Metal-polymer Interface Research Articles

Photoelectron Spectroscopy Investigation of Interface Formation Between poly(2-methoxy-5-(2′-ethyl-hexyloxy)- 1,4-phenylene vinylene) and Aluminum

AbstractAl was thermally evaporated onto the conjugated semiconductor polymer, poly(2-methoxy-5,2′- ethyl-hexyloxy-phenylene vinylene) (MEH-PPV) in successive steps. Interface formation and chemical interactions between the polymer and Al were investigated by X-ray Photoelectron Spectroscopy (XPS) and synchrotron source Ultraviolet Photoelectron Spectroscopy (UPS). XPS analyses (angle resolved and sputtering depth profile) proved the formation of an insulating aluminum oxide layer at the polymer metal interface. The C-O bonds in the polymer decreased as more Al was evaporated on the MEH-PPV. Also reaction between Al and the carbon backbone of the polymer leading to the creation of carbon-metal bonds was observed. UPS analysis suggested the possibility of Al reacting with the phenylene ring of the polymer.

The properties of free polymer surfaces and their influence on the glass transition temperature of thin polystyrene films

We present a detailed study of free polymer surfaces and their effects on the measured glass transition temperature (T(g)) of thin polystyrene (PS) films. Direct measurements of the near-surface properties of PS films are made by monitoring the embedding of 10 and 20 nm diameter gold spheres into the surface of spin-cast PS films. At a temperature T = 378 K( > T(g)), the embedding of the spheres is driven by geometrical considerations arising from the wetting of the gold spheres by the PS. At temperatures below T(g) (363 K < T < 370 K), both sets of spheres embed 3-4 nm into the PS films and stop. These studies suggest that a liquid-like surface layer exists in glassy PS films and also provide an estimate for the lower bound of the thickness of this layer of 3-4 nm. This qualitative idea is supported by a series of calculations based upon a previously developed theoretical model for the indentation of nanoscale spheres into linear viscoelastic materials. Comparing data with simulations shows that this surface layer has properties similar to those of a bulk sample of PS having a temperature of 374 K. Ellipsometric measurements of the T(g) are also performed on thin spin-cast PS films with thicknesses in the range 8 nm < h < 290 nm. Measurements are performed on thin PS films that have been capped by thermally evaporating 5 nm thick metal (Au and Al) capping layers on top of the polymer. The measured T(g) values (as well as polymer metal interface structure) in such samples depend on the metal used as the capping layer, and cast doubt on the general validity of using evaporative deposition to cover the free surface. We also prepared films that were capped by a new non-evaporative procedure. These films were shown to have a T(g) that is the same as that of bulk PS (370+/-1 K) for all film thicknesses measured (> 7 nm). The subsequent removal of the metal layer from these films was shown to restore a thickness-dependent T(g) in these samples that was essentially the same as that observed for uncapped PS films. An estimate of the thickness of the liquid-like surface layer was also extracted from the ellipsometry measurements and was found to be 5+/-1 nm. The combined ellipsometry and embedding studies provide strong evidence for the existence of a liquid-like surface layer in thin glassy PS films. They show that the presence of the free surface is an important parameter in determining the existence of T(g) reductions in thin PS films.

Surface modification of polytetrafluoroethylene for adhesive bonding

The chemical bonding at metal–polymer interface is believed to play an important role in adhesion. The interfacial bonding and consequently adhesion are directly influenced by the way in which the interface is formed. Modification of a polytetrafluoroethylene surface (PTFE) has developed into an attractive way to chemically alter a polymer. Such synthetic approaches potentially can maintain a polymer's desirable bulk properties but can provide new different interfacial properties. There are many methods by which polymer surfaces can be chemically modified. Most common examples of this chemistry are reactions that introduce a single metal atom, a single type of functional group or mixture of functional groups. This paper briefly summarizes some existing as well as some newer examples from our laboratory of an alternative metal coating chemistry by which PTFE surface has covered with a thin and uniform metal sodium layer. In order to study the nature of the bond formation at metal/polymer interface, we have investigated the interaction of sodium atoms with a polytetrafluoroethylene fragment. Our results suggest that sodium bound to a PTFE polymer chain is energetically favorable. The compound formation is accompanied by charge transfer between sodium and polymer. PTFE with new interfacial properties is well recognized in many areas. Requirements to join dissimilar materials are more and more frequent in new technologies. The paper also deals with a study of a joint between aluminium pieces and polytetrafluoroethylene (PTFE) pieces. The way of their joining is described. The joint was performed through soldering with adhesive that covered the joining surfaces of the pieces, processed at a certain roughness, in a well established thickness layer. The joint was achieved through particular procedure of soldering, simultaneous with plastic deformation of the metal. Various tests of the joint were carried out under conditions simulating service conditions. A piece joint was tested in terms of its strength, thermal resistance and tightness. The tests showed a very good tightness of the joint at high vacuum (1.333 × 10 −9 mbar), in conditions of repeated thermal shocks, in the range of temperatures between − 195.5 and 270 °C. The study took into account the requirements of the particular tests and relevant foreign standards, including ASTM and DIN.

Effects of cathode thickness and thermal treatment on the design of balanced blue light-emitting polymer device

The interface between layered conjugated polymer and electrode is a most important factor to improve the performance and lifetime of polymeric light-emitting devices (PLEDs). In this work, a blue PLED with improved stability was achieved by the combination of optimized cathode structure as well as thermal treatment of light-emitting polymer (LEP). Experimental evidence of the initial luminance “settling in” stage was found to be dependent upon the cathode structure, while the long-term slope of luminance as a function of elapsed time is governed by the annealing conditions. Our study revealed the importance of extrinsic design of device for the improvement of PLED stability. Experimental data shows that a blue PLED annealed at 170°C and 6nm LiF at LiF∕Ca∕Al cathode retained the best lifetime, which can be explained by the improved polymer–metal interface and LEP’s charge mobility.

Formation of defect structures in Au-polysilane interfaces probed by low energy positron beams

Formation and accumulation of defect structures at interfaces between polysilanes and vacuum-evaporated gold (Au) electrodes are discussed quantitatively by low energy positron annihilation spectroscopy. The size distribution of the defects at the interface is analyzed based on the values of ortho-positronium ( o-Ps) lifetime ( τ 3), and no effect of the evaporation process is observed in the polymer films. The intensity of o-Ps ( I 3) indicates no considerable change before and after Au evaporation on dialkyl-substituted polysilanes, however, the values of I 3 is increased ∼20% in phenyl-substituted polysilane (PMPS) by the evaporation. The I 3 dependence on incident positron energy suggests the formation of the defects not only at an Au-PMPS interface but also in PMPS bulk phase as deep as 400 nm from the interface. Phenyl ring dissociation from the polymer backbone will play a significant role in the selective formation of the defects in PMPS. This is the first report on the direct measurement of defect structures at conjugated polymer–metal interface with non-destructive way, implying that electrode fabrication by vacuum evaporation affects the solid state structure of polymers.

Schottky emission at the metal polymer interface and its effecton the polarization switching of ferroelectric poly(vinylidenefluoride-trifluoroethylene) copolymer thin films

In metal/insulator/metal sandwich structure, the interface between the metal and the insulator thin films plays significant roles in the functional performance of the thin films. In ferroelectric poly(vinylidene fluoride-trifluoroethylene) copylmer thin films metallized by different metals, it is found that the polarization switching is asymmetric due to the work function difference of the metal electrodes, and the degree of asymmetry increases with the increasing work function difference. For the samples with high work function metals as both or at least one of the electrodes, the polarization switching is much faster. The current–voltage curves at various temperatures indicated the interface-controlled asymmetric phenomena are mainly determined by the Schottky emission and charge injection at the metal/insulator polymer interface.

Constitution and Properties of Nanocomposites Prepared by Thermal Decomposition of Silver Salts Sorbed by Polyacrylate Matrix

Constitution and dispersity of the products of thermal decomposition of silver nitrate ammonia complex sorbed by polyacrylate matrix are studied by the methods of small- and wide-angle X-ray scattering, optical, photoelectron and Auger-electron spectroscopies. It is shown that, at temperatures of 140–150°C, the complete decomposition of the complex occurs with the formation of nanoparticles and charged silver clusters in polymer bulk. No initial or intermediate products were observed. The average size of obtained nanoparticles is equal to 5 nm. The particles with the size less than 5 nm are amorphous according to X-ray data. The stabilization of nanoparticles occurs due to the adsorption of acrylic copolymer (presumably, via oxygen atoms) on their surfaces. Upon long-term storage in air, the self-diffusion of silver particles and clusters takes place from the surface to composite bulk caused by the detachment of oxygen-containing groups occurring at the metal–polymer interface.

Liquid metals as electrodes in polymer light emitting diodes

We demonstrate that liquid metals can be used as cathodes in light emitting diodes (pLEDs). The main difference between the use of liquid cathodes and evaporated cathodes is the sharpness of the metal–polymer interface. Liquid metal cathodes result in significantly sharper metal–organic interfaces than vapor deposited cathodes, due to the high surface energy of the metals. The sharper interface in pLEDs with liquid metal cathodes is observed by neutral impact collision ion scattering spectroscopy and low energy ion scattering spectroscopy measurements. The influence of interface sharpness on device performance was studied by comparing current–voltage-light characteristics of devices with OC1C10 paraphenylenevinylene (PPV) as electroluminescent polymer and indium tin oxide (ITO) as hole injection electrode, and different cathodes. Comparison of devices using a liquid Ga cathode and an evaporated Al cathode showed that light emission for the liquid Ga cathode is two orders of magnitude larger than for the evaporated Al cathode, and that the external light efficiency is increased by an order of magnitude. Since the work function of Ga and Al is nearly the same, the poor performance for evaporated Al LEDs is attributed to the formation of an interfacial layer where Al has diffused into, and reacted with, the PPV. This interfacial layer has poor electrical conduction compared to pure PPV, and contains quenching sites which reduce light emission. Low work function liquid metal cathodes were studied by using liquid Ca and Ba amalgams. The improved performance of liquid amalgam pLEDs is attributed to the different structure of the metal–polymer interface. The enormous increase in light and current through the amalgam devices compared to those using pure Hg demonstrate that less than 1 ML of a metal with a low work function at the polymer-cathode interface can have a dramatic effect on the performance of the devices. Devices with a liquid Ca amalgam cathode showed an increase of the current (by 50%) and brightness (80%) compared to devices with an evaporated Ca cathode, which is ascribed to reduced diffusion of Ca into the emissive PPV layer.

Electropolymerised acrylic coatings for polymer-metal adhesion enhancement

Electrochemical polymerisation (ECP) of acrylic monomers produces thin coatings on metal substrates and is a potentially useful method for forming adhesion promoting tie-layers at a polymer-metal interface. Uniform, passive films of poly (methyl methacrylate) and poly (glycidyl acrylate) have been formed via a cathodic free radical mechanism on stainless steel electrodes from aqueous electrolytes. The thickness of these films was found to increase with electrolysis time and the passive nature has been demonstrated by cyclic voltammetry studies. Adhesion tests were performed to compare the adhesion strength and failure mechanisms of various adhesives to coated and uncoated stainless steel substrates. The results indicate that ECP tie-layers can significantly increase the adhesive bond strength and alter the failure mechanisms observed. Electropolymerised acrylic coatings on metal substrates are thus seen as a promising approach for pretreatment of metals for adhesion enhancement.

Al/PET adhesion: role of an excimer laser pretreatment of PET prior to aluminum thermal evaporation

The modifications induced on PET, poly(ethylene terephthalate), by an excimer laser irradiation performed below the polymer ablation threshold were studied. The laser treatment of the polymer before aluminum thermal evaporation improved the Al-PET adhesion. The enhancement of the metal-polymer interface strength is attributed to surface morphological changes rather than to chemical modifications of the PET surface. Surface oxidation under UV irradiation was found to degrade the adhesion.

Surface Micromechanics of Polymer Coated Aluminium Sheets during Plastic Deformation

The plastic deformation of coated aluminium sheets has been investigated by D. Raabe and coworkers. Grain-scale micromechanical effects at the sheet surface or metal-polymer interface are essentially determined by orange peel and ridging phenomena. This supplies information about roughness, friction conditions during forming, strain localization, optical appearance and failure of constructional materials. The heterogeneity of microstructure and microstrains in the aluminium sheets and the resulting surface roughness of the coated and uncoated materials after plastic straining were investigated in a set of Al-Mg-Si (AA6022, T4 condition) aluminium samples of identical composition but different history.

Surface reactions of vapor phase titanium atoms with halogen and nitrogen containing polymers studied using in situ X-ray photoelectron spectroscopy and atomic force microscopy

The surface reactions of vapor phase Ti with polytetrafluorethylene (PTFE), polyvinylchloride (PVC) and nitrogen-modified polyethylene (PE) have been studied using in situ X-ray photoelectron spectroscopy (XPS) and ex situ atomic force microscopy (AFM). Titanium reactions with PTFE and PVC surfaces lead to the simultaneous formation of a titanium halide salt and TiC. Titanium reactions with both PTFE and PVC also produced significant morphological changes at the polymer surface. During Ti metallization of PTFE, defluorination did not produce any CF or CF 3 species normally observed during defluorination reactions of metal atoms with PTFE. The amount of titanium fluoride present at the metal–polymer interface was enhanced by post-metallization X-ray irradiation. Results on the Ti surface reactions are also compared and contrasted with Fe and Cu reactivity with PTFE and PVC. The reactions of Ti with nitrogen-modified PE lead to the simultaneous formation of TiN and TiC. Experimental evidence, however, suggests that the different nitrogen containing functional groups present in the nitrogen-modified PE were not equally reactive towards Ti metallization.

Metallization of poly(vinylchloride) by Fe, Ni, Cu, Ag, and Au

The evolution of the metal-polymer interface during the physical vapor deposition of Fe, Ni, Cu, Ag, and Au on poly(vinylchloride) (PVC) has been studied in situ using x-ray photoelectron spectroscopy. At low metal exposures, all of the metals studied reacted with PVC, forming a metal chloride. For those metals with more than one stable oxidation state, the salt produced corresponded to the metal’s lowest stable oxidation state, e.g., CuCl instead of CuCl2. Higher metal exposures resulted in deposition of a metallic overlayer. For Fe, Ni, Cu, and Au the variation in the metal’s reactivity (measured by the metal/metal-chloride ratio as a function of metal coverage), was observed to increase in the order Fe≈Ni&amp;gt;Cu&amp;gt;Au.

On the Complementary Photoluminescent and Electroluminescent Images of Poly(phenylenevinylene)-Based Light-Emitting Diodes with a Thin Active Layer

A clear complementary relationship between photoluminescent (PL) and electroluminescent (EL) images was observed for organic light-emitting diodes (OLEDs) based on poly(phenylenevinylene) (PPV) and dye-doped PPV. So-called ‘black spots' (dark circular regions observed on the active area of running OLEDs) become bright ones, when the photoluminescence of the same area is excited. A very small thickness of the active layer (ca. 10 nm) was the crucial point to observe this anticorrelation between EL and PL. A substantial increase of the PL yield (‘anti-burning' effect) was observed after strong light exposure (ca. 10 mJ/cm2) of the polymer covered by an aluminium layer. The same light exposure without aluminium protection resulted in complete photobleaching of the polymer. The presence of a thin insulating layer between the polymer and aluminium was proposed to be responsible for these effects. This layer prevents electron injection and PL quenching due to exciton dissociation at the metal-polymer interface. The former effect leads to black spots in the EL image, the latter one gives rise to bright spots on the PL image situated on the same places. The intermediate layer can be also induced by light exposure. A very efficient energy transfer from the polymer to the dye and to the J-aggregates of the dye was demonstrated in PPV/dye composite films.

Piezoelectric actuation of crack growth along polymer–metal interfaces in adhesive bonds

A new experimental technique for determining mechanical properties of polymer–metal interfaces was developed by replacing the conventional mechanical testing machine with a piezoelectric actuator. The actuator was made from a thin ferroelectric ceramic beam attached to a bilayer polymer-metal composite specimen. The trilayer specimen was loaded by applying ac electric fields on the piezoelectric actuator to drive crack growth along the polymer-metal interface. Subcritical crack growth was observed along the epoxy/aluminum interface, and the growth rate was found to depend on the magnitude of the applied electric field. The fracture mechanics driving force for the crack growth was computed from the finite element analysis as a function of crack length, applied field, material properties, and specimen geometry. Kinetics of the crack growth was correlated with the piezoelectric driving force.

Asymmetry and rectification in the tunnel current of a nanometer-sized metal-conjugated polymer–metal junction

Electron transport processes of a nanometer metal-conjugated polymer–metal tunnel junction have been probed using a scanning tunneling microscope. The tunnel current of the junction shows two effects. The appearance of an asymmetry in the tunnel current indicates that the junction transport mechanism is different from that for which tunneling occurs directly between two metallic electrodes. Thus, understanding of the asymmetry and hence the transport mechanism demands a detailed description of the metal–polymer interface. By applying the theories of the metal–semiconductor interface to the tunnel junction, we show the presence of an asymmetric electrostatic potential-energy profile, which, together with the metal-induced gap states in the polymer, gives rise to the observed asymmetry in the tunnel current. In some cases, a threshold of anomalously large currents enhances the current asymmetry to give rise to rectification, indicating carrier excitations and carrier multiplication processes in the junction. Our results show that a detailed description of the interface electronic structure is essential to understanding electron transport in devices based on organic molecules.

New Technique for Determining Mechanical Property of Polymer/Metal Interface

ABSTRACTA new experimental technique for determining mechanical properties of the polymer-metal interface was developed by replacing conventional mechanical testing machine with a piezoelectric actuator. Using this technique, fatigue crack growth behavior of epoxy/aluminum interface was studied as a function of electric field, crack length and cyclic frequency. The crack growth rate was found to depend on the magnitude of the applied electric field and decrease with testing frequency.

Reactions and Intermediates at the Metal-Polymer Interface as Observed by XPS and NEXAFS Spectroscopy

Potassium or chromium were evaporated by means of a Knudsen effusion cell under ultra-high vacuum conditions onto a number of common polymers, prepared as stretched foils and spin-coated films. The metal-polymer interface was studied by X-ray Absorption and X-ray Photoelectron Spectroscopy. Evaporated samples were analyzed without exposure to the atmosphere. Different general types of reactions of the metal atoms with the polymers were observed. With deposited K and Cr a redox process including the transfer of substrate oxygen atoms across the interface was found. The formation of π-electron complexes and covalent metal-carbon bonds were obtained exclusively with chromium. Aromatic rings, carbonyl groups and, to a lesser extent, ether linkages are scissioned by metal-polymer interactions. The fourfold substitution of aromatic rings and the exclusive existence of C ─ O ─ C structures within polyphenylene ether (PPE) make this polymer most stable toward reactions with chromium. In contrast, bisphenol-A polycarbonate is susceptible to redox reactions with the carbonate group and to forming Cr complexes with aromatic rings using the chromium 3d orbitals. The result is a succession of complex formation, followed by Cr substitution onto aromatic rings and destruction of rings with the formation of chromium carbides.

Photochemical functionalization of polymer surfaces for subsequent metallization

Plasma-treated polymers typically show changes of surface morphology as well as modifications in their chemical composition. Both effects are known to have an influence on deposition and adhesion of metal coatings, although the exact mechanisms are not yet understood. Besides high-energy electrons and chemically active species, the generated UV radiation is one major component of plasma surface modification. The energy of ultraviolet photons is sufficiently high to induce bond scissions in polymeric materials, which result in subsequent chemical reactions.In this study, excimer radiation from lasers (e.g. KrF, 248 nm) or lamps (e.g. KrCl, 222 nm) was used for the formation of polar functional groups on the surface. The treatment was carried out at atmospheric pressure, mostly in air. The chemical and physical properties of the irradiated surface are different compared with untreated areas.The resulting functional groups allow for a spontaneous deposition of molecular thin films from aqueous solutions. In particular, these can be complexes from noble metals. Noble metals catalyze deposition in electroless metallization baths. A process for selective metal deposition on polymers has been developed based on this principle.After deposition, the main issue is the adhesion that can be achieved in the polymer–metal interface. It is determined by chemical or physical interaction between the polymer surface and the adjacent layer, as well as by typographical features.

Novel instrumentation for evaluating stone impact wear of automotive paint systems

Stone impact damage to painted automotive exteriors is an area of significant concern both to the automobile manufacturer and the paint supplier. Damage occurs by stones that are either launched by rear tires of heavy-duty trucks or stones dropped from moving gravel trucks. Stone impact can result in removal of one or more paint layers and in some cases can lead to delamination at the metal–polymer boundary. Removal of paint layers (referred to as chipping) is a cosmetic issue while delamination at the metal–polymer interface can lead to corrosion. Various methods have been described in the literature for rapid assessment of stone impact durability. These methods are associated with one or more drawbacks. These include: (A) being phenomenologically inappropriate; (B) not associated with quantitative metrics; (C) inadequate control of relevant parameters and (D) not being reproducible. This paper describes novel second generation instrumentation for precise and quantitative assessment of impact durability. This device overcomes many of the drawbacks associated with conventional methods. A statistically designed experiment to assess impact durability is also described.