High-performance structural adhesive joints, such as those found in aerospace components, must exhibit both high initial strength and excellent durability in severe environments. These properties are largely governed by the bonding across and the stability of the metal (oxide) - polymer interphase. Modern adherend surface treatments commonly provide the required adequate strength and durability. Factors that can degrade or limit joint strength and durability are discussed along with means to improve or, at least, to prevent reduction of these properties. Finally, areas in which advances are needed: increased understanding of bonding phenomena, improved reliability of bonded joints, and minimized environmental burdens are suggested.

Diffusion of cations along the polymer/metal interface controls the rate of blistering of polymer coatings on metals exposed to electrolytes. Cations are driven by both concentration and electrical potential gradients. A theoretical and experimental study was carried out on the diffusion of sodium ion along the polymer coating/steel interface under an applied potential. Mathematical models, consisting of initial and propagation stages, are derived based on a moving boundary diffusion problem. Model variables include ion diffusivity, potential gradient and distance between defects and delamination sites. Models are solved to predict ion fluxes and concentration in the blistering areas. Experimental data are analyzed to extract model parameters. Model predictions agreed well with experimental data and practical observations

Fusion bonded epoxies (FBE) in combination with cathodic protection (CP) are widely used to protect the soil side of buried pipelines from corrosion. Cathodic disbonding of the epoxy coating is a degradation process involving accelerated loss of adhesion of FBE because of chemical and electrochemical processes due to CP. Results are presented from our study to determine the mechanism of cathodic disbonding.

Disordered glass microsphere-epoxy composites have been used in a study of diffusional, electrical and mechanical effects of interfaces in polymer-matrix composites exposed to pure water. Mass gain measurements on composites manufactured from 10 μm silane-treated microspheres indicate initial near-Fickian diffusion with water saturation times on the order of 500 h. However, electrical measurements indicate water transport at rates at least 100 times more rapid. This behaviour is interpreted in terms of a cellular microstructure with areas of low cross-link density separating highly cross-linked areas. Rapid water transport can thus occur in areas of low cross-linking, even without the contribution of connected clusters of particles where rapid interfacial water transport occurs substantially ahead of the main diffusion front. Reductions in ultimate tensile strength and fracture energy in dry and water-saturated tensile test specimens are observed with increasing volume fraction of glass spheres but with a distinct plateau between about 6% and 12% Vf. This can be explained in terms of secondary cracking below the percolation threshold which causes toughening of the composite. However, a few % above pc (≍ 16%), most particles belong to the percolating cluster and the primary crack can grow without hindrance.

The influence of polymer coatings on the mechanical properties of optical fibers is reviewed. Current research is directed at the chemical mechanisms that contribute to mechanical strength degradation. In this review, a chemical kinetics model is applied to describe the influence of polymer adhesion on chemical corrosion of the glass fiber (i.e., its zero stress and stress assisted corrosion).

This work examines the usefulness of the single-fiber fragmentation test in studying the durability of fiber/matrix interfaces/interphases. This test measures the critical length/diameter ratio (L/D) of the fiber fragments formed in the test and relates this length to the interface's strength, or ability to transfer load. In the work reported here, we immersed samples of epoxy containing a single-glass fiber - that was previously sized with an epoxy-compatible coating - in either 65 or 75 °C water and tested after different times of exposure. In general, this ratio increased as a function of time of exposure to water. During exposure at 75 °C, the fibers' L/D in the samples did not increase significantly until after the sample reached its “apparent” equilibrium content of water ∼ (3.0 wt%). Because there was no significant measurable change in the tensile modulus between wet and dry samples, we cannot attribute these differences in L/D to changes in the resin's properties due to plasticizing by water. A small percentage of samples exposed at 65 °C did not show a significant increase in L/D, and in these cases the moisture produced a marked roughening of the fiber surface along the fiber/matrix interface. One possible explanation is that the attack by moisture degrades the interface, thus reducing its strength with a corresponding increase in the L/D. To varying degrees, however, the attack by moisture also degrades the E-glass fiber. This attack by moisture roughened the surfaces of the fibers and increased the distribution and/or size of the critical flaws, thus reducing both the strength of the fiber and the L/D. Based on our preliminary results, it appears that the singlefiber test has the potential to be useful for studying the durability of the resin/matrix interface providing that the influence of the environmental agent on all of the components of the model composite: resin, fiber, and interface/phase, is considered.

A diepoxide containing a disulfide linkage, capable of being reductively cleaved was synthesized. This compound was cured with standard amine curing agents to form intractable network polymers, which were then decomposed into soluble fragments, allowing the effects of network formation on cure chemistry to be evaluated. This system was then used to investigate the effects of an alumina interface on epoxy network formation.

The preparation of a hybrid conducting polymer/high-temperature superconductor device consisting of a polypyrrole coated YBa2Cu3O7-δ microbridge is reported. Electrochemical techniques are exploited to alter the oxidation state of the polymer and, in doing so, it is found for the first time that superconductivity can be modulated in a controllable and reproducible fashion by a polymer layer. Whereas the neutral (insulating) polypyrrole only slightly influences the electrical properties of the underlying YBa2Cu3O7-δ film, the oxidized (conductive) polymer depresses Tc by up to 15K. Thus, a new type of molecular switch for controlling superconductivity is demonstrated.

The viscoelastic properties of model epoxy resins were investigated using a tensile stress relaxation technique. Simultaneously with the applied tensile strain, the specimens were subjected to torsional oscillations in a specially designed instrument. The crosslinking density of the resins was controlled by the length of the aliphatic chain of the diamine hardener. Smooth master curves were obtained by Computed Aided Superposition (CAS) using a software written especially for this work. The shape of the master curves changed significantly as a function of the frequency of torsional oscillations. Based on previously published data, as well as on pertinent information from literature, it was concluded that the variations in the shape of the master curves reflect corresponding changes in the spectrum of relaxation times of the investigated resins. It is suggested that the observed phenomena are caused by physical processes, such as short-range orientation and physical aging, which lead to densification of the network and a corresponding increase in its stiffness.

Bonding of chromium to the polyimide, PMDA-ODA, surface is still subject to debate. In an attempt to clarify this problem, we have performed density functional theory calculations on a model molecule, phthalimide, which contains the most reactive functionalities of the polyimide PMDA part. Considering only the low spin case, we find that chromium bonds preferentially to the phenyl ring. However, when we release the spin polarisation and optimise the structure, we find that the absolute stable configuration is that of chromium in a quintet state at a carbonyl group. The energy difference is 0.30 eV. The complete optimised structures are determined. The infrared spectrum have been calculated for phthalimide and compared to experimental spectra. The agreement is excellent. A vibrational analysis for the Cr/phthalimide system, in both configurations (Cr on C=O and Cr on phenyl), in their stable spin states, is presented.

We have used X-ray Photoelectron Spectroscopy to study the chemical interactions at the interface formed during in situ deposition of Ti atoms on triazine, polyimide (PMDAODA), and polystyrene surfaces. For deposition on thin triazine films (∼ 100Å) we observe that titanium carbide is the dominant product, while oxides and nitrides are formed as well. Aging in air causes the carbide and nitride to convert to the more thermodynamically stable oxide. Titanium carbide is also the primary species at the Ti/polyimide and Ti/polystyrene interface. In all cases the reaction of Ti atoms with different sites in the polymer is nonselective.

A family of new high performance thermoplastic polyimide adhesives have been developed that can be directly laminated to ceramic substrates, polyimide films, metals, and/or sputter metallized for eventual circuit fabrication. We have also developed new heat-sealable composite polyimide films that have improved CTE, that have good dimensional stability and that can also be directly bonded to ceramic and glass-ceramic substrates. With these new material sets various constructions of ceramic-polyimide rigid-flex systems can be conceived to meet the more demanding needs of the electronics industry. We have laminated these films to metals and various ceramic substrates such as alumina, aluminum nitride, glass, etc. and investigated the bond stabilities under thermal and humidity aging conditions.

Enhancement of the adhesion of thin copper films on polytetrafluoroethylene substrates was found when the substrate surface was irradiated with a pulsed UV excimer laser prior to metal deposition. The interaction between the laser and the polymer was examined by characterizing the neutral and charged species emitted from the surface during irradiation. The nature of the species emitted indicates that significant chemical modification of the polymer surface occurs. In addition to chemical modification, the interaction with the laser also alters the surface morphology of the polymer. Irradiation at fluences of ∼ 0.6 J/cm2 results in an overall planarization of the surface, whilst irradiation at higher fluences results in the formation and enlargement of voids and localized melting.

Backside laser irradiation of metal coated quartz wafers can be used for transferring material imagewise to a target cleanly in absence of chemical vapors. However, the transfer of metal from a quartz surface requires a high energy, resulting in poor quality images in some cases.

Use of a fluorinated polymer layer between the polycarbonate substrate and a Te-Se metal alloy is known for clean fabrication of metal holes in write-once-read-only memory media. Polymer thin films can be used similarly in backside laser irradiation for metal ejection from substrate surfaces.

With an ArF excimer laser at 193 nm, gold and aluminum were ejected imagewise from Teflon AF, poly(methacrylonitrile) and quartz surfaces. The threshold fluences of 230 Å thick gold films from polymer surfaces were 7.9 to 9.7 mJ/cm2, whereas the threshold fluence from a quartz surface was determined to be 17.8 mJ/cm2. For aluminum on top of quartz and aluminum coated polymers, higher threshold fluences were found compared to gold ranging from 12.96 to 22.7 mJ/cm2.Also, interfacial reactions between the polymer films and the metal layer occured and have been studied by SEM and XPS.

The interface formed between metals, Ti and Cr, and polymers, epoxy and triazine, have been studied, nondestructively, using x-ray absorption spectroscopy. The metals were sputtered onto the polymer surfaces. Titanium reacts extensively, up to Ti thicknesses of 100 Å while Cr remains primarily metallic. In situ heating at 200°C increases the extent of reaction for both metals. Heating has a greater effect on metal/epoxy interfaces than metal/triazine. Titanium and Cr were ion implanted into the polymer in order to determine the interactions of isolated metal atoms with the polymer. Titanium and Cr appear to form oxides as the final reaction product, and the Ti is tetrahedrally coordinated.

Since the interphase plays an important role in the strength and durability of a bondline, it is essential to develop a fundamental understanding of the interphasal region in order to predict and control its structure and properties. Therefore, a key aspect to consider it's the influence of specific chemical interactions on the evolution of the structure of the interphase. Surface analysis techniques have been very effective in studying this problem. However, such experimental approaches do not yet have the specificity and resolution to obtain the atomic densities as a function of distance from the surface. In this work we employ molecular dynamics calculations to study adsorption of both monomers and polymer networks. The density profiles, molecular structure, and mobility of epoxy, amine, and polymer networks on the surfaces of graphite and alumina have been examined. We have considered the influence of changes in surface chemistry on the interfacial structure of the adsorbed species. For all systems investigated the presence of a surface resulted in pronounced ordering of atomic species in the near surface region. Specific chemical interactions such as hydrogen bonding further enhance this tendency and leads to near-surface structure which is markedly different from the bulk.

In situ Fourier transform infrared (FT-IR) spectroscopy in an attenuated total reflection mode has been employed to examine the chemical modification of ozone-cleaned, native oxide on single crystal Si(100). This planar model for silica is subjected to solution modification using silane reagents which impart variable polarity to the modified surface through different termini of the modifier. These chemical modifications are followed in real time using FT-IR as a diagnostic of surface preparation; either by examining the appearance of a band for an IR chromophore of the modifier which is attached, or by monitoring the loss of the surface silanol groups consumed in the reaction. Following modification, polymer solutions (acrylates, siloxanes) are introduced into the cell and the dynamics of the adsorption followed by a chromophore of the polymer. Not only can the total amount of bound polymer be determined, but also in cases of strong interactions between the polymer and surface modified (e.g. OH... O=C) the bound fraction can be determined. Correlations between surface polarity and these experimentally determined quantities give insight in to the configuration(s) of the adsorbed polymers.

Overlayers of Cr and Ni deposited at 22 °C and at -100 °C onto self-organized molecular assemblies of HS(CH2 )11 CN on Au have been studied by X-ray photoelectron spectroscopy (XPS). For the lowest overlayer coverages, the C ls and N is core levels show evidence for a chemical interaction between the deposited metal and the CN end-group. At 22 °C and for coverages of 0.6 nm or more, Cr/CN shows a C ls low binding energy, carbide-like shoulder. The degree of metal penetration is assessed based on the attenuation of the C ls and Au 4f intensities, and on the positions of the Cr and Ni 2p levels. In general, the least penetration and the largest binding energy shifts of the C, N, and the Cr or Ni core levels are observed for Cr/CN, while and Ni/CN shows smaller shifts and greater penetration.

Molecular dynamics simulations of alkane chains chemically tethered to silica surfaces are presented. The system was modeled after the stationary phases of chromatographic columns. The interphase properties were computed as functions of chain length, surface bonding density, and temperature. At densities appropriate for chromatography, the chains undergo a gradual transition with increasing temperature from a glassy state to a liquid-like state. The simulations are consistent with extensive experimental data including neutron scattering, NMR, IR, and EPR methods. The implications for chromatographic retention are discussed. In a second series of studies, we explored driving forces for observed ordering on the solid surface in terms of the various components of the chain-chain and chain-surface forces. Interfacial z profiles are sensitive functions of each of the forces. Finally, we present preliminary Monte Carlo results on origins of tilt behavior in self assembled monolayers of rigid chain species on metal surfaces.

X-ray and neutron reflectivity was used to determine the kinetics of adsorption and characterize the concentration profile of a low molecular weight trichlorosilane endfunctionalized polystyrene adsorbed onto polished silicon wafers. Higher adsorbed amounts were obtained from a cyclohexane solution of the polymer rather than toluene, with kinetics that followed a stretched exponential behavior. For moderately high grafting densities the polymer concentration profile in deuterated toluene (a good solvent) was best modelled using a small tailed parabola. In deuterated cyclohexane (a poor solvent) the brush profile steepened but was substantially smoother than a step, while in D20 (a non-solvent) it became step-like.