Metal-polymer Interface Research Articles

Adhesion between aluminum and copolymers of ethylene and vinyltrimethoxysilane

AbstractThe adhension between aluminum and poly(ethylene‐co‐vinyltrimethoxysilane) (EVS) and poly(ethylene‐co‐butylacrylate‐co‐vinyltrimethoxysilane) (EVSBA), respectively, have been studied. For comparison an ordinary low density polyethylene (LDPE), a poly(ethylene‐co‐butylacrylate) (EBA), and an ionomer regarded as a bonding polymer were studied as well. The peel strength of laminates obtained by pressing were measured by a T‐peel test. The structure of the fracture surfaces were investigated by reflection‐IR, ESCA, and SEM. The peel strength of the LDPE and the EBA samples were 100 and 700 N/m, respectively. Although the amount of vinylsilane was low, about 0.2–0.3 mol %, its presence had a pronounced influence on the adhesion: 1800 and 3000 N/m for EVS and EVSBA, respectively. This is even higher than the value observed for the ionomer, 1560 N/m. Although there was a marked difference in surface topology, the SEM and ESCA analysis showed that the fracture was cohesive for both EVS and EVSBA. Immersion in water at 85°C increased the peel strength even more, especially in the case of EVSBA (up to 9000 N/m), in contrast to what is normally observed with aluminum polyethylene laminates. The results suggest that strong and nonhydrolyzable bonds, e.g., covalent bonds, have been formed across the polymer‐metal interface for the ethylene copolymers containing vinylsilane.

Fluorinated Monomolecular Assemblies: Model Systems to Probe Chemical Interactions at the Polymer-Metal Interface

AbstractMonomolecular assemblies of n-perfluorocarboxylic acids [CF3(CF2)nCOOH, n=6, 7, and 8] have been adsorbed at silver by spontaneous adsorption from dilute alcoholic solutions. Infrared reflection spectroscopy indicates that the monolayers form by the reactive chemisorption of the carboxylic acid to a carboxylate salt. Ellipsometrically-determined film thicknesses increase linearly with an increase in the perfluorocarbon chain length. Contact angle measurements indicate that the packing density of the films increases with increasing chain length, resulting in an interface with a low surface free energy. The structure and properties of these interfaces are compared to those for monolayers of n-alkanoic acids. The potential utility of such assemblies to probe fundamental chemical and physical interactions at polymer-metal interfaces are also discussed.

Fluorinated Monomolecular Assemblies: Model Systems to Probe Chemical Interactions at the Polymer-Metal Interface

AbstractMonomolecular assemblies of n-perfluorocarboxylic acids [CF3 (CF2)nnCOOH,n=-6,7, and 8] have been adsorbed at silver by spontaneous adsorption from dilute alcoholic solutions. Infrared reflection spectroscopy indicates that the monolayers form by the reactive chemisorption of the carboxylic acid to a carboxylate salt. Ellipsometrically-determined film thicknesses increase linearly with an increase in the perfluorocarbon chain length. Contact angle measurements indicate that the packing density of the films increases with increasing chain length, resulting in an interface with a low surface free energy. The structure and properties of these interfaces are compared to those for monolayers of n-alkanoic acids. The potential utility of such assemblies to probe fundamental chemical and physical interactions at polymer-metal interfaces are also discussed.

Influence of metallic electrodes on electrical tree initiation in polyethylene

The nature of electrical tree initiation in polyethylene is treated with a focus on the influence of the nature of metallic electrodes on the initiation voltage. It is shown that the latter parameter varies linearly with the work function of the oxide-covered metallic electrode. This suggests that charge injection in polymers depends on the oxide layer at the metal-polymer interface.

Fracture of Metal-Polymer Interfaces with Fine-Line Geometries

AbstractThe fracture behavior of metal-polymer line structures as a function of dimensions was investigated using a stretch-deformation technique. The effects of line orientation, line width and film thickness are reported in this paper. When the line orientation is parallel to the stretching direction, only formation of cracks normal to the lines is observed. However, when the line is perpendicular to the stretching direction, delamination becomes the dominant mode of fracture. Wide lines (16μm) exhibit larger shear stress at the edge of the metal-polymer interface, thus delaminate earlier than narrow lines (4μm). By decreasing the metal film thickness, the depth of stress penetration at the interface decreases, making the propagation of cracks more difficult in thin films than in thick films.Finite element analysis was carried out to account for the experimental observations and good agreement was obtained. In the analysis, the plastic deformation characteristics of the metal and the polymer have been specifically taken into account. In comparison with a linear elastic analysis, the linear model predicts significantly higher stress levels and local concentrations than the nonlinear model.

CHEMISTRY AND MICROSTRUCTURE AT METAL-POLYMER INTERFACES

This paper summarizes the present knowledge of the chemistry and microstructure at metal-polymer interfaces formed on PMDA-ODA polyimide. The nature of chemical bonding was deduced from electronic structures observed by photoemission spectroscopies and interpreted by molecular orbital calculations. Bonding at the initial coverages of the metals studied are well described by the formation of metal-polymer complexes as a result of delocalized charge transfer from the metal to the polymer repeat unit. The microstructure was observed directly by transmission electron microscopy and ion scattering techniques. The morphology was found to be strongly dependent on the chemical reactivity of the interface. This correlation between chemistry and microstructure is important for understanding adhesion at the metal-polymer interface.

Chemistry of the Metal-Polymer Interfacial Region

In many polymer-metal systems, chemical bonds are formed that involve metal-oxygen-carbon complexes. Infrared and Mössbauer spectroscopic studies indicate that carboxylate groups play an important role in some systems. The oxygen sources may be the polymer, the oxygen present in the oxide on the metal surface, or atmospheric oxygen. Diffusion of metal ions from the substrate into the polymer interphase may occur in some systems that are cured at elevated temperatures. It is unclear whether a similar, less extensive diffusion occurs over long time periods in systems maintained at room temperature. The interfacial region is dynamic, and chemical changes occur with aging at room temperature. Positron annihilation spectroscopy may have application to characterizing the voids at the metal-polymer interface.

Delocalized bonding at the metal-polymer interface

This paper summarizes our current understanding of the nature of the chemical bond formed at the interface between a deposited metal atom and an underlying polyimide surface. The approach in these studies is based on the use of quantum chemical calculations to interpret photoemission spectroscopy results. By focusing on the initial reaction between a chromium atom and the PMDA-ODA polyimide repeat unit, the bonding is demonstrated to be delocalized, arising from the formation of a charge-transfer complex between the metal atom and the PMDA unit of the polyimide. Stabilization of the complex involves the transfer of electronic charge from the metal d states of chromium to the lowest unoccupied molecular orbital of the π system of the PMDA unit of the polyimide. The complex proposed is energetically favored over that involving a direct local interaction between the chromium atom and one of the carbonyl functional groups. The distribution of single-particle electron energy levels deduced from molecular-orbital calculations can account for the spectroscopy results. The formation of such delocalized metal-polymer complexes is also inferred from a related study of the chromium/PMDA-PDA interface.

Adhesion of polymers to metals: A review of the results obtained studying a model system

AbstractFollowing a rapid overview of the various mechanisms involved in the process of polymer adhesion to metals, modelling is attempted in order to describe, at molecular scale, the metal—polymer interface in its technological reality. In particular, the concept of an active site is explained and the role of these sites in bonding mechanisms is illustrated.The model is then verified in laboratory using experimental and theoretical techniques appropriate for the problem posed. Electrochemistry in organic environment is used to impart to the surfaces a homogeneous acid or basic character (i.e. electron acceptor or donor) and to set up an intense anisotropic electrical field allowing nonrandom simulation of the fields induced by the roughness of the potential of the surface of an industrial object. It is thus demonstrated that the establishment of a strong metal—polymer bond results from so‐called Lewis acid—base reactions activated by the interfacial electrical field.Finally, the ageing study of the metal—polymer system thus obtained shows that the interface is stable relative to atmospheric oxygen and that vacuum pyrolysis converts the polymer into a carbon fibre strongly bonded to the metal.

On the mechanism of electrical conduction in plasma-polymerised thiophene thin films

High-field dark conduction in (DC discharged) plasma-polymerised thiophene thin films is reported. A contact-limited Schottky mechanism is found to predominate at fields above 3*105 V cm-1. The dominance of the Schottky mechanism is confirmed by employing dissimilar-electrode configurations. The metal-polymer interface barrier heights calculated are 1.22 eV for Al, 1.03 eV for Cu and 0.879 eV for Au.

Investigation of a metal-polymer interface by X-ray photoelectron spectroscopy

A method has been developed for studying metal-polymer interfaces by means of X-ray photoelectron spectroscopy, which exploits the effect of differential charge formed on the metal-polymer interface by shifting the potential of the metal phase. As an example, the fracture surface of a copper-plated-acrylonitrile/1,3-butadiene/styrene terpolymer was studied.

Contact of Polymer Latex Particles with Metals

Abstract Polymer latex has been widely used in decorative paints and adhesives. It is now beginning to find applications in steel protection where coalescence to form an impermeable coating is a prime requirement. The mechanism by which latex particles coalesce into a coherent and adhering coating is described. Electron microscope observations demonstrate that latex spheres deform elastically as coalescence occurs, this elastic deformation being driven by interfacial attractive forces. Direct measurement of the deformation allows the calculation of the interfacial energy both for the polymer-polymer interaction and for the metal-polymer interface.

Properties of the metal–polymer interface observed with space-charge mapping techniques

A space-charge mapping technique was used to measure the space charge and internal electric fields in metal–poly(ethylene terephthalate)–metal samples subjected to electric field stressing. From these results the role of electronic injection and transport in the observed electrical properties of capacitor structures could be inferred. Below breakdown fields, there was no evidence that the dark current can be attributed to electronic processes. As breakdown fields were approached, a decrease in the injected space charge trapped adjacent to the electrode was observed. This behavior was attributed to the onset of tunneling into high-energy electronic states in the polymer. This process may produce large current densities leading to dielectric breakdown.

The effect of heat on the molecular weight of poly(ethyl 2‐cyanoacrylate) adhesive

AbstractSteel–steel ethyl 2‐cyanoacrylate adhesive bonds have been thermally aged and the polymeric material isolated from the glue line. Molecular weight measurement by gel permeation chromatography suggest that a significant degree of post‐curing occurs followed by a slight decrease in molecular weight. This decrease in molar mass is not considered large enough to explain the observed decrease in bond strength. It is postulated that poly(ethyl 2‐cyanoacrylate) undergoes thermal degradation in a manner similar to that reported for poly(methyl methacrylate). The loss in bond strength is thought to be due to the disruption of the polymer–metal interface by monomer molecules produced during the thermal depolymerization.

Trente Iznos : L. I. Kuxenova, V. M. Samylkin, V. I. Tolokonnikov and L. M. Rybakova, studies on structural changes of the surface layers of aluminium bronzes during friction, 5 (2) (1984) 227

Water absorption is believed to be one of the main causes leading to the deterioration and degradation of protective organic coatings. Water uptake in coatings on metal may be measured, for example, by changes in electrical impedance (e.g. capacitance and resistance) as a function of time. Generally coating capacitance is expected to increase during initial stages of water uptake (due to the greater dielectric constant of water) while coating resistance is expected to decrease (due to the lower resistivity of water compared to most polymers). However, here we present evidence that water ingress is not the main determinant of damage in epoxy-phenolic can coatings. Thus, as the degree of cure is increased, the water saturated coating capacitance and resistance both increase while the time-to-failure also increases. We suggest that these observations are due, respectively, to an increase in the free polymer volume (permitting more water uptake) and to an increase in the charge transfer resistance at the metal-polymer interface (due to a higher density of polymer-to-substrate bonding). These results and interpretations are supported by local electrochemical impedance spectroscopy which has confirmed water absorption, coating failure and increased coating resistance for highly cured capacitive systems, at the microscale.

Effect of Interfacial Characteristics of Metal Clad Polymeric Substrates on Electrical High Frequency Interconnection Performance

AbstractEtched metallic conductor lines on metal clad polymeric substrates are used for electronic component interconnections. Significant signal losses are observed for microstrip conductor lines used for interconnecting high Frequency (above 20 GHz) devices. At these frequencies, the electronic signal travels closer to the metal-polymer interface due to the skin effect. Copper-teflon interfaces were characterized by SEM and AES to determine the interfacial properties. Data relating roughness of the copper film to signal losses was compared to theory. Films used to enhance adhesion, were also found, to a lesser extent, to contribute to these losses.

Influence des charges d'espace sur les caracteristiques courant—tension, dans les feuilles minces de polymeres

This contribution begins with the description of the electron transfer model through the metal—polymer interface barrier potential. It is presumed that the barrier structure depends upon the applied electric field (Schottky effect), modified by the fixed space charges (homo- or heterocharges) accumulated in the vicinity of the metallic electrod. Therefore, the variation of the barrier height with the applied field leads to a static exponential current — electrical field characteristic I = φ( E). This model seems to be valid for electric fields greater than 10 7 V/m. The second part of this work is devoted to experimental characteristics interpretation. The semi-theoretical values found for the density of the space charges and for their space dimensions are in agreement with those mentioned in the scientific literature. We try to demonstrate that the shapes and slopes of the I = φ( E) characteristics are essentially dependent on the nature of the space charges.

Electrical field-effect of polyvinylidenefluoride (PVF2)

By means of modified field-effect measurements the properties of space charge distribution in polymers at the polymer-metal interface have been investigated. In general, these properties are due to excess charge and/or dipole orientation. The superposition of both effects is investigated in PVF2 (phase-II) in order to show whether the conformational PVF2 properties in the crystalline or in the noncrystalline regions control the space charge distribution. In the temperature region 60 °C <T < 90 °C and for times greater than 60 sec the space charge properties are significant influenced by additional dipole relaxations which result from the beginning chain mobility within the crystallites of PVF2. In the time interval 1 sec <t < 60 sec charge carrier injection at the PVF2-gold contact can be described by the Richardson-Schottky theory. Based on this theory, a plot of reduced current versus reduced field is presented displaying all experimental data obtained at various temperatures and field-strength on a single curve. No clear distinction can be given in this case between bulk-limited and electrode-limited processes.

Electrodes covered with thin, permeable polymer films

Because of the recent interest in electrodes modified with thin polymer films the question of permeability of such films to electrolytes and electroactive species is discussed. Glow-discharge polymer films prepared on platinum electrodes from 4-vinylpyridine were used as model systems. They were investigated mainly with electrochemical techniques. ln particular, the analysis of the electrode impedance over a frequency range of 10–3000 Hz gave valuable information about the structure of the metal—polymer interface in presence of the electrolyte. The films prepared on the anode of the glow discharge were found to act as semi-permeable membranes. Hydrogen could be oxidized on the platinum—polymer interface of these polymer covered electrodes at rates comparable to uncovered platinum, while the films were impermeable to iron ions. The matrix prepared on the cathode was more rigid due to cross-links, and thus less permeable. The polymer matrix was more or less electroactive itself. This was probably due to quinone-type groups formed with oxygen and water after the glow-discharge polymerization process. The oxidation and reduction of these groups gave rise to a Warburg-type contribution to the electrode impedance.

Application of XPS to the study of polymer-metal interface phenomena

XPS has been used to identify the mechanism of adhesion and adhesion failure of metals to rubber. The polymer was cured in contact with a series of metals and alloys. The rubber-metal interface was analyzed by XPS and the results were compared with those of bond strength measurements. High bond strengths are obtained only if an interfacial film of sulfidic reaction products is formed which becomes an integral part of the bond. It is postulated that metal sulfidization affects the rubber chemistry in the interface region leading to an increased crosslink density of the polymer. Several different types of adhesion failure have been observed. CuNi alloys do not bond well because of poor adhesion between the interfacial NiS layer and the metal substrate. The very strong bond formed by CuZn alloys fails during aging in air of high humidity or in steam as a result of partial dezincification of the unreacted brass leading to a Weak Boundary Layer consisting of ZnO and Cu x S. Minor changes in compound formulation may result in drastic differences of aged adhesion retention of brass, however. Good compounds form homogeneous sulfide films containing ZnS and Cu x S which passivate the brass. Poor compounds form irregular sulfide films which contain ZnO as well and which do not passivate the metal. Addition of a few per cent of Ni to brass results in a much improved aged adhesion retention. XPS analyses suggest that the nickel addition inhibits brass dezincification leading to a suppression of interfacial film growth during bond aging.