Metal-polymer Interface Research Articles

Local polarization switching kinetics in thin-film P(VDF-TrFE) (76:24) studied by time-resolved LIMM

Extending the well-established laser intensity modulation method (LIMM) by means of sequential frequency modulation represents a new approach for obtaining polarization depth profiles at both high spatial (nanometer) and temporal (millisecond) resolution. Thin films of the archetypal, organic, ferroelectric copolymer poly(vinylidene fluoride-co-trifluoroethylene) P(VDF-TrFE) were investigated in this manner to assess local switching kinetics. For subcoercive field conditions of 5 MV/m, the central regions of 2000 nm thick samples show a “bulk”-like, uniform polarization switching behaviour at a time-scale of 250 ms. In contrast, a 100 nm thick region near the metal-polymer interface reveals clear deviations from bulk switching kinetics. This likely reflects local charge generation/injection and trapping effects.

Optimization of laser-assisted joining through an integrated experimental-simulation approach

An integrated experimental-simulation study is carried out to optimize of laser-assisted joining of metals and plastics. An empirical model that predicts the local strength based on the temperature distribution was developed. Experimental mechanical tests were involved to determine the influence of the main process parameters on the strength of the joints. These test results were also employed to calibrate the empirical model. A numerical FE model was developed to predict the temperature distribution at the metal-polymer interface by varying laser power, scanning speed, and beam shape. From results, it was observed that the proposed methodology is capable to predict with good accuracy the strength of the joints under the assumed conditions. The onset of defects in the joint and the local strength can also be predicted. Results indicate that this approach can be readily employed for the optimization of the process conditions as well as the optimization of the laser beam shape and dimension. A similar approach can be easily extended to other processes, where the objective is to produce uniform thermal fields, such as laser welding and hardening.

Superficial and Inner Examination of a Microwave-Irradiated Dental Acrylic Resin and Its Metal-Polymer Interface.

The aim of this study is to conduct an extended surface and cross-section characterization of a denture base acrylic resin subjected to 500, 650, and 750 W microwave irradiation for 2, 3, and 5 min to assess its morphological modifications. A commercial heat-cured powder was polymerized according to the manufacturer's specifications and distributed into 20 circular samples. A stainless-steel wire was partially embedded in half of the discs, in order to investigate the metal-polymer interface. High-resolution scanning electron microscopy (SEM) imaging, white light interferometry, roughness measurements and Fourier transform infrared spectrometry were employed for morphological and structural evaluation of the irradiated polymer. Superficial adaptation was discovered after 5 min exposure at 500 W, 650 W, and 750 W, revealing significant roughness correction for 750 W. SEM characterization revealed the inner alteration of the resin for the 750 W protocol and a metal-polymer gap developed regardless of the irradiation conditions. The considerable temperature fluctuations that the samples were subject to during the experiments did not essentially change the poly(methyl-methacrylate) bond structure.

Metal-polymer interface influences apparent electrical properties of nano-structured polyaniline films.

The interface between the conductive polymer, polyaniline (PAn-Cl), and gold, platinum, or an interceding layer of electrodeposited platinum on gold or platinum, markedly influences the apparent electrical properties and the electronic to ionic transition in physiological buffers. Polyester-supported, sputter-deposited gold and platinum thin films were laser patterned to yield co-planar Thin Film Electrodes (TFEs) suitable for platinization and deposition of PAn-Cl nanofibers. Electrodeposition of platinum from chloroplatinic acid (50 mC cm-2) onto gold produced larger feature sizes and larger surface roughness (23.5 nm) when compared to platinization of platinum (15.2 nm) and both similarly reduced interfacial impedance in water and physiologically relevant buffers, PBS and HEPES. UV-Vis characterization produced absorption edges (DI water 2.36 eV, PBS 2.64 eV, and HEPES 2.66 eV) reflective of the ionic strength of the medium. Thin films (23 ± 2 μm) of PAn-Cl nanofibers were deposited onto Au, Pt, Au|Pt, Pt|Pt TFEs and each characterized by Electrical Impedance Spectroscopy (EIS) over the range 106-10-1 Hz at RT in air, DI water, PBS, and HEPES buffers and by multiple scan rate cyclic voltammetry (MSRCV) in PBS. Platinized gold and platinized platinum decorated with PAn-Cl behaved quite differently in these test environments confirming a role for the contacting surface roughness/nano-topography in influencing apparent electrical properties. Equivalent circuit modeling of EIS data revealed a modified Randles circuit (R(QR)) of low chi-square values (<0.05) that rationalized the capacitance and membrane resistance and confirmed that platinization of gold served to increase the PAn-Cl apparent resistance while platinization of platinum served to decrease the PAn-Cl apparent resistance.

Study of surface appearance and composition effect on AISI 304 and 304L stainless steel wear against nylon wire

The objective of this research was to study effects of surface appearance, chemical composition and direction on wear resistance of polymer-metal interface by using AISI 304, AISI 304L stainless steel and nylon wire as specimens. The analysis of wear resistance was compared before and after the sliding between these specimens. Optical microscope was used to investigate the surface appearance specimens. It was found that sliding direction and surface roughness affected on wear rate, while the chemical composition of AISI 304 and 304L stainless steel had limited effect. The increase of wear rate was correlated to the increase of surface roughness in parallel direction, but wear rate decreased in perpendicular direction. For the AISI 304 stainless steel, wear rate in perpendicular direction was higher than in parallel direction for 0.57 mg, 0.53 mg and 0.39 mg at 0.2 µm, 0.5 µm and 1.0 µm of surface roughness respectively. However, for AISI 304L stainless steel, wear rate in perpendicular direction at 0.2 µm and 0.5 µm had a similar result, except for at 1.0 µm of surface roughness that the wear rate in perpendicular direction was less than in parallel direction for 0.56 mg. This was considered to be related to the amount of the contact area and displacement between nylon wire and steel during the sliding.

On Diagnostics of Double Electric Layer in the Zone of Metal–Polymer Adhesion Contact Using the Positron-Annihilation-Probe Method

Application of the positron-annihilation-probe technique for nondestructive diagnostics of the double electric layer (DEL) at the polymer–metal interface and, hence, of the electric component of polymer adhesion to the metal is described. Separated curves of angular correlation of positron-annihilation radiation (ACPAR) for metal, polymer, and metal with a polymer layer allow detecting the presence of a strong electric field of a DEL at the nickel–epoxy-resin interface. The value of the DEL potential is determined to be 15 keV. The method of a positron-annihilation probe allows identifying the presence of a DEL on the internal interfaces of materials and estimating its energy parameters so as to correctly reflect the value of the contribution of the electrostatic component to the interphase interaction.

Assessment of steel corrosion and deadhesion of epoxy barrier paint by scanning Kelvin probe

Abstract Marine paints are complex polymeric systems containing layers of different nature that significantly delay the corrosion failures. In order to obtain a rapid ranking in the corrosion protection provided by the paints, it is important to apply sensitive and non-invasive methods for early detection of under paint corrosion. Scanning Kelvin Probe (SKP) was used to determine the mechanism(s) of corrosion and paint de-adhesion. This technique is sensitive to the metal-polymer interface and gives a quick assessment of cathodic delamination or anodic undermining. Depending on the type of exposure the mode of corrosion de-adhesion was evaluated and compared with SKP data obtained under polarization experiments by impressed current in aqueous electrolyte. After exposure, pull off test and Scanning Electron Microscopy – Energy Dispersive X ray Spectrometry (SEM-EDX technique) were applied to determine the paint adhesion stability around the defect and to analyse the steel-paint interface.

Predictions of micromechanics models for interfacial/interphase parameters in polymer/metal nanocomposites

In this paper, the polymer-metal interfacial/interphase parameters (PMIP) in polymer/metal nanocomposites are studied by modeling the mechanical properties. In this regard, the experimental results of yield strength, Young's modulus and elongation at break can be compared with the micromechanical models to evaluate the PMIP. The good agreement obtained between the experimental data of samples and the predictions confirms the applicability of models for polymer/metal nanocomposites. Many calculated parameters show the existence of a strong interphase in the reported samples. It is concluded that the fine morphology of nanoparticles and the strong interaction/adhesion at the polymer-metal interface can produce the significant PMIP in the polymer/metal nanocomposites.

Significance of combined functional nanoparticles for enhancing tribological performance of PEEK reinforced with carbon fibers

Tailoring a robust and slippery tribo-film on polymer-metal interface is a potential approach for improving the tribological performance. In this work, hybrid soft-hard nanoparticles were compounded into polyetheretherketone (PEEK) reinforced with carbon fibers. Effects of nanoparticle combinations on tribological performance of the composites and tribo-film’s functionality were explored. It was demonstrated that hybrid oxide nanoparticles, i.e. Bi2O3-SiO2 and CuO-SiO2, accelerated formation speed of tribo-films. The soft nanoparticles, i.e. Bi2O3 and CuO, having low-melting points expedited tribo-sintering of wear products on the interface. Moreover, combination of WS2-SiC nanoparticles resulted in a “slippery” tribo-film withstanding harsh rubbing conditions. Hard species endowed the tribo-film a high load-carrying capability, while the soft ones imparted the tribo-film an easy-to-shear characteristic. Striking stretching and re-orientation of PEEK molecules in tribo-film were identified and corroborated the “slippery” characteristic. It is expected that this work can pave a route for developing new self-lubrication composites under extreme running conditions.

Adhesion and failure analysis of metal-polymer interface in flexible printed circuits boards

As device miniaturization in microelectronics is currently requested in the development of high performance device, which usually include highly-integrated metal-polyimide multilayer structures. A redistribution layer (RDL) process is currently emerging as one of the most advance fabrication techniques for on-chip interconnect and packaging. One of the major issues in this process is the poor adhesion of the metal-polyimide interfaces particularly in flexible circuit boards due to the flexibility and bendability of devices. In this study, low pressure O2 plasma treatment was investigated to improve the adhesion of metal-polyimide interfaces, using inductively coupled plasma (ICP) treatment. We identified that the adhesion of metal-polyimide interfaces was greatly improved by the surface roughness control providing 46.1 MPa of shear force in the ball shear test after O2 plasma treatment, compared 14.2 MPa without O2 plasma treatment. It was seemingly due to the fact that the adhesion in metal-polyimide interfaces was improved by a chemical conversion of C=O to C−O bonds and by a ring opening reaction of imide groups, which was confirmed with FT-IR analysis. In the finite element numerical analysis of metal-polyimide interfaces, the O2 plasma treated interface showed that the in-plane stress distribution and the vertical directional deformation agreed well with real failure modes in flexible circuits manufacturing.

Metallization and electrical characterization of platinum thin film microelectrodes on biocompatible polydimethylsiloxane substrates for neural implants

Microfabrication technology in the biomedical field has provided microelectrode arrays for neural implants with new development opportunities. The need for more complex physiological functions and miniaturization, as well as the use of new materials for more flexible electrodes, can now be satisfied. PDMS (Polydimethylsiloxane) substrates are elastic, biocompatible and permeable to oxygen, in addition to being a highly stable material. However, the implementation of microfabrication techniques such as deposition and patterning processes on these new substrates is not straightforward. This paper will describe the development of a reliable method to metalize thin film microelectrodes on a highly flexible medical grade PDMS layer that is suitable for long-term implantation. Platinum (Pt) microelectrodes were deposited by physical vapor deposition and pattern transfer by lift-off was chosen. Standard photolithography was used to pattern a conventional positive photoresist and was optimized to improve adhesion and to avoid cracks in the resist. The electrical behavior of the metal-polymer interface was analyzed using multiplexed DC measurements. The resistance values of seven samples and a control were acquired sequentially. Special attention was paid to the connectorization from the flexible microelectrodes to a rigid substrate. Measurements were carried out in an air-protected environment as well as in a biological environment designed to mimic the environment of the human body. Long-term stability of the Pt-PDMS interface was strongly influenced by the electrode configuration and its connection. A characteristic electrical behavior was observed for a straight-electrode configuration. This configuration demonstrated significant drift of resistance values of more than 4% during the initial 56h. By contrast, a stable behavior was observed for a loop electrode design, with only small variations of less than ±0.5% caused by thermal fluctuations.

Role of Sputter Deposition Rate in Tailoring Nanogranular Gold Structures on Polymer Surfaces.

The reproducible low-cost fabrication of functional polymer-metal interfaces via self-assembly is of crucial importance in organic electronics and organic photovoltaics. In particular, submonolayer and nanogranular systems expose highly interesting electrical, plasmonic, and catalytic properties. The exploitation of their great potential requires tailoring of the structure on the nanometer scale and below. To obtain full control over the complex nanostructural evolution at the polymer-metal interface, we monitor the evolution of the metallic layer morphology with in situ time-resolved grazing-incidence small-angle X-ray scattering during sputter deposition. We identify the impact of different deposition rates on the growth regimes: the deposition rate affects primarily the nucleation process and the adsorption-mediated growth, whereas rather small effects on diffusion-mediated growth processes are observed. Only at higher rates are initial particle densities higher due to an increasing influence of random nucleation, and an earlier onset of thin film percolation occurs. The obtained results are discussed to identify optimized morphological parameters of the gold cluster ensemble relevant for various applications as a function of the effective layer thickness and deposition rate. Our study opens up new opportunities to improve the fabrication of tailored metal-polymer nanostructures for plasmonic-enhanced applications such as organic photovoltaics and sensors.

A bulk and localised electrochemical assessment of epoxy-phenolic coating degradation

Abstract 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.

Investigating Polymer-Metal Interfaces by Grazing Incidence Small-Angle X-Ray Scattering from Gradients to Real-Time Studies.

Tailoring the polymer–metal interface is crucial for advanced material design. Vacuum deposition methods for metal layer coating are widely used in industry and research. They allow for installing a variety of nanostructures, often making use of the selective interaction of the metal atoms with the underlying polymer thin film. The polymer thin film may eventually be nanostructured, too, in order to create a hierarchy in length scales. Grazing incidence X-ray scattering is an advanced method to characterize and investigate polymer–metal interfaces. Being non-destructive and yielding statistically relevant results, it allows for deducing the detailed polymer–metal interaction. We review the use of grazing incidence X-ray scattering to elucidate the polymer–metal interface, making use of the modern synchrotron radiation facilities, allowing for very local studies via in situ (so-called “stop-sputter”) experiments as well as studies observing the nanostructured metal nanoparticle layer growth in real time.

Chemical Sculpturing of Al Micro-Particles for Polymer Composites and Universal Polymer-Polymer Joints

Composite materials attract more and more attention due to their unique properties in many fields. For certain applications metal particle-polymer composites are very attractive, where e.g. lightweight heat-conductive composites are needed. For particle-polymer composites the typical problem is the deterioration of the mechanical properties of the polymer matrix due to poor adhesion at the polymer-metal particle interface. Our approach solves this problem by generating a mechanical interlocking surface structure on the metal particle surface via wet chemical etching which we call sculpturing. In this work commercially available granular Al micro-particles with a typical size distribution between 200 - 800 µm and a purity of 99.7 % is used. These Al particles are structured in an optimized aqueous HCl based electrolyte. The key idea is to balance the passivation and dissolution of the metal surface in such a way that the desired interlocking structures form. Similar to HNO3-HF based Si etchants, the employed electrolyte makes use of in-situ formed reaction products controlling the structuring homogeneity and reaction speed. Figure 1a) provides a top view on Al particles sculptured in such way with a magnification view in Fig. 1b). The sculptured surface structure consists of a multitude of rectangular structures with flat faces in a step-like arrangement or as free-standing hooks. These rectangular structures vary in size and are tilted towards each other depending on the crystal orientation. The EDX analysis confirms that the surface of these Al micro-particles after sculpturing is covered only by a native oxide layer. By adjusting the chemical fabrication process the Al micro-particles could be fully or partially sculptured. Correspondingly, the Al micro-particles showed a change in their optical properties depending on the etching duration. Fully-sculptured Al micro-particles are dark gray in color, partially structured ones are light-gray, and the untreated ones are silver colored. With these sculptured Al micro-particles polymer composites can be formed as shown in Fig. 1c. Here, a densely packed polythiourethane (PTU)-sculptured Al particle composite is formed in the shape of a dog bone. Tensile test results show that the addition of sculptured particles results in a strongly increased Young’s modulus and reaching at least the maximum tensile strength of the pure polymer, unlike in case of the addition of untreated Al micro-particles. In case of polydimethylsiloxane (PDMS) which is well-known for its poor adhesion to metals, polymers and ceramics, the effect on the increase of Young’s modulus is even far more pronounced as well as a tremendous increase in the maximum tensile strength. The fracture surfaces of all sculptured Al micro-particle-polymer composites showed always cohesive failure where even the particles themselves are torn apart, which is the best reachable adhesion strength. Another very important application for sculptured Al micro-particles could be adhesive-free joining of immiscible polymers like ethylene-vinyl acetate (EVA) and polyoximethylen (POM) or other low surface energy polymers. Here, the sculptured micro-particles are partially imprinted into the thermally-softened POM surface and then joined with the softened EVA. Owing to the hook and step structures of the sculptured particles a robust connection between the Al micro-particles and any polymer matrix is obtained. No matter how the sculptured particles are partially imprinted in the polymer surface, they stick to it and hinder delamination. This is impressively shown via tensile testing, where only cohesive failure within the EVA component is found, while the direct joint without particles and via untreated particles fail adhesively directly at the POM-EVA interface. Fig. 1: SEM top view on chemically sculptured a) Al micro-particles, b) in high magnification of the structured Al micro-particle surface and c) Al micro-particle filled PTU composite in the shape of dog bone. Figure 1

Plasma polypyrrole micro-coatings on metallic stents

Stainless steel stents were coated with biocompatible polypyrrole doped with iodine and synthesized by plasma to add a polymeric layer on the metallic surfaces that may reduce some implant rejection reactions in the human circulatory system due to the implantation of metallic surfaces. Trying to increase the adhesion in the polymer–metal interface, this study used erosion, oxidation and sensitization of the metallic stents with an in situ posterior polymerization of the monomer. All processes were carried out by plasma which increased the reactivity of the metallic surfaces producing chemical bonds between polymers and stents with a high possibility to resist the blood conditions without peeling or detachment that may expose the metallic surface to the blood. The results showed that the coatings contain pyrrole rings joined among them in different configurations which originated crosslinked polymers. The erosion did not change, substantially, the density of protrusions, but changed only their width and height, and the coatings acquired the morphology of the metallic surface with a 3.7 nm roughness and protrusion separation of 35.6 nm. The coated stents were immersed in phosphate-buffered saline (PBS) solutions with pH ~7.4 at 37 °C to partially simulate the chemical conditions in arteries and veins, in which the coating remained adhered on the stent up to 2 months without significant degradation. The immersion in PBS static fluids caused a roughness decrease from 3.7 to 2.4 nm with a protrusion separation of 35.4 nm, however, in PBS dynamic fluids, the erosion increased the roughness and protrusion separation up to 9.3 and 43.9 nm, respectively.

Structural modeling and mechanical behavior of Metal-Porous-Polymer-Composites (MPPCs) with different polymer volume fractions

Since the past five decades, the study of new materials has become attractive in both scientific research and industrial applications. For purpose of enhancing mechanical properties of metallic foam, a new composite entitled “Metal-Porous-Polymer-Composite (MPPC)” consisting of open-cell metallic foam and filling polymers was developed by infiltrating the filling polymers throughout the connected pores in open-cell metallic foam. Based on a three-dimensional (3D) structural modeling, microscopic structural models of Al metallic foam and MPPCs with spherical pores and Kelvin’s pores are established in the finite element code. Introducing the elastoplastic mechanical properties, the polymer–metal interfacial behaviors and the proper loads and boundary conditions on these structural models of Al metallic foam and MPPCs, the numerical compressive behaviors of Al metallic foam and PA6/Al & LDPE/Al MPPCs are successfully performed. The results show that the mechanical properties of PA6/Al & LDPE/Al MPPCs are mainly influenced by the filling polymers, the kind of pores, the pore size, the polymer–metal interface, the polymer volume fraction and the distribution of pores, respectively. It means that a good understanding can be provided concerning the deformation mechanism and engineering applications of both Al metallic foam and MPPCs.

Mechanical interlocking to improve metal–polymer adhesion in polymer-based neural electrodes and its impact on device reliability

The polymer–metal interface is one of the most critical parts in polymer-based neural implants in relation to their long-term reliability. In this study, we aim to suggest a feasible fabrication method using mechanical interlocking to improve polymer–metal adhesion in polymer-based neural electrodes and evaluate its impact on device reliability in vitro. Liquid crystal polymer (LCP) is selected as the base material for the polymer-based neural electrode due to its extremely low water absorption rate (<0.04 %). Test samples with mechanical interlocked interface between LCP and a noble metal are designed and fabricated using micro-patterning technologies, in this case photolithography, electroplating, and laser machining. After the metal patterns with undercut profile cross sections were fabricated using a dual photolithography process and electroplating, the LCP and the metal formed a mechanical interlocking pattern during the lamination process. In a 180° peel test, the average maximum adhesion force of the samples with and without mechanical interlocking was 19.24 and 14.27 N, respectively. In vitro accelerated soak tests, which consist of interdigitated electrode patterns and a customized system for measuring the leakage current, are carried out to evaluate the long-term reliability of the LCP-based neural electrodes. Samples with and without interlock failed after 224 and 185 days, respectively, in a 75 °C saline environment. Scanning electron microscopy images revealed that the interlocked LCP–metal interfaces remained intact after water leakage. The results demonstrate the effect of the fabrication method using mechanical interlocking, which can be applicable to other polymer-based neural electrodes for long-term implantation.

Solution-Processed p-Dopant as Interlayer in Polymer Solar Cells.

We report here an original approach to dope the semiconducting polymer-metal interface in an inverted bulk-heterojunction (BHJ) organic solar cell. Solution-processed 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), is deposited on top of a P3HT:PC61BM layer before deposition of the top electrode. Doping of P3HT by F4-TCNQ occurs after thermally induced diffusion at 100 °C of the latter into the BHJ. Diffusion and doping are evidenced by XPS and UV-vis-NIR absorption. XPS highlights the decrease in Fluorine concentration on top of the BHJ after annealing. In the same time, a charge transfer band attributed to doping is observed in the UV-vis-NIR absorption spectrum. Inverted polymer solar cells using solution-processed F4-TCNQ exhibit power conversion efficiency of nearly 3.5% after annealing. This simple and efficient approach, together with the low annealing temperature required to allow diffusion and doping, leads to standard efficiency P3HT:PC61BM polymer solar cells, which are suitable for printing on plastic flexible substrate.

Nanotextured Polymer Substrate for Flexible and Mechanically Robust Metal Electrodes by Nanoimprint Lithography.

Metal thin film electrodes on flexible polymer substrates are inherently unstable against humidity and mechanical stresses because of their poor adhesion properties. We introduce a novel approach for improving the adhesion characteristics of metal-polymer interface based on the nanostructuring of the polymer substrate by using nanoimprint lithography. The adhesion characteristics of metal-polymer interface were measured by accelerated test, cyclic bending test and double cantilever beam (DCB) test. The interface of Au/Ti dual layer thin film and nanoimprinted PMMA substrate shows over 2.03 and 1.95 times higher adhesion energy (G(c)) than that of Au/Ti dual layer thin film and plane PMMA substrate in air and wet environments, respectively. The adhesion energy between metal thin film and polymer substrate was dramatically improved by the increased surface roughness and mechanical interlocking effect of numerous nanoscale anchors at the edges of nanoimprinted surface, which was verified by both experiment and numerical analysis.