XPS Studies of Polymer Surface Modifications and Adhesion Mechanisms

XPS has been used to elucidate the mechanisms of surface modification of low density polyethylene by electrical (“corona”) discharge treatment and by chromic acid treatment. The use of derivatisation techniques for improving the precision of functional group analysis is described. These techniques also allow the role of specific interactions in adhesion to discharge treated surfaces to be investigated. The role of residual Cr on the adhesion of deposited metal to polymer surfaces is discussed.     

Surface Silanization of Polyethylene for Enhanced Adhesion

Low density polyethylene has been treated using a novel surface treatment process “SICOR” (“SIIane-on-CORona” treated polymer) in order to enhance adhesion with a range of adhesives including polyure-thane, methacrylate and cyanoacrylate. The process comprises two steps, i.e corona discharge followed by application of an organo-functional silane. The incorporation of surface hydroxyl groups onto the polymer surface enables organo-silane to create the hydrogen or covalent bonds with the oxidized polymer surface. The possibility of the creation of these bonds has been investigated using FTIR, XPS and wettability studies. The adhesion enhancement due to the new process is significant. Frequently, the strength increase exceeds 200% compared with the corona discharge treatment and more than 300% compared with LDPE priming using the “Loctite 770” polyolefin primer. The process is shown to be as good as, or better than, plasma treatment in terms of the strength increase following substrate treatment prior to adhesive bonding.     

Effect of Surface Energetics of Substrates on Adhesion Characteristics of Poly (p-xylylenes)

In investigating the effect of the surface energetics of substrate materials on the adhesion characteristics of poly(p-xylylene) and poly(chloro-p-xylylene) by the “Scotch Tape” method, it was found that if the substrates had not been preconditioned (treated with argon or a methane plasma), the adhesion was poor. The characteristics of water resistant adhesion that were observed when coated substrates were boiled in 0.9% sodium chloride solution were found to vary from excellent (when the polymer did not peel from the substrate after three cycles of 8 hours of boiling and 16 hours at room temperature) to poor (when the polymer peeled off almost immediately). It was noticed that water resistant adhesion depends on the hydrophobicity of the substrate material (the greater the hydrophobicity, the greater the adhesion) and is not related to the dry adhesive strength of poly(p-xylylene). The oxygen glow discharge treatment of the substrates decreased both the dry and wet adhesive strength of the polymer. The effect of the argon glow discharge treatment depended on the surface energetics of the substrate, and the methane glow discharge treatment increased both the dry and wet adhesive strength of the polymer. These preconditioning processes are discussed in terms of the sputtering of the material from the wall of the reactor in contact with the plasma and the deposition of the plasma polymer of the sputtered material on the substrate surface.     

Adhesion Mechanisms at Amine-Cured Epoxy/Aluminium Interfaces

Because the structure and the chemical composition of the interface can have a large effect on the adhesion properties of polymeric materials to metallic surfaces, many investigations have concentrated on the study of the interphase region. However, the complexity of the materials often leads to the use of model compounds to mimic the interfacial reaction. We have presented a critical discussion of three different approaches which have been used to understand the adhesion mechanism at amine-cured epoxy/aluminium interfaces: i) fracture of “real world” joints; ii) deposition of model (amino-alcohol) molecules on “real world” substrates; i) deposition of model (amino-alcohol) molecules on clean, oxidised and hydroxylated Al (100) surfaces. We have shown that model compounds can adequately duplicate the interface chemistry observed in “real world” joints. However, a detailed understanding of the exact nature of the interactions and of the role of the different reactive sites can only be achieved through studies performed on a model surface under controlled ultrahigh vacuum conditions.     

Functionalization of Polymer Surfaces Using Excimer VUV Systems and Silent Discharges. Application to Electroless Metallization

New approaches for electroless plating of nonconductive polymers or polymer-based materials are described. In this work, polyimide substrates were surface-functionalized (i) in nitrogenated (ammonia at reduced pressure) and oxygenated (air at atmospheric pressure) atmospheres under assistance of vacuum-ultraviolet (VUV) irradiation (use of a xenon silent discharge excimer source) or (ii) directly in air at atmospheric pressure using a dielectric-barrier discharge (DBD) device. After functionalization, the substrates were “activated” by dipping in a dilute acidic PdCl₂ solution or by spin-coating of a thin metal-organic film (from a solution of palladium acetate (PdAc) in chloroform). The catalytic activity of the so-deposited palladium species toward the electroless deposition of nickel was studied before and after a VUV post-irradiation (in air at atmospheric or reduced pressure) with a view to understanding better the role of the reducer (sodium hypophosphite) within the electroless bath.

This work confirms the specific interest of grafting nitrogenated functionalities onto polymer surfaces for attaching covalently the palladium-based catalyst (in particular in the case of the PdCl₂ route), forming thus strong Pd - N - C bonds at the metal/polymer interface. This results from the strong chemical affinity of palladium toward nitrogen. On the other hand, when oxygenated functionalities are surface-grafted, the conventional two-step procedure using SnCl₂ and PdCl₂ solutions can be proposed due to the strong chemical affinity of tin toward oxygen. The Ni deposits obtained under these different conditions pass the standard Scotch^®-tape test and, therefore, exhibit a good practical adhesion. For this same purpose, it is interesting to note that the DBD treatment operating in air at atmospheric pressure causes an increase of the surface roughness and, therefore, an improvement in adhesion of metallic films when their initiation is catalyzed through the PdAc route. In addition, this work demonstrates that extensive research still has to be performed to understand and improve the Ni/polymer adhesion when the PdAc route associated with a VUV irradiation is considered.     

Model Studies of the Effect of Surface Roughness and Mechanical Interlocking on Adhesion

The apparent strength of adhesion has been measured for a soft elastic layer adhering to model porous substrates, consisting of rigid plates containing regular arrays of cylindrical holes. Two contributions to the apparent strength have been identified and compared with the predictions of a simple theoretical treatment. The first is adhesion to the surface itself. Because “rough” surfaces have greater area for bonding, the strength of adhesion was increased by as much as twenty-fold. The second arises from the work of breaking deeply embedded or entangled strands in order to detach the overlayer. Contributions from this mechanism were as much as several hundred times the (low) intrinsic strength of adhesion. Satisfactory agreement was obtained with theory in both cases. Measurements were also made using cloth substrates, when the adhering layer penetrated the cloth completely. The work of detaching and breaking permeating strands was again much larger than the intrinsic strength of adhesion, in reasonable agreement with theoretical predictions.     

Bonding of Polyethylene to Gold

X-ray photoelectron spectroscopy (XPS) has had a profound impact on the surface chemical analysis of polymers, particularly in the identification of chemical species resulting from surface treatments that render so-called “unbondable” or non-polar polymers such as polyethylene suitable for adhesive bonding. ¹ This has occurred because XPS is a sensitive probe for examining the chemical nature of organic and inorganic surfaces. Recently, several investigators have adopted the XPS techniques as a primary method for attempting to answer some questions which are basic to adhesion science.2 These are, i) Is it necessary to oxidize polymer surfaces prior to adhesive bonding or to generate such polar surfaces in situ in order to obtain a strong adhesive joint?, and ii) Does one have to invoke covalent bonding at the polymer-metal interface to explain a strong adhesive joint?     

Investigation of the locus and mechanism of adhesion failure during peel tests on copper-chromium coated polyimide systems

Auger depth profiling and X-ray photoelectron spectroscopy (XPS) techniques were used to elucidate the adhesion process in copper-chromium coated polyimide. Metal layers were sputter-deposited on polyimide, which was modified by glow discharge under various conditions. The results showed that glow discharge could increase the chromium coverage on polyimide, improve or deteriorate adhesion at various interfaces, and change the locus of adhesion failure. Auger depth profiles could explain the various failure modes by revealing the chemistry at each interface. Additional information was provided by XPS results obtained from surfaces generated by failure. The effects of accelerated aging on this metal-polymer system were also studied.     

Changes in the Peeled Surfaces of Copper Thin Films Deposited on Polyimide Substrates After Heat Treatment

Peel strength between a copper (Cu) thin film and a polyimide (pyromellitic dianhydride-oxydianiline, or PMDA-ODA) substrate is reduced by heat treatment at 150°C in air. In this work, we investigated the peel strength, the morphology of the interface between Cu films and polyimide substrates using optical microscopy and electron microscopy, and chemical change of the interface using Auger electron spectroscopy (AES) and micro X-ray photoelectron spectroscopy (XPS). The analysis showed that CuO “lumps” were present on the peeled surface of PMDA-ODA after heat treatment at 150°C in air. The peeled surfaces of other polyimide substrates were also analyzed: biphenyl dianhydride-para phenylene diamine (BPDA-PDA) and biphenyl dianhydride-oxydianiline (BPDA-ODA). CuO lumps were present on the peeled surface of BPDA-ODA after the heat treatment, but not that of BPDA-PDA. Compared with the adhesion strength for the Cu thin film, the adhesion strength was high for the Cu/PMDA-ODA and Cu/BPDA-ODA laminates, but the adhesion strength was very low for the Cu/BPDA-PDA laminate. This low strength is the reason that CuO lumps were not detected on the peeled surface of the BPDA-PDA substrate. These CuO lumps were related to the adhesion degradation of the Cu/polyimide laminates after the heat treatment.     

Characterizing the appearance of medium glossy surfaces

For characterizing the overall appearance of a coating system mainly two different techniques are used in the automotive and paint industry. The mechanical profilometry combined with Fourier techniques and the “wave-scan” instruments. In 2006 the new “wave-scan dual” was introduced by Byk-Gardner for detecting the appearance of surfaces with lower gloss (medium glossy), e.g. primer-surfacers and EC. Within the framework of different studies results of evaluations of applicability and limitations of the “wave-scan dual” for characterizing the appearance of medium glossy surfaces will be presented.     

Role of helium plasma pretreatment in the stability of the wettability,adhesion, and mechanical properties of ammonia plasma-treated polymers. Application to the Al-polypropylene system

The stability of a plasma-treated polymer surface is an important issue, but very often a surface rendered wettable by the treatment is found to revert to a less wettable state with time. The purpose of this work was to minimize the ageing phenomenon by stabilizing the surface layer via crosslinking using an inert gas discharge. The stability of the wettability, adhesion, and mechanical properties of treated polypropylene (PP) has been investigated by a comparative study of two different plasma treatments (i.e. an NH₃ plasma and a He plasma pretreatment carried out before the NH₃ plasma; He plasma is well known to crosslink polymer surfaces). The aluminium-polypropylene (Al-PP) interfaces present very different features depending on the gas treatment. The role of the treatment time has been pointed out and under our experimental conditions, a treatment time of the order of a few seconds seems to be an overtreatment leading to degradation of the adhesion and mechanical properties. A broad interphase was obtained for an NH₃-overtreated PP, in contrast to the abrupt one formed when pretreated with the helium gas followed by NH₃ treatment. Good correlations between wettability and mechanical properties with adhesion measurements were established.     

Review of surface pretreatments for aluminium alloys

The pretreatment of aluminium to enhance adhesion has been the subject of a very large amount of research. In this review, 41 mechanical, chemical, electro chemical or other treatments specifically designed to modify the surface of aluminium to enhance bond durability have been identified. These may be combined with a range of chemical “add-ons”, such as primers, coupling agents or hydration inhibitors to stabilise the surface during storage or to further enhance bond durability. Furthermore, a diverse range of surface analytical techniques have also been identified, these range from simple wettability or optical inspection methods to more complex and costly ultra high vacuum based techniques, for example, AES and XPS/ESCA. These techniques provide useful data in adhesion studies, for example, in studying surface chemistry and topography before and after treatment, or, in failure analysis. An indication is given, wherever possible, as to how the treatments perform in comparative durability trials.     

Proliferation rate of fibroblast cells on polyethylene surfaces with wettability gradient

Surface wettability on anchorage‐dependent cells has an important role in cell growth rate. In our previous studies, we prepared a wettability gradient on polyethylene (PE) surfaces using corona discharge treatment from a knife‐type electrode whose power increased gradually along the sample length. The PE surfaces were oxidized gradually with increasing power and characterized by Fourier transform infrared spectroscopy, contact angle goniometry, and electron spectroscopy for chemical analysis. The purpose of this study is to determine the rate of proliferation on polymer surfaces with different wettability. The behavior of cell growth for NIH/3T3 fibroblast cells attached on the polymer surfaces with different hydrophilicity was investigated using wettability gradient PE surfaces prepared by corona discharge treatment. They were investigated for the number of grown cells from 24 to 60 h in terms of surface wettability. From the slope of cell number on PE gradient surface versus culture time, the proliferation rates (number of cell/cm² · h) were calculated. It was observed that the proliferation rate was increased more on positions with moderate hydrophilicity of the wettability gradient surface than on the more hydrophobic or hydrophilic positions, i.e., 1111 (number of cell/cm² · h) of 57° of water contact angle at the 2.5‐cm position (P < 0.05). This result seems closely related to the serum protein adsorption on the surface: the serum proteins were also adsorbed more on the moderately hydrophilic surface. In conclusion, surface wettability plays an important role in cell adhesion, spreading, and proliferation on the polymer surfaces. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 599–606, 2004     

Re: Contact Theory of Adhesion

Recently we have developed a contact theory of adhesion published under the title “Interfacial Contact and Bonding in Autohesion”.¹ The theory was formed for the case of polymers in a highly viscous state, where it is hard to bring them together for bonding. For low viscosity materials coalescence occurs very rapidly and contact is not a problem. However, most cases of adhesion that give us problems belong to the former case where establishment of contact between the two surfaces is the primary requirement. The rate of contact formation is controlled by the shape of the surfaces in contact, the viscous or viscoelastic properties of the material comprising them and the contact pressure. This is handled by extending Hertz' contact theory² by applying Alfrey's and Gurnee's elastic-viscous-viscoelastic analogy. Where-ever contact is established the interfacial bond strength may be obtained from molecular primary or secondary bond forces. This was obtained by using Good's⁴ 6-12 Lennard-Jones potential treatment which is an extension of Hamaker's calculations.⁵ Good has brought in the repulsive component in addition to the attractive forces. In case of polystyrene it is found that the intermolecular van der Waals' forces are sufficient to yield interfacial bond strength values equivalent in magnitude to the tensile strength of the polymer.     

Surface Pretreatment of Zinc and its Adhesion to Epoxy Resins

An epoxy resin based on the diglycidyl ether of bis-phenol A cured with piperidine and containing 7 or 15% rubber toughening agent has been bonded to zinc and the adhesion measured by calculating the critical strain energy release rate, Gc, from single edge notched (SEN) and tapered double cantilever beam (TDCB) specimens. The zinc has been pretreated to give surfaces ranging from relatively flat to “microfibrous” ones covered with dendrites of the metal. The microfibrous surfaces give highest values of Gc with both modified and unmodified resins. The fracture surfaces have been studied with SEM, XPS and EPMA and the failure mode related to the joint toughness.     

Fracture Toughness of a Silane Coupled Polymer-Metal Interface: Silane Concentration Effects

Fracture toughness of joints made from a glassy, 343,000 molecular weight polystyrene block bonded to chromic-sulfuric acid etched or phosphoric acid anodized aluminum are investigated. The fracture tests are performed with a 90-degree peel apparatus under “dry” laboratory conditions and “wet” conditions created by submerging the apparatus in a temperature controlled water bath. The bond strengths are controlled using various concentrations of styrl silane coupling agent added directly into the styrene monomer solution that polymerizes against the aluminum. Ellipsometric measurements on smooth silicon surfaces verify that the thickness of bound polymer is controlled by the silane to polystyrene mole ratio. X-ray photoelectron spectroscopy (XPS) analysis of fractured surfaces indicates that the fracture is near the aluminum surface. Both the wet and dry fracture energy as a function of bound polymer thickness on acid etched aluminum joints resemble quite closely the adhesion literature results obtained by fracturing pairs of fused, immiscible glassy polymers. Reasons for this similarity are discussed.     

Determination of the degree of adhesion in plasma-treated polyethylene/paper laminates

It is shown that an oxygen or ammonia plasma treatment significantly improves the adhesion between low density polyethylene (LDPE) and cellulose. Plasma treatment of the polymer was more effective than treatment of the paper, and ammonia plasma seemed somewhat more effective than the oxygen treatment. The adhesion between polyethylene and cellulose was evaluated at room temperature using a non-linear double cantilever beam test. The effect of the discharge treatment on the surface composition of LDPE and cellulose was characterized using electron spectroscopy for chemical analysis. The improved adhesion may be due to an improved penetration of the porous paper surface by the LDPE-melt and to an increase in the interfacial attraction forces as a result of the introduction of polar groups in the surfaces.     

Improved adhesion of low-density polyethylene/EVA foams using different surface treatments

The adhesion between a polyurethane (PU) adhesive and four foams containing different low-density polyethylene (LDPE)/ethylene vinyl acetate (EVA) blends was improved by using different surface treatments. UV-ozone, corona discharge, and low-pressure oxygen plasma treatments for different times were used to increase the surface energy of the foams. The low-pressure oxygen plasma was the most successful surface treatment to promote the adhesion of the foams. A reduced length of treatment was needed to improve the adhesion of the foams containing higher EVA content. The surface treatments produced a noticeable decrease in contact angle values due mainly to the creation of different carbon–oxygen moieties and to the formation of cracks/heterogeneities on the foams surfaces. After oxygen plasma, removal of non-polar material from EVA surfaces allowed to expose acetate groups which are likely to be important for increasing the adhesion of the foams.     

Adhesion Between Metals and Polymers as a Three-Dimensional System

Adhesion science in a technical sense is the study of reactions in boundary layers. From a macroscopic point of view the result is the adhesive joint strength dependent on the magnitude of the adhesion forces without hints on the nature of these forces. So the question of the nature of adhesion has at least to be answered for technical applications by using other measurement techniques. From the microscopic point of view adhesion is of interdisciplinary nature, where molecules or atoms act with each other across the interface. Mainly adhesive bonds are based on these interactions of different bodies like metals and polymers or other material discontinuities. So far we can speak about a “chemical adhesion”. But in practice there we realize a “technical adhesion” with more or less sharp discontinuities.     

Immobilization of polyethylene oxide surfactants for non-fouling biomaterial surfaces using an argon glow discharge treatment

A non-fouling (protein-resistant) polymer surface is achieved by the covalent immobilization of polyethylene oxide (PEO) surfactants using an inert gas discharge treatment. Treated surfaces have been characterized using electron spectroscopy for chemical analysis (ESCA), static secondary ion mass spectrometry (SSIMS), water contact angle measurement, fibrinogen adsorption, and platelet adhesion. This paper is intended to review our recent work in using this simple surface modification process to obtain wettable polymer surfaces in general, and non-fouling biomaterial surfaces in particular.