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.     

Titania Coatings on Polyethylene Terephthalate: Adhesion and XPS Studies

Titania coatings have been deposited on polyethylene terephtalate (PET) by the r.f. magnetron sputtering method in an oxygen-argon plasma from a titanium target and in a pure argon plasma from a titania target.

The dependence of the structural properties and the composition of the deposited films on the sputtering pressure and the r.f. power have been studied. In order to improve the adhesion strength between the titanium oxide films and their substrate, various cold plasmas are used to treat the polymer surface. These treatments' influence on the adhesion is studied by using the fragmentation test. The best results are obtained with a carbon dioxide plasma. The adhesion of the titania coating on the PET film also depends strongly on the deposition conditions. The highest values are reached when the titanium oxide films are deposited by the reactive sputtering process and when the elaboration parameters combine a total pressure as low as 0.8 Pa and a power density of 2.54 W cm^-2. The titania/PET interface, investigated by XPS, suggests the formation of Ti-O-C bonds in the first stage of the deposition of the titania films obtained by the reactive magnetron sputtering process, while no chemical reaction seems to occur between the PET and the titanium oxide film sputtered under a pure argon plasma.     

Laser-Enhanced Gas Phase Surface Modifications of Teflon AF1600 for Increased Copper Adhesion

Multilevel interconnect devices, made of alternating layers of a low permittivity polymer (e.g., Teflon AF1600^TM) and a low resistivity metal (e.g., copper), are increasingly being used in microelectronics in order to decrease the RC signal transmission time delay. The mechanical stability of the multilevel interconnects is related to the adhesion developed at the metal-dielectric interface. Since Cu/Teflon AF1600 adhesion is moderate and may not satisfy the requirements of the microelectronics industry, new treatments of the fluoropolymer surface are needed to improve it. In this note, we present several surface modifications, such as the formation of reactive sites during intense X-ray exposure, and S- or N-grafting, activated by UV radiation in the presence of H₂S and NH₃; copper is well known to react with both thiols (R—SH) and amines (R—NH₂) to form strong bonds. Both X-ray exposure and N-grafting lead to enhanced adhesion.     

等离子体改性聚丙烯膜表面的XPS研究

介绍用等离子体技术对PP膜(PPF)表面进行改性处理。所用等离子体气氛分别为Ar、N_2、O_2和空气,处理压力13.3Pa、功率80W、处理时间10min。通过XPS(X射线光电子能谱)研究了改性前后聚合物表面的元素组成、相对含量变化、表面官能团的类型。采用曲线拟合分峰技术对谱图进行数学处理。结果表明,在氧气氛中处理PPF膜,表面氧含量可达18.93%、在氮气氛中处理,表面氮量可达4.76%。处理后的PPF表面,由于氧和氮与碳形成了活性基团,C—O、C—NH_2、C=O、—CONH—和COOH,从而提高了PPF表面的活性。此外,通过处理功率和压力对PPF表面结构影响的实验表明,两者对PPF表面的O和N含量无明显影响。     

XPS Structural Studies of Nano-composite Non-platinum Electrocatalysts for Polymer Electrolyte Fuel Cells

The chemical structure of non-platinum electrocatalysts obtained from cobalt porphyrins (CoTMPP or CoTPP) by pyrolysis is investigated by X-ray Photoelectron Spectroscopy (XPS). The catalysts represent highly dispersed, self-supported nano-composites that demonstrate electrocatalytic performance for oxygen reduction and practically no activity in methanol electro-oxidation. High-resolution Co2p, C1s, N1s and O1s XPS spectra acquired from precursors and electrocatalysts pyrolyzed at various experimental conditions were curve-fit using (a) individual peaks of constrained width and shape as well as (b) experimentally obtained photopeaks from the precursor with additional peaks required for a complete curve fit. Principal Component Analysis (PCA) applied to quantitative results from the curve-fits of both types of spectra facilitates visualization and identification of the chemical species that are formed or destroyed, and simplifies evaluation of critical correlations. As a result of these studies it is established that the catalyst is a nano-composite of highly dispersed pyropolymer with some remaining N _x -centers inserted into a graphite-like matrix. Approximately 50% of the metal is distributed as Co²⁺, associated with N₄-centers. The remaining cobalt is present in crystallites of metallic Co. A thin layer of CoO coats these metallic cobalt phases. The developed methodology, described herein, combines model curve-fits and principal component analysis (PCA), resulting in a quantitative and unambiguous understanding of the chemical composition and structure of complex electrocatalysts.     

Surface Characterization of Plasma Treated Carbon Fibers and Adhesion to a Thermoplastic Polymer

The surface chemistry of IM7 carbon fibers was characterized by x-ray photoelectron spectroscopy (XPS). The fiber surface energetics were determined from a two-liquid tensiometric method. The adhesion between as-received and plasma-treated carbon fibers and polyethersulfone (PES) was measured by the microbond pull-out test.

The surface characterization techniques showed that the effect of any plasma treatment is attained within less than 15 seconds. It was found that both argon and air plasmas increased the oxidation state of the fiber surface and that they reduced the dispersive component (γ_s^d) of the fiber surface free energy considerably. The ammonia plasma treatment resulted in a cleaning of the surface. This plasma treatment was also effective in improving the fiber/matrix adhesion of quenched samples. A similar adhesion enhancement between as-received fibers and PES is obtained by annealing the samples above the Tg of the polymer. The air plasma treatment did not have any significant effect on the fiber/matrix adhesion.     

Surface Characterization of Plasma Treated Carbon Fibers and Adhesion to a Thermoplastic Polymer

The surface chemistry of IM7 carbon fibers was characterized by x-ray photoelectron spectroscopy (XPS). The fiber surface energetics were determined from a two-liquid tensiometric method. The adhesion between as-received and plasma-treated carbon fibers and polyethersulfone (PES) was measured by the microbond pull-out test.

The surface characterization techniques showed that the effect of any plasma treatment is attained within less than 15 seconds. It was found that both argon and air plasmas increased the oxidation state of the fiber surface and that they reduced the dispersive component (γ_s ^d) of the fiber surface free energy considerably. The ammonia plasma treatment resulted in a cleaning of the surface. This plasma treatment was also effective in improving the fiber/matrix adhesion of quenched samples. A similar adhesion enhancement between as-received fibers and PES is obtained by annealing the samples above the Tg of the polymer. The air plasma treatment did not have any significant effect on the fiber/matrix adhesion.     

Reversible Switching of Surface Wettability and Water Adhesion on a Polymer Nano-composite Coating

Abstract

Super-hydrophobic polymer/ZnO nano-composite coatings were fabricated by a simple spray-coating method. When the prepared super-hydrophobic coating was directly illuminated by UV light, the surface was fully covered with the hydrophilic groups, which created a state of super-hydrophilic wettability. When UV light illuminated the surface through a mask, the surface maintained its super-hydrophobic property, but the adhesion of a water droplet changed from sliding easily to a highly sticky movement. More importantly, the initial surface wettability and water adhesion can be re-established by heat treatment, and these processes were repeated, with full reproducibility, for a number of cycles.     

Surface Modification of Polymer Films for Improved Adhesion of Deposited Metal Layers

Plasma treatment and vacuum Al deposition on films from biaxially oriented polypropylene is a multistep large scale industrial process, mainly ending up in packaging film laminates. As atmospheric plasma treatment processes suffer from lack of reproducibility, low pressure plasma treatment processes that can be operated in-line with the metal deposition are being developed. Process development is difficult, because the final packaging film laminate has to deliver optimum properties of adhesion as well as of the barrier against oxygen and water vapor permeation. As a typical production run involves tens of thousands to up to one hundred thousand square meters of film, experiments on an industrial scale are expensive, so smaller scale experimental processes are needed, which so far do not match well enough with industrial process characteristics. Moreover, bonding mechanisms between the treated substrate film and the deposited Al layer are not sufficiently understood. This paper describes the sequence in development and optimization of substrate films and plasma treatment that has been performed on an experimental as well as on an industrial scale. A sufficient correlation between experimental and industrial scales was achieved, which helps to perform development and optimization on a small scale before scaling up to industrial processes. However, improvement is still needed both in fundamental understanding of the aluminum–polypropylene interface as well as in experimental equipment and methodology.     

AFM Scratching for Adhesion Studies of Thin Polymer Coatings on Steel

Abstract

Atomic force miscroscopy (AFM) scratching at constant applied forces was used to quantify the adhesion of polymer coatings to cold rolled steel (CRS) and to study the effectiveness of a pretreatment for improving the adhesion. The pretreatment was a phosphate-free zirconia-based coating. Thin layers of commercially available epoxy, acrylic and polyester-based polymer coatings, were applied to polished or pretreated cold rolled steel substrates and the surface was scratched at the edge of the polymer coatings with the AFM tip at increasing values of normal loads until the coating was removed. Adhesion strengths were determined from the minimum tip-sample interaction force and number of cycles (scans) at a particular applied force. The pretreatment significantly improved adhesion of the epoxy and acrylic-based coatings on CRS. Adhesion of the acrylic-based coating was found to be better than the epoxy coatings on the bare as well as pretreated steel. Adhesive strength of the polyester-based coating was inconclusive because it was very easily removed on application of small forces using the AFM tip. The AFM scratching technique was found to provide a quick, easy and effective way to make quantifiable comparisons in relative adhesive strengths of polymer coatings and the effect of pretreatments.     

Surface Energy and Adhesion Studies on Acrylic Pressure Sensitive Adhesives

The surface energy and adhesion dynamics of pressure sensitive adhesives-like networks (PSA-LNs) as mimics for PSAs were studied using JKR-based contact mechanics and peel tests. Acrylic acid (AA) was co-polymerized with 2-ethyl hexyl acrylate (2-EHA) and 1,6-hexane diol diacrylate (HDDA) to create PSA-LNs. The measured surface energy (27 to 31 mJ/m²) was sensible as surmised from their structure. Acrylic acid content increases the surface energy, threshold adhesion energy and adhesion hysteresis of PSA-LNs. Measurements of adhesion dynamics showed a dependence of adhesion energy to the 0.6–0.8 power of crack speed, depending upon the model chosen for analysis of the data. When compared with actual pressure-sensitive adhesive tape peel tests, the adhesion dynamics data predicted the peel strength. This study shows a direct relationship between threshold adhesion energy, crack propagation mechanics and peel strength measurements.     

Surface Energy and Adhesion Studies on Acrylic Pressure Sensitive Adhesives

The surface energy and adhesion dynamics of pressure sensitive adhesives-like networks (PSA-LNs) as mimics for PSAs were studied using JKR-based contact mechanics and peel tests. Acrylic acid (AA) was co-polymerized with 2-ethyl hexyl acrylate (2-EHA) and 1,6-hexane diol diacrylate (HDDA) to create PSA-LNs. The measured surface energy (27 to 31 mJ/m²) was sensible as surmised from their structure. Acrylic acid content increases the surface energy, threshold adhesion energy and adhesion hysteresis of PSA-LNs. Measurements of adhesion dynamics showed a dependence of adhesion energy to the 0.6-0.8 power of crack speed, depending upon the model chosen for analysis of the data. When compared with actual pressure-sensitive adhesive tape peel tests, the adhesion dynamics data predicted the peel strength. This study shows a direct relationship between threshold adhesion energy, crack propagation mechanics and peel strength measurements.     

Investigation on the Adhesion of Polymer Particles to the Surface of a Semiconductor

In this work the role of electrostatic forces in the adhesion of particles of dielectrics to a solid surface was studied. The experiment consisted in measuring simultaneously the force of adhesion and the charges of the electric double layer arising upon contact. The measurements were made with specially developed units—a pneumatic adhesiometer and a charge-spectrometer. The objects of study were polymer powders used in electrography, whose adhesion to thin layers of selenium samples were varied by varying the illumination and were estimated by their volume resistance in the dark and in the light. The shift in the Fermi level of the selenium caused by the change in the output work is accompanied by a change in adhesion of the particles. Experiments on the separation of polymer particles and films from selenium surface on illumination revealed spectral dependence of adhesion. In accordance with the electronic theory of adhesion, the effects observed may be attributed to the change in charge density of the electric double layer.     

Adhesion and Surface Analysis

In the last 25 years, surface sensitive analytical techniques have made a major contribution to our understanding of adhesion phenomena and problems. There are several areas where these techniques have provided important information including the identification of failure modes, the chemistry of a substrate before and after pretreatments, the stability of surfaces and interfaces, the identification of surface contaminants, the interaction across an interface and the nature of interphases. X-ray photoelectron spectroscopy (XPS or ESCA), Auger electron spectroscopy (AES) and static secondary ion mass spectrometry (SSIMS) have proved to be especially useful. Many examples of the usefulness of these techniques are given.     

Surface Energetics and Adhesion

The relationships between surface energetics and adhesion are critically reviewed. New data that confirm such relationships, for peel tests as well as lap shear tests, are presented. The effect of hydrothermal aging of aluminum surfaces on surface energetics can be used to predict degradation in bond strength. The mechanism of failure for elastic adhesives (such as Scotch ® tape) in peel tests may be essentially the same as for more brittle adhesives (such as epoxies) in lap shear tests. This mechanism may involve brittle fracture that forms a critical flaw at the adherend-adhesive interface (on a microscopic level), followed by crack propagation which then may include considerable elastic and plastic deformation. The locus of propagation (fractography) is generally not (but may be) relevant to the problem of how to remedy mechanical weakness in an adhesive joint, since the local region of critical flaw formation rather than the general surface area determines the joint strength.     

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.     

HMX塑料粘结炸药的粘接研究

测试了水和甲酰胺对 HMX塑料粘结炸药 (PBX)药柱表面的接触角 ,它们的接触角分别为 82°和 32°;计算了 HMX药柱的表面能为 91 m N/ m,这表明 HMX塑料粘结药柱表面为低能表面。比较了几种结构胶粘剂粘接药柱的剪切强度 ,使用环氧胶时为 7.6MPa。试验发现胶粘剂种类、厚度对 HMX塑料粘结剂药柱爆速的影响较小     

Surface Roughness and Particle Adhesion

One of the important mechanisms affecting particle adhesion is the geometry of the contact between the particle and the surface. Atomic force microscopy can measure both this geometry and the particle adhesion. The positional dependence of adhesion of a point probe to a variety of rough surfaces has been measured. The Derjaguin approximation predicts that the adhesion fluctuations are proportional to the surface curvature fluctuations, if the adhesion is dominated by long range forces. Atomic force microscopy adhesion maps have directly verified this linearity.     

Surface Roughness and Particle Adhesion

One of the important mechanisms affecting particle adhesion is the geometry of the contact between the particle and the surface. Atomic force microscopy can measure both this geometry and the particle adhesion. The positional dependence of adhesion of a point probe to a variety of rough surfaces has been measured. The Derjaguin approximation predicts that the adhesion fluctuations are proportional to the surface curvature fluctuations, if the adhesion is dominated by long range forces. Atomic force microscopy adhesion maps have directly verified this linearity.     

Adhesion in Polymer/Steel Sandwiches

Polymer/steel sandwiches are able to reduce the nuisance due to vibrations and noise in automotive applications, for example. Thin layers of polymer are inserted between two metal sheets. The deformation of the polymer is responsible for the damping properties of the sandwiches and, therefore, the viscoelastic behavior of the polymer is of major importance. However, adhesion between the two materials is also required. The polymer studied in the present work is a copolymer of ethylene and vinyl acetate (EVA) containing 28 wt% of vinyl acetate grafted with maleic anhydride (1 wt%). A wedge test is used to measure the interfacial strength and the durability of the adhesive bond. The influence of the surface treatment of the steel substrate on the adhesive behavior and the effect of water has been studied. FTIR surface analysis after cleavage helped us to identify the nature of the interfacial bonds.