Micrographs of the spiral fractures are given, along with a qualitative explanation for the failure process as observed in adhesives and coatings.

Flexible Backbone Aromatic Polyimide Adhesives

Abstract Continuing research at Langley Research Center on the synthesis and development of new inexpensive flexible aromatic polyimides as adhesives has resulted in a material identified as LARC-F-SO2 with similarities to polyimidesulfone (PISO2) and other flexible backbone polyimides recently reported by Progar and St. Clair. Also prepared and evaluated was an endcapped version of PISO2. These two polymers were compared with LARC-TPI and LARC-STPI, polyimides researched in our laboratory and reported in the literature. The adhesive evaluation, primarily based on lap shear strength (LSS) tests, involved preparing adhesive tapes, conducting bonding studies and exposing lap shear specimens to 204°C air for up to 1000 hrs and to a 72-hour water boil. LSS tests at RT, 177°C and 204°C were performed before (controls) and after these exposures. The type of adhesive failure as well as the Tg was determined for the fractured specimens. The results indicate that LARC-TPI provides the highest LSSs, 33 MPa at RT, 3...

Flexibilized Copolyimide Adhesives

Abstract Two copolyimides, LARC-STPI and STPI-LARC-2, with flexible backbones were prepared and characterized as adhesives. The processability and adhesive properties were compared to those of a commercially available form of LARC-TPI. Lap shear specimens were fabricated using adhesive tape prepared from each of the three polymers. Lap shear tests were performed at room temperature, 177°C, and 204°C before and after exposure to water-boil and to thermal aging at 204°C for up to 1000 hours. The three adhesive systems possess exceptional lap shear strengths at room temperature and elevated temperatures both before and after thermal exposure. LARC-STPI, because of its high glass transition temperature provided high lap shear strengths up to 260°C. After water-boil, LARC-TPI exhibited the highest lap shear strengths at room temperature and 177°C, whereas the LARC-STPI retained a higher percentage of its original strength when tested at 204°C [68% versus 50% (STPI-LARC-2) and 40% (LARC-TPI)]. These flexible th...

Adhesive bonding study of amorphous LARC-TPI

A novel form of LARC™-TPI has been developed which does not exhibit a crystalline melt as was encountered in earlier fonns of this material. Being amorphous allows the powder to be consolidated at temperatures near its glass transition temperature of 260°C (500°F). Adhesively bonded titanium lap shear specimens were fabricated at temperatures as low as 260°C (500°F). At lower temperatures, the pressures needed to create a well-consolidated bond were high, but as the bonding temperature was increased, the pressure could be decreased. High quality, high strength bonds were prepared at a temperature as low as 280°C (536 °F) with a pressure of only 0.69 MPa (100 psi). At a 350°C (662°F) bonding temperature, only 0.10 MPa (15 psi) was needed to develop very high lap shear strengths.

Moisture dependence of positron annihilation spectra in nylon-6

Abstract Positron annihilation time spectra have been measured in nylon-6 specimens as a function of their moisture content. The measured average longlife component lifetime values are: 1722 ± 47 ps (dry), 1676 ± 40 ps (14.6% saturation value), 1719 ± 26 ps (29.3% saturation value), 1720 ± 35 ps (50% of saturation value), 1857 ± 35 ps (78.1% of saturation value) and 1936 ± 57 ps (saturated). It is noted that the longlife component lifetime at first decreases and then increases with increasing moisture content in the specimens. This behavior is quite different from that observed in earlier studies of various epoxy, polyamide, and polyimide materials, where the longlife components lifetime decreased linearly with increasing moisture content. The longlife component intensity on the other hand, decreases steadily as the moisture content increases from 0 to 100% of the saturation value. A possible explanation for these anomalous features is discussed.

A thermoplastic polyimidesulfone

A polymer system has been prepared which has the excellent thermoplastic properties generally associated with polysulfones, and the solvent resistance and thermal stability of aromatic polyimides. This material, with improved processability over the base polyimide, can be processed in the 260–325°C range in such a manner as to yield high quality, tough unfilled moldings; strong, high-temperature-resistant adhesive bonds; and well consolidated, graphite-fiber-reinforced moldings (composites). The unfilled moldings have physical properties that are similar to aromatic polysulfones which demonstrates the potential as an engineering thermoplastic. The adhesive bonds exhibit excellent retention of initial strength levels even after thermal aging for 5000 hours at 232°C. The graphite-fiber-reinforced moldings have mechanical properties which makes this polymer attractive for the fabrication of structural composites.

Processing Characteristics of TEEK Polyimide Foam

High performance polymeric foams have experienced a growing demand in aerospace applications such as cryogenic insulation, flame-retardant panels and structural sub-components. The Next Generation Launch Technology (NGLT) program requires foams capable of retaining their structural integrity while providing insulating capabilities in a liquid hydrogen environment. NASA Langley Research Center (LaRC) and Unitika Ltd co-developed a polyimide foam processed from a fine powder of salt-like foaming precursor isolated from solution via solvent evaporation. This paper reports the foaming characteristics of this precursor based on rheometric, calorimetric, thermogravimetric and chromatographic characterization methods. The degree of foaming varied significantly as a function of process heating rate and hold temperatures. Results from the battery of measuring techniques employed afford a complete picture of the foaming and curing mechanism for the subject polyimide foam.