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At least 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Physical aging and solvent effects on the fracture of LaRC-TPI adhesives

When amorphous materials are quenched below their glass transition temperature, excess enthalpy is trapped in the glassy material because the viscosity is too great to allow the material to remain in volumetric equilibrium. Over time, this excess free volume is reduced as the material slowly approaches its equilibrium configuration. This process, known as physical aging, leads to substantial changes in the constitutive behavior of polymers, as has been widely discussed in the literature. Less is known about the effects of this physical aging process on fracture and fatigue properties of aged materials. The original goal of the summer was to investigate the effects of physical aging on the fracture and fatigue behavior of LaRC-TPI, a thermoplastic polyimide developed at NASA-Langley. Preliminary results are reported, although a lack of equipment availability prevented completion of this task. In the process of making specimens, the current LaRC-TPI was observed to be extremely susceptible to environmental stress cracking. A study of the unique failure patterns resulting from this degradation process in bonded joints was conducted and is also reported herein.

Dillard, David A.↗
NASA NTRS1993

Crosslinked polyimides prepared from N-(3-ethynylphenyl)maleimide

The compound N-(3-ethynylphenyl)maleimide (NEPMI) was used to prepare thermally stable, glassy polyimides which did not exhibit glass transition temperatures below 500 C. NEPMI was blended with the maleimide of methylene dianiline (BMI) and heated to form the polyimide. NEPMI was also mixed with Thermid 600 R, a commercially available bisethynyl oligomeric material, and heated to form a thermally stable, glassy polyimide. Lastly, NEPMI was blended with both BMI and Thermid 600 R to form thermally stable, glassy polyimides.

Gerber, Margaret K.↗
NASA NTRS1993

Oxygen plasma resistant phosphine oxide containing imide/arylene copolymers

A series of oxygen plasma resistant imide/arylene ether copolymers were prepared by reacting anhydride-terminated poly(amide acids) and amine-terminated polyarylene ethers containing phosphine oxide units. Inherent viscosities for these copolymers ranged from 0.42 to 0.80 dL/g. After curing, the resulting copolymers had glass transition temperatures ranging from 224 C to 228 C. Solution cast films of the block copolymers were tough and flexible with tensile strength, tensile moduli, and elongation at break up to 16.1 ksi, 439 ksi, and 23 percent, respectively at 25 C and 9.1 ksi, 308 ksi and 97 percent, respectively at 150 C. The copolymers show a significant improvement in resistance to oxygen plasma when compared to the commercial polyimide Kapton. The imide/arylene ether copolymers containing phosphine oxide units are suitable as coatings, films, adhesives, and composite matrices.

Jensen, Brian J.↗
NASA NTRS1992

Toughening of PMR composites by gradient semi-interpenetrating networks

The toughening of the PMR-15 and LARC RP-46 high temperature thermosetting polyimides is presently attempted through the construction of a semiinterpenetrating network at ply interfaces through the use of the Matrimid 5218 thermoplastic polyimide powder, whose 315-320 glass transition temperature is compatible with the PMR matrices. The 60 vol pct fiber composites thus prepared for the two resins, with and without toughening, were comprehensively characterized in flexure, tension, intralaminar and short beam shear, compression and quasi-isotropic short-block compression, as well as modes I and II interlaminar fracture toughness and compression after impact.

Johnston, N. J.↗
NASA NTRS1993

The effect of high-energy electron radiation on poly(arylene ether)s

Thin films of four experimental poly(arylene ether)s of similar chemical structure were exposed to 1 MeV electrons while under high vacuum. The films received total exposures of 5 x 10 exp 7 and 1 x 10 exp 9 rads at a dose rate of 5 x 10 exp 7 rads/h and a pressure of 2 x 10 exp -7 torr. Films exposed to 5 x 10 exp 7 rads showed dramatic changes in molecular weight distribution. After exposures of 1 x 10 exp 9 rads the films were only partially soluble in chloroform and exhibited no detectable changes in the glass transition temperatures. Thin-film tensile properties were also altered by the exposure to electron radiation. The effect of the exposures as determined by various analyses is discussed.

Connell, J. W.↗
NASA NTRS1993

Polyimides containing pendent trifluoromethyl groups

Several new polyimides containing trifluoromethyl groups were prepared from the reaction of various aromatic dianhydrides and two new diamines containing trifluoromethyl groups, 4,4'-bis(3-amino-5-trifluoromethylphenoxy)biphenyl and l,4-bis(3-amino-5-trifluoromethylphenoxy)benzene. The diamines were prepared from the aromatic nucleophilic displacement of the disodium salts of 4,4'-biphenol or hydroquinone with 3,5-dinitrobenzotrifluoride followed by hydrogenation of the resultant dinitro compounds. The thermally cured polyimides exhibited glass transition temperatures between 186 and 262 C. By thermogravimetric analysis, the polyimides exhibited 5 percent weight losses at 484-527 C in nitrogen and 452-506 C in air.

Havens, S. J.↗
NASA NTRS1992

Lower temperature curing thermoset polyimides utilizing a substituted norbornene endcap

Methoxycarbonyl bridgehead substituted nadic diacid monomethyl ester, when used as an endcapping monomer, lowered the cure temperature of thermoset PMR polyimides without seriously affecting other desirable properties, such as glass transition temperature and thermal oxidative stability. The C-13 CP/MAS NMR of model compounds was used to follow the cure of resin systems using both the unmodified nadic endcap and the methoxycarbonyl-substituted endcap. Rheological analysis and differential scanning calorimetry DSC also provided evidence for the lower curing nature of the substituted endcap. Two regioisomers of the bridgehead-substituted endcap were isolated, and their chemical structures were elucidated by X-ray crystallography. The model compound and molecular modeling studies conducted ruled out the possibility of regioisomeric imide formation in the substituted endcaps.

Waters, John F.↗
NASA NTRS1988

The effect of diamic acid additives on the dielectric constant of polyimides

The effect of six selected diamic acids additives (including 2,2-prime bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride-aniline (An); 4,4-prime-oxydiphthalic anhydride-An, 3,3-prime diaminodiphenyl sulfone-phthalic anhydride (PA); 4,4-prime-oxydianiline-PA; 2,2-bis 4(4-aminophenoxy)phenyl hexafluoropropane-PA; and 2,2-bis 4(3-aminophenoxy)phenyl hexafluoropropane-PA) on the dielectric constants of low-dielectric-constant polyimide resins was evaluated. It was found that the effect of the incorporation of the diamic acids on reducing the dielectric constant of polyimides may be limited as the dielectric constant of the base resin itself becomes very low. The additives were found to lower the resin's values of glass transition temperature, with no effect on thermooxidative stability.

Stoakley, Diane M.↗
NASA NTRS1987

Selected spectroscopic and magnetic properties of lanthanide complexes in polyimide XU-218

Polyimide XU-218 films containing approximately 5 wt pct of Eu(III), Gd(III), Tb(III), and Er(III) were prepared, and the effects of complexing each of the metals with the following four ligands were investigated: N-phenylphthalamate (NPPA), 2,4-pentanedionate (AcAc), 1,3-diphenyl 1,3-propanedionate (DBM), and a new hexa-aza-macrocyclic (MAC) ligand. The tris-chelated complexes of the mononegative ligands NPPA, AcAc, and DBM produced transparent, flexible films, which had magnetic and spectral properties very similar to those of the parent lanthanide complexes, while complexes of MAC showed problems due to the presence of lattice water and yielded dark brittle films. AcAc caused little or no effect on the glass transition temperature (Tg), while NPPA and DBM complexes lowered Tg to 269-290 C, and MAC indicated moisture by inflexion at 95-100 C with a true Tg at 320 C. All lanthanide-containing films were paramagnetic.

Stoakley, D. M.↗
NASA NTRS1992

Chemistry and properties of poly(arylene ether 1,3,4-oxadiazole)s and poly(arylene ether 1,2,4-triazole)s

Poly(arylene ether)s containing l,3,4-oxadiazole and 1,2,4-triazole units were prepared by the aromatic nucleophilic displacement reaction of bisphenol oxadiazole and bisphenol triazole compounds with activated aromatic dihalides. The polymers exhibited glass transition temperatures (Tg) ranging from 182 to 242 C, and several polymers exhibited melting transitions (Tm) ranging from 265 to 390 C. Inherent viscosities ranged from 1.02 to 3.40 dl/g, indicating relatively high molecular weights. Thin films exhibited tensile strengths, moduli, and elongations at 23 C of 90-110 MPa, 2.7-3.6 GPa, and 4-7 percent, respectively. Titanium-to-titanium tensile shear specimens of a poly(arylene ether 1,3,4-oxadiazole) exhibited tensile shear strengths at 23 and 150 C of 22.1 and 17.9 MPa, respectively.

Connell, J. W.↗
NASA NTRS1991

Chemistry and adhesive properties of poly(arylene ether)s containing heterocyclic units

Novel poly(arylene ether)s containing heterocyclic units were prepared, characterized, and evaluated as adhesives and composite matrices. The polymers were prepared by reacting a heterocyclic bisphenol with an activated aromatic dihalide in a polar aprotic solvent, using potassium carbonate. The polymerizations were generally carried out in N,N-dimethylacetamide at 155 C. In some cases, where the polymers were semicrystalline, higher temperatures and thus higher boiling solvents were necessary to keep the polymers in solution. Heterocyclic rings incorporated into the poly(arylene ether) backbone include phenylquinoxaline, phenylimidazole, benzimidazole, benzoxazole, 1,3,4-oxadiazole, and 1,2,4-triazole. The polymers were characterized by differential scanning calorimetry, solution viscosity, X-ray diffraction, thin film, and adhesive and (in some cases) composite properties. The glass transition temperatures, crystalline melt temperature, solubility, and mechanical properties varied depending upon the heterocyclic ring. The chemistry and properties of these materials are discussed.

Connell, John W.↗
NASA NTRS1993

Thermal processing of omicron-terphenyl - A Raman study

Raman spectra from omicron-terphenyl sealed in glass capillary tubes were acquired over temperatures ranging from 233 to 343 K. Over this temperature range, omicron-terphenyl can exist in the solid-crystalline, molten, supercooled-liquid, and amorphous-glassy states. Representative structure-sensitive spectra are presented along with a qualitative description. The lattice spectral feature occurring at a Raman shift of 123/cm displays abrupt changes in profile and intensity as melting and glass transition occur. The intensity ratio of the C-C stretching 996/cm line and C-H stretching line at 1008/cm displays a change related to the transformation of the ordered-crystalline phase to the disordered-liquid or glassy state. At a Raman shift of 1162/cm the solid-crystalline state shows a splitting due to the effect of the crystal field. The possibility of using these structure-sensitive Raman characteristics for in situ diagnostics during materials processing is discussed.

Biswas, A.↗
NASA NTRS1994

Polyimides Containing Pendent Ethynyl Groups

Series of novel diamines containing pendent ethynyl groups synthesized and reacted with various dianhydrides to form polyamide acids cyclodehydrated to form polyimides containing pendent ethynyl groups. Copolyimides of these diamines and other diamines also prepared. Upon heating, ethynyl groups in these polymers react with one another to form cross-linked molecular structures. As result of curing reaction, resistance to solvents, glass-transition temperature, and modulus of elasticity of polymer of this type generally increase, accompanied by corresponding decrease in tensile elongation and toughness. Cured polymers used as films, moldings, adhesives, and composite matrices.

Hergenrother, Paul M.↗
NASA NTRS1994

Aromatic Polyimides Containing Meta-Biphenoxy Moieties

Synthesis of two novel monomers and subsequent incorporation into aromatic polyimides yields polyimides containing meta-biphenoxy moieties exhibiting stability at high temperatures and having glass-transition temperatures lower than state-of-the-art polyimides containing para-biphenoxy moieties. Because of outstanding thermal stability, low density, resistance to radiation, electrical-insulating capability, toughness, and flexibility, linear aromatic polyimides used increasingly for applications in aerospace and electronics industries and possibly in others.

St. Clair, Terry L.↗
NASA NTRS1994

Acetylene- and Phenylacetylene-Terminated Poly(Arylene Ether Benzimidazole)s (PAEBI's)

Polymers prepared by first synthesizing polymers terminated with hydroxy groups, then reacting them with either 4-ethynylbenzoyl chloride or 4-fluoro-4'-phenylethynylbenzophenone. Endcapped polymers thermally cured to yield materials with attractive combination of properties. Cured acetylene-and phenylacetylene-terminated PAEBI's exhibit higher glass-transition temperatures and better retention of mechanical properties at high temperatures. Cured acetylene- and phenylacetylene-terminated polymers exhibit excellent adhesion to copper foil and polyimide film. Potentially useful as adhesives, coatings, composite matrices, fibers, films, membranes, and moldings.

Connell, John W.↗
NASA NTRS1994

Modified LaRC(TM)-IA Polyimides

Modified versions of thermoplastic polyimide LaRC(TM)-IA incorporate various amounts of additional, rigid moieties into backbones of LaRC(TM)-IA molecules. Modified versions more resistant to solvents and exhibit higher glass-transition temperatures, yet retain melt-flow processability of unmodified LaRC(TM)-IA.

St. Clair, Terry L.↗
NASA NTRS1994

Oligomers Terminated By Maleimide And Acetylene

Oligomeric molecules terminated with maleimide and acetylene groups synthesized and thermally treated to form cross-linked polymers exhibiting high or undetectable glass-transition temperatures and high thermo-oxidative stabilities. Compounds used to make thermally stable, glassy polymers.

St. Clair, Terry L.↗
NASA NTRS1994

Phenylethynyl-Terminated Polyimides

Phenylethynyl-terminated polyimides exhibit properties suitable for films, moldings, adhesives, and composite-material matrices. Polyimides synthesized at various molecular weights, selected to obtain desirable properties for specific applications. Thermally cured to form cross-linked molecular structures typically increasing resistance to solvents, glass-transition temperatures, and moduli of elasticity. Also have low molecular weights and low melt viscosities for easier processing into composites and adhesive bonds.

Jensen, Brian J.↗