The synthesis of soluble imide oligomers of controlled molecular weight endcapped with the 3-phenylethynyl aniline endcapping agent is described. The 3-phenylethynyl aniline was employed to afford a higher curing temperature (380 - 420°C) which widens the processing window compared to commercially available acetylene endcapped polyimides. The polyimides were synthesized via solution imidization techniques, using the ester-acid of various dianhydrides and various aromatic diamines. The “one pot” procedure utilizes NMP as the solvent and o-dichlorobenzene as the azeotroping agent, and reproducibly exhibit fully imidized soluble polyimides with Mw/Mn values of ∼2.0 determined by gel permeation chromatography. Thermally cured samples display good solvent resistance and exhibit glass transition temperatures comparable to their high molecular weight linear analogs. These polyimides show excellent thermal stability at 700°F aged in air up to 100 hours. Mechanical testing has been performed on a pyromellitic dianhydride (PMDA) and a phosphine oxide-based system. The flow properties have been studied by parallel plate rheology and the cure cycle has been monitored using 13CNMR.

As part of a program to develop high performance structural resins for aerospace applications, a variety of poly(arylene ether benzimidazole)s were prepared by the nucleophilic displacement reaction of activated aromatic difluorides with bis(hydroxyphenylbenzimidazole) monomers. Glass transition temperatures as high as 364°C and room temperature unoriented thin film tensile strength and modulus as high as 22.8 ksi and 652 ksi, respectively, were observed. The polymer prepared from the reaction of 1,3-bis(4-fluorobenzoyl)benzene and 5,5′-bis[2-(4-hydroxyphenyl)benzimidazole] was selected for more extensive evaluation. To improve the processability, the molecular weight was reduced by upsetting the stoichiometry. Polymers were endcapped with benzimidazole, ethynyl and phenylethynyl groups. The chemistry, physical, and mechanical properties of these materials are presented.

In an effort to improve the processing characteristics of addition-type polyimide matrix resins, the use of flexibilized three-ring and four-ring aryl diamines was investigated. A series of fourteen diamines containing carbonyl and methylene, as well as thio connecting groups was synthesized. The diamines were polymerized with the dimethyl ester of 3,3,′4,4′-benzophenonetetracarboxylic acid, using the monomethyl ester of nadic acid as an end cap. The effect of diamine structure on the solubility and rheological properties during cure, as well as glass transition temperature and thermooxidative stability of neat resins was determined. Graphite fiber-reinforced composites were fabricated from selected resin formulations, and composite thermomechanical properties were investigated.

New polymers with exceptional properties are needed for applications in high-performance structures, novel electrical, optical and electro-optical devices, and for multi-functional smart materials. Concurrently, new computational capabilities and methods for properties prediction and analysis have enabled the study of a variety of polymer chain architectures to examine the principles that govern their high-performance properties. By semi-empirical and ab initio computational methods, flexible, stiff-chain, rigid-rod, and biological structures could be analyzed. Single chain molecular stress-strain curves for axial tension and compression were calculated, and the strain dependence of the molecular modulus and vibrational frequencies were compared to measurements of molecular deformation, such as IR and Raman spectroscopy. However, of special interest is the distinctly different response of alpha-helical biopolymer chains to strain. Indeed, in this study we compare on a theoretical basis the ‘spring-like’ microscopic mechanical response of alpha-helical biopolymers having a reinforcing intra-molecular hydrogen bonding network to analogous synthetic extended chain polymers, especially poly(para-phenylene terephthalamide) (PPTA) [KEVLARTM]. The theoretical verification of the absence of compressive buckling in alpha-helical biopolymer chains rationalizes the molecular elasticity and resistance to ‘kinking’ of those strands, manifested by the prevalence in Nature for coiled coils. The understanding of the structure-tofunction relationship in biopolymers explaining the role of the alpha-helix in these systems as a requirement for superior compressive mechanical properties, may enable new guidance for the synthesis of motifs consistent with molecular frameworks optimized by Nature.

Although benzobisazole rigid-rod polymers are well recognized for their outstanding tensile and modulus properties as well as their excellent thermal and thermal-oxidative stabilities, they can only be fabricated from acidic solvents. Benzobisirnidazole polymers containing pendent benthiazole or sulfonic acid groups exhibit partial solubility in DMSO. Reaction of these systems with sodium methylsulfinylmethide in DMSO abstracts the acidic proton from the benzobisimidazo unit and forms the polyanion. Subsequent reaction of the polyanion with 1,3-propanesultone provides water soluble systems. Random copolymers containing benzobisimidazo and benzobisthiazole repeat units were prepared in polyphosphoric acid and subjected to the derivatization reaction. Benzobisimidazo content in the copolymers varied from 10–0 mole percent. Minimum ionic charge along the rigid-rod backbone to obtain water solubility was determined to be approximately 10 mole percent as reflected in the benzobisimidazole repeat units.

This paper represents a review of the literature on high performance films. Topics discussed include properties of major polyimide films, the preparation of polyimide films via vapor deposition techniques, special extrusion methods for liquid crystalline polymerlpolyimide blends, factors affecting color in polyimide films, and barrierf ilms.

New negative-type photosensitive polyamic acids, precursors of polyimides, developable with aqueous alkaline solutions were synthesized. Among diamine monomers having a photofunctional group, 2′-(methacryloyloxy)ethyl 3,5-diaminobenzoate yields high molecular weight polyamic acids. A photosensitive polymer composition consisting of Michler's ketone and a thermally stable peroxide, 3,3′,4,4′-tetrakis(t-butyldioxycarbonyl)-benzophenone, results in high photosensitivity. The photosensitive coating of the composition is soluble in aqueous alkaline solutions as well as in organic aprotic polar solvents and either can be used as a developer of the coating. However, resolution of the patterns is quite different especially with thick films. Solvent developers result in poor resolution whereas aqueous alkaline developers give patterns of high aspect ratio even with thick films. This is probably due to the difference in the diffusion efficiency and swelling property of the polymer to the developers.

Effects of copolymerizing with an ordinary diamine monomer on photosensitivities and film properties of the polyimides derived from their precursors are also discussed.

Polymers based upon inorganic elements are becoming more important in the field of high performance materials because of their outstanding mechanical, thermal, and chemical stabilities. Polyphosphazenes, which are based upon a nitrogen-phosphorus backbone, show promise as new engineering materials. The large number of different chemical structures possible allows the synthesis of phosphazene polymers for specific applications. Initial tests on one phosphazene-based set of materials show that they are very hard, stiff, and have excellent resistance to organic chemicals.

Much work has recently been done on the interphases between fibres and polymers and it is now clear that they are usually brittle. This requires a re-examination of our basic ideas on how composites achieve their strength. The Kelly-Tyson approach, which is based on ductile metal behaviour, has never received unequivocal support. In addition, work during the last ten years in the author's laboratories indicates that it is incorrect in one of its major predictions: that the stress-strain plots of short aligned fibre composites should be curves. In this paper we present the evidence and introduce a new failure model which eliminates these difficulties.

X-ray photoelectron spectroscopy (XPS), tensiometric wetting, and atomic force microscopy (AFM) were used to investigate the surfaces of PAN based carbon fibers. Percent weighted dipole moments (%WDM) were computed from the deconvoluted core level carbon XPS peaks. Tensiometric wetting separated the polar and dispersive components of the fiber surface energies thus allowing the computation of % polar energies. AFM provided quantitative measurements of the fiber surface rugosities. Scanning electron microscopy (SEM) revealed adhesive interfacial fractures for PMR-15 (polyimide) composites of fibers with low %WDM and % polar energies; cohesive fractures for those with high %WDM and % polar energies. Increasing interlaminar shear strengths correlated with increasing %WDM and % polar energies. The fibers with higher %WDM also had rougher surfaces which probably expose more graphite basal plane dipole sites in addition to providing mechanical interlocking.

Initial results are presented of in-situ wide angle x-ray scattering (WAXS) studies of structure and orientation development in the draw zone during fiber spinning of poly(p-phenylene benzobisoxazole) (PBO) in polyphosphoric acid (PPA). The diffraction patterns show strong [100] and [002] reflections. The coherence lengths found from the [100] and [002] reflections are consistent with those found by electron diffraction and suggest important elements of the microfibrillar morphology may already be present in the texture of the extrudate before coagulation. Increasing extrusion rate is found to produce little improvement in molecular orientation. Molecular orientation initially increases with increasing draw ratio from 1 to 10 but, remains constant for higher draw ratios. These results are compared to an affine deformation model during elongational flow.

A modification of the transverse photothermal deflection technique is used to determine the thermal diffusivity of high-conductivity carbon fibers. A chopped, cw Ar-ion laser beam is focused to a point on the fiber to cause localized heating of the fiber and the surrounding transparent fluid medium. The fluid medium immediately adjacent to the fiber sample is probed with a weak He-Ne laser that grazes the fiber surface and is deflected by the synchronous thermal lens produced near the fiber surface. The pump laser is stepped along the length of the fiber to produce a photothermal signal that is proportional to the temperature gradient along the length of the fiber. The theory necessary to predict the magnitude and phase of the probe beam deflection as a function of distance along the fiber is derived. This signal can then be analyzed by a simple method to determine the thermal diffusivity of the carbon fiber.

The use of fibrous reinforcement to improve the strength and deformation properties of concrete is now well established. The concept of fibre reinforcement is to use the deformation of the matrix under stress to transfer load to the fibre. Substantial improvements in static and dynamic strength properties could then be achieved if the fibres are strong and stiff, and loaded to fracture, provided there is, of course, a minimum fibre-volume fraction.

Besides fibres like asbestos, glass and steel, different kind of synthetic fibres such as polyethylene, polypropylene, polyamide and others are recently used for cementitious composites.

Together with general aspects of synthetic fibre concrete composites, original results concerning the study done on a hybrid composite based on steel and polypropylene fibres will be presented and discussed.

The plasma polymerization of allylamine onto PAN-based carbon fibers has been investigated by XPS and SEM. Fiber surfaces were cleaned and oxidized by treating in an O2 plasma for 5 minutes. XPS characterization of pretreated samples showed surfaces functionalized with COH, C=O, and COOH species. Fibers were coated with poly(allylamine) by introducing a monomer-saturated stream of Ar gas into the plasma chamber and reacting for 2–30 minutes. Some samples were further treated in an O2 plasma to determine the stability of the polymer coating. N(1s) spectra from the fiber surfaces showed two nitrogen peaks at 399.9 eV and 400.7 eV, which may be assigned to amine and amide groups, respectively. C(1s) spectra showed a decrease with treatment in the amine carbon peak at 287.2 eV and a corresponding increase in the peak at 288.7 eV, assigned to carboxyl and/or amide carbon. The increase in higher binding energy carbon corresponded with an increase in the 400.7 eV N(1s) peak and gave evidence for the conversion of amines to amides. Total O(1s) intensity initially decreased following 2 minutes of allylamine treatment but increased with further treatment. In addition to the carbon fiber samples, a silica wafer was coated and analyzed by XPS. Quantitative XPS results revealed that all the nitrogen present on the surface was in the form of amine and not amide species. Oxygen in the form of carboxyl species was also present on coated silica surfaces. Most likely, oxygen dissolved in the allylamine monomer was carried into the plasma chamber by the Ar gas. Comparing the silica wafer with the fiber samples revealed treatments were strongly sample dependent. Even after only 2 minutes of treatment, fiber samples showed the presence of amide and substituted amide species. SEM revealed coatings filled in the fiber grooves and left clumps of material on the surface. Micrographs of fibers treated for longer times revealed surfaces covered with particles < 0.25 μm in diameter. Finally, the XPS N(1s)/C(1s) and O(1s)/C(1s) intensity ratios indicated that some polymer remained on the fiber surface even after 15 minute post O2 plasma treatment. These experiments illustrate that plasma polymerization can be effective for functionalizing carbon fiber surfaces, but that optimization of conditions is necessary to improve wetting and control amine/amide functionality.

The compressive properties of PAN-based carbon fibers were measured with the tensile recoil method. The recoil fracture morphology was also examined. In addition to the axial compression, flexure during recoil affects the test results. Compressive failure may be caused by shear failure and/or bending buckling of the microfibrils/fibrils. Study of the evolution of compressive and other mechanical properties discloses that, in general, the compressive strength bears a similar tendency to the tensile strength and torsion modulus up to the carbonization temperature of 1500 °C. However, the relative rate of evolution is different. To some extent, the bending buckling mechanism may relate to the structural units which govern the tensile strength.

High-modulus PPTA and PBZT fibers have been surface-modified by a series of reactions to allow single filaments to be wetted by sodium silicate solutions as well as to exhibit good adhesion to the resulting glass coating. Fiber surfaces have been characterized by X-ray photoelectron spectroscopy. PBZT and PPTA fibers showed little or no decrease of their tensile properties as a consequence of the surface-modification. Glass coatings have been applied to single filaments by dip-coating in an aqueous sodium silicate solution followed by air- and vacuumdrying. Coated PPTA and PBZT fibers with shear moduli as high as 5 GPa have been prepared. The compressive strain at failure of PPTA filaments coated with a 0.5 μm thick air-dried coating was raised to 0.6%, compared to 0.4% for uncoated filaments.

Understanding elastic interactions that occur between an embedded optical fiber and its host matrix is important for implementation of optical strain sensors into aerospace structures. A previously developed analytical model predicted the effects of fiber coating thickness and elastic modulus on the strain transfer from an isotropic matrix to an embedded coated fiber optic.To verify this, a series of tungsten coatings were magnetron sputtered onto bare intrinsic Fabry-Perot sensors that were embedded into a neat epoxy resin block. The block was then compression loaded and the optical strain response measured. Comparisons between the responses of both coated and uncoated sensors indicated that the experimental measurements of strain transfer in embedded fiber-optic devices yielded good agreement with the analytical model.

The adhesion of spuncast polyimide layers is critical to the success of manufacturing multilayer polyimide structures. We have measured the interpenetration between a spuncast layer of the meta isomer of poly(amic ethyl ester) (PAE) precursor of PMDA 4,4′ ODA and the corresponding partially-imidized PAE base layer using forward recoil spectrometry (FRES). Measurements made with FRES show that interpenetration decreases rapidly as the base layer imide fraction f increased, with a corresponding decrease in adhesion measured with a T-peel test. In the region of f > 0.8, differences in interfacial adhesion are still observed while the interfacial widths are too narrow to be differentiated with FRES. For this reason, the use of other high resolution techniques for profiling the polyimide-polyimide interface such as nuclear reaction analysis, dynamic secondary ion mass spectrometry, and neutron reflectometry will be necessary to accurately correlate interpenetration with adhesion.

The basic concepts of in-situ molecular composite formation are presented with a polyazomethine/nylon 6 system as an example. All the encouraging attributes of molecular reinforcement - increased Tg, solvent resistance and mechanical properties were observed. This provides a commodity resin, such as nylon 6, with properties normally associated with higher priced engineering/specialty resins.

The phase behavior of a blend containing poly(hydroxypropyl L-glutamine) (PHOPG), a rod-like polypeptide, and a coil-like random copolymer with 1.5% hydrogen bonding monomer is studied using light scattering, DSC, and optical microscopy. The results show that this system has an LCST. The phase behavior of this binary system is complicated by a glass transition and an ordering transition. Evidence is also observed of a possible change in phase demixing process, from nucleation and growth to spinodal decomposition, for a 50/50 blend with a change in temperature.