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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1999

On the Oxidative Degradation of Nadic End-Capped Polyimides: Synthesis and Characterization of Model Compounds for End-Cap Degradation Products - 3

The oxidative degradation of PMR (for polymerization of monomeric reactants) polyimides at elevated temperatures was followed by cross-polarized magic angle spinning (CP-MAS) NMR. Labeling of selected sites in the polymers with C-13 allowed for direct observation of the transformations arising from oxidation processes. The formation of several degradation products has been proposed to be occurring in the cross-links derived from the nadic end caps on oxidation. Model compounds have been synthesized and characterized by CPMAS NMR with both normal and delayed decoupling to distinguish between protonated and unprotonated carbons. Comparison of these spectra to predicted chemical shifts of proposed products for the aged polymer provides further insight to degradation occurring in the cross-linked moieties.

Meador, Mary Ann B.↗
NASA NTRS1992

Benzonorbornadiene end caps for PMR resins

Several ortho-disubstituted benzonorbornadiene derivatives are described. These molecules contain acid, ester, or anhydride functionality permitting their use as end caps in PMR (polymerization of monomer reactants) polyimide systems. The replacement of the currently used norbornenyl end caps with benzonorbornadienyl end caps affords resins of increased aromatic content. It also allows evaluation of some mechanistic aspects of PMR cross-linking. Initial testing of N-phenylimide model compounds and of actual resin formulations using the benzonorbornadienyl end cap reveals that they undergo efficient thermal crosslinking to give oligomers with physical properties and thermal stability comparable to commercial norbornene-end-capped PMR systems.

Panigot, Michael J.↗
NASA NTRS1993

Molecular-Weight-Controlled, End-Capped Polybenzimidazoles

Novel molecular-weight-controlled end-capped poly(arylene ether benzimidazole)s (PAEBI's) prepared by nucleophilic displacement reaction of di(hydroxyl)benzimidazole monomers with activated aromatic dihalides. Polymers prepared at various molecular weights by upsetting stoichiometry of monomers and end-capped with monohydroxybenzimidazole. Exhibit favorable physical and mechanical properties, improved solubility in polar aprotic solvents and better compression moldability. Potential applications as adhesives, coatings, films, fibers, membranes, moldings, and composite matrix resins.

Connell, John W.↗
NASA NTRS1985

All-aromatic biphenylene end-capped polyquinoline and polyimide matrix resins

Biphenylene end-capped polyquinoline and polyimide resins afford low void content graphite-reinforced composites with good initial properties. However, with both resins, rapid degradation occurs during oxidative isothermal aging at elevated temperatures. The degradation is not observed during isothermal aging under a nitrogen atmosphere which suggests that the biphenylene end-cap (or the resulting crosslink/chain extension structures) is not particularly thermooxidatively stable. The nature of the thermooxidative instability is currently under investigation.

Droske, J. P.↗
NASA NTRS1980

Addition polyimide end cap study

The characterization of addition polyimides with various end caps for adhesive applications at 120-250 C environments is discussed. Oligometric polyimides were prepared from 3,3',4,4'-benzophenone tetracarboxylic dianhydride and 3,3'-methylenedianiline which were end-capped with functionally reactive moities which cause crosslinking when the oligomers are heated to 200-400 C. The syntheses of the oligomers are outlined. The thermolysis of the oligomers was studied by differential scanning calorimetry and the resulting polymers were characterized by differential thermal analysis and adhesive performance. The adhesive data include lap shear strengths on titanium 6-4 adherends both before and after aging for 1000 hours at 121 C and/or 232 C.

St.clair, T. L.↗
NASA NTRS2004

Better End-Cap Processing for Oxidation-Resistant Polyimides

A class of end-cap compounds that increase the thermo-oxidative stab ility of polyimides of the polymerization of monomeric reactants (PM R) type has been extended. In addition, an improved processing proto col for this class of end-cap compounds has been invented.

Meador, Mary Ann B.↗
NASA NTRS1984

A study of the effects of end-cap molecular species on environmental characteristics of polimidesulfones

To improve the environmental stability and mechanical properties of polyimidesulfone (PISO2), it was decided to investigate the effects of various types of end caps on its thermo-mechanical and related properties. It was noted that end caps are effective in reducing the environmental damage susceptibility of PISO2 samples, apparently due to their ability to react with free end groups which are believed to be moisture pickup sites. Phthalic anydride, aniline and aminobenzophenone were the end caps used in this study.

Singh, J. J.↗
NASA NTRS1997

Elucidation of the Cross-Link Structure of Nadic-End-Capped Polyimides Using NMR of C-13-Labeled Polymers

Solid NMR of C-13 isotope-labeled samples of PMR-15 was used to follow the cross-linking reaction of the nadic end cap. Some samples were labeled on one of the carbon atoms of the nadic end cap, and others on the methylene carbon atom of the methylenedianiline portion of the polymer. NMR spectra were run on these samples both before and after cross-linking. In this way, direct evidence of the major products of cross-linking under normal cure conditions is provided. The majority (approximately 85%) of the cross-linking derives from olefin polymerization through the double bond of the end cap. Approximately 15% of the products could come from a pathway involving a retro-Diels-Alder reaction. However, all of the products could be explained by a biradical intermediate without a retro-Diels-Alder reaction. Evidence is also presented that the methylene moiety in the methylenedianiline part of the polymer chain also participates in the cross-linking, albeit to a small extent, by a radical transfer reaction. Different cure conditions (higher temperatures, longer times) could change the relative distribution of the products.

Meador, Mary Ann B.↗
NASA NTRS2003

Processable Polyimides Containing APB and Reactive End Caps

Imide copolymers that contain 1,3- bis(3-aminophenoxy)benzene (APB) and other diamines and dianhydrides and that are terminated with appropriate amounts of reactive end caps have been invented. The reactive end caps investigated thus far include 4-phenylethynyl phthalic anhydride (PEPA), 3- aminophenoxy-4-phenylethynylbenzop henone (3-APEB), maleic anhydride (MA), and 5-norbornene-2,3-dicarboxylic anhydride [also known as nadic anhydride (NA)]. The advantage of these copolyimides terminated with reactive groups, relative to other polyimides terminated with reactive groups, is a combination of (1) higher values of desired mechanical-property parameters and (2) greater ease of processing into useful parts.

Jensen, Brian J.↗
NASA NTRS2003

Reevaluation of Tetrahydrophthalic Anhydride as an End Cap for Improved Oxidation Resistance in Addition Polyimides

Several substituted 1,2,3,6-tetrahydrophthalic anhydride end caps - including the 3-phenyl, 3-methoxy, 3-trimethylsilyloxy, and 3,6-diphenyl analogs - were synthesized via the Diels-Alder condensation of the corresponding butadienes and maleic anhydride. These anhydrides, as well as the commercially available 3-hydro and 4-methyl analogs, were each ground together with methylenedianiline in a 2:1 ratio and heated gradually from 204 C to 371 C, with the thermolysis followed by NMR. Generally speaking, a transformation via monoimide to bisimide was observed in the lower temperature range, followed by competition between crosslinking and aromatization. We believe that this competition produces a substantial percentage of aromatic product, with the concomitant lowering of the relative amount of crosslinking and is responsible for improving both thermal oxidative stability of tetrahydrophthalic end capped polyimides and their substantial frangibility. The thermolysis of the tetrahydrophthalimides under inert atmosphere dramatically lowers the amount of aromatization hence, the mechanism for aromatization is an oxidative one.

Meador, Mary Ann B.↗
NASA NTRS1983

Characterization of geometric isomers of Norbornene end-capped imides

Three geometric isomers from the thermal isomerization of methylene-4,4' bis(endo-N-phenylbicyclo/2.2.1/hept-2-ene-5,6-di carboximide) (I) were chromatographically separated and isolated in order to investigate the thermal cure of norbornene end-capped imide oligomers, which display considerable promise for use in various aerospace adhesive and composite applications. Endo-endo (I), endo-exo (II), and exo-exo (III) configurations were assigned to each compound based on the results of NMR spectroscopy. Several chromatographic, spectroscopic, and thermal techniques were then used to characterize these three isomers which serve as model compounds for norbornene end-capped polyimides. It was found that each compound thermally isomerized to an equilibrium mixture of all three compounds prior to cure. It is proposed that these compounds react by different mechanisms in air and nitrogen.

Young, P. R.↗
NASA NTRS1994

Phenylethynyl End-Capping Reagents And Reactive Diluents

Compounds containing phenylethynyl group serve as thermally reactive polymer end caps and reactive diluents. Useful in preparation of adhesives, composite matrices, and molding compounds. These reagents transform arylene ether oligomers and polymers into readily processable reactive materials that convert thermally to thermosets. Compounds synthesized for subsequent use in making thermoset polymers. Phenylethynyl group found to offer several unexpected advantages over ethynyl-based analog.

Bryant, Robert G.↗
NASA NTRS1991

Chemistry and properties of controlled molecular weight end-capped LaRC-CPI

As part of a program on structural resins for high temperature aerospace applications, a controlled molecular weight end-capped semi-crystalline polyimide, designated LaRC-CPI, was evaluated. Neat resin moldings, adhesive specimens and composites were fabricated at temperatures of 365-375 C under pressures of 200-500 psi. Adhesive and composite specimens provided good mechanical properties at temperatures as high as 232 C.

Hergenrother, Paul M.↗
NASA NTRS2005

Tetrahydrophthalic Anhydrides as Addition Curing Polyimide End Caps: Thermal Isomerization of Methylendianiline 3,6-Diphenyltetrahydrophthalic Bisimides

In depth NMR studies confirm that heating a 1:2 mixture of cis, cis, cis 3,6-diphenyltetrahydrophthalic anhydride (end cap 9c) with methylenedianiline at 316 C initially yields the corresponding highly congested cis, cis, cis 3,6-diphenyltetrahydrophthalic bisimide 11, which is converted at this temperature to the observed product, the less hindered trans, cis, trans isomer 12.

Frimer, Aryeh A.↗
NASA NTRS1981

Characterization of initial cure reactions in propargyl and nadic end capped model compounds

Imide model compounds containing propargyl and nadic groups were studied to obtain a fundamental understanding of the reaction of these groups attached to imide oligomers. The initial cure reactions were examined by a variety of characterization techniques including high pressure liquid chromatography, infrared spectroscopy, thermal analyses, and mass spectroscopy. The initial step in the cure of propargyl end capped model compounds probably involved the formation of a new terminal acetylenic group. Configurational changes involving endo/exo isomerism was found in the nadimide model compounds. Nadimide compounds heated in air and in nitrogen appeared to cure by different mechanisms.

Young, P. R.↗
NASA NTRS1997

Oxidative Degradation of Nadic-End-Capped Polyimides: Evidence for Reactions Occurring at High Temperatures - 2

The oxidative degradation of PMR (for polymerization of monomeric reactants) polyimides at elevated temperatures was followed by cross-polarized magic angle spinning (Cp-MAS) NMR. C-13 labeling of selected sites in the polymers allowed for direct observation of the transformations arising from oxidation processes. As opposed to model compound studies, the reactions were followed directly in the polymer. The labeling experiments confirm the previously reported oxidation of the methylene carbon to ketone in the methylenedianiline portion of the polymer chain. They also show the formation of two other oxidized species, acid and ester, from this same carbon. In addition, the technique provides the first evidence of the kind of degradation reactions that are occurring in the nadic end caps. Several PMR formulations containing moieties determined to be present after oxidation, as suggested by the labeling study, were synthesized. Weight loss, FTIR, and natural abundance NMR of these derivatives were followed during aging. In this way, weight loss could be related to the observed transformations.

Meador, Mary Ann B.↗
NASA NTRS1985

Process of end-capping a polyimide system

A process of endcapping a polyimide system with an endcapping agent in order to achieve a controlled decrease in molecular weight and melt viscosity along with predictable fracture resistance of the molded products is disclosed. The uncapped system is formed by combining an equimolar ratio of 4,4'-bis (3,4-dicarboxyphenoxy) diphenylsulfide dianhydride (BDSDA) and 1,-bis (aminophenoxy) benzene (APB) dissolved in bis (2-methoxyethyl)ether. The endcapped system is formed by dissolving APB in bis-(2-methoxyethyl)ether, adding the BDSDA. By varying the amount of endcapping from 0 to 4%, molecular weight is decreased from 13,900 to 8660. At a processing temperature of 250 C, there is a linear relationship between molecular weight and viscosity, with the viscosity decreasing by two orders of magnitude as the molecular weight decreased from 13,900 to 8660.

Stclair, T. L.↗