Adhesion buried interfaces

Although the above experiments involved exposure to the environment of unbonded surfaees, the same proeess oeeurs for buried interfaces within an adhesive bond. This was first demonstrated by using electrochemical impedance spectroscopy (EIS) on an adhesive-covered FPL aluminum adherend immersed in hot water for several months [46]. EIS, which is commonly used to study paint degradation and substrate corrosion [47,48], showed absorption of moisture by the epoxy adhesive and subsequent hydration of the underlying aluminum oxide after 100 days (Fig. 10). After 175 days, aluminum hydroxide had erupted through the adhesive. 

The characterization of the surface chemistry of the modified polymer is one step in understanding the mechanism for cells adhesion. The next crucial step is to determine the nature and extent of chemical interactions between the overlayer of interest and the modified polymer surface. This step presents a challenge because cmrently there are no techniques available with the sensitivity to characterize chemical interactions for an atomically thin buried interface. Several approaches have been used to analyze buried interfaces. Ion sputter depth profiling (typically done with Ar ions) in conjunction with XPS can be used to evaluate a buried interface for overlayers >10 nm [2]. 

While the macroscopic concepts of hardness, adhesion, friction, and slide have evolved over the last two centuries, atomic level understanding of the mechanical properties of surfaces eluded researchers. The discovery of the atomic force microscope in recent years promises to change this state of affairs. Being able to measure forces as small as 10 newton or as large as 10 newton [5] over a very small surface area (few atoms) and by simultaneously providing atomic spatial resolution, this technique permits the study of deformation (elastic and plastic), hardness, and friction on the atomic scale. The buried interface between moving solid surfaces can be studied with spectroscopic techniques on the molecular level. Study of the mechanical properties of interfaces is, again, a frontier research area of surface chemistry. 

Fig. 1.1 The problem that exists in the analysis of the buried interface a region responsible for adhesion that is nanometers thick buried between thick layers of adhesive or coating and substrate materials.

Fig. 31.1 Schematic representation of the measurement of eiectro-chemicai potentiais at buried interfaces (here cathodic deiamination of an adhesive iayer from an iron substrate) with the scanning Keivin probe. Voita potentiai difference A P, Keivin current i(t) induced by the vibration of the needie (reference), compensation voitage Uq.

Adhesion is a consequence of the chemical or physical interaction between two surfaces, one of which is a solid and the other a liquid, temporarily more mobile. As a consequence of the way in which adhesion is achieved practically. the interface or interphase region, where the bonds responsible for adhesion occur, is buried below many im, or even mm, of solid sub.stratc and solidified (generally crosslinked) polymer. The dimensions of the interphasc region are likely to be of the order of nm at the most (unless an extended mechanical interphase is present of the type found in the adhesive bonding of anodized aluminum alloys) so that direct examination of interphase chemistry is best considered as an exercise in the analysis of a deeply buried interface. This situation is encountered frequently by those working in microelectronics and in corro,sion and oxidation research. Removal of material is invariably ac-... 

In numerous applications of polymeric materials multilayers of films are used. This practice is found in microelectronic, aeronautical, and biomedical applications to name a few. Developing good adhesion between these layers requires interdiffusion of the molecules at the interfaces between the layers over size scales comparable to the molecular diameter (tens of nm). In addition, these interfaces are buried within the specimen. Aside from this practical aspect, interdififlision over short distances holds the key for critically evaluating current theories of polymer difllision. Theories of polymer interdiffusion predict specific shapes for the concentration profile of segments across the interface as a function of time. Interdiffiision studies on bilayered specimen comprised of a layer of polystyrene (PS) on a layer of perdeuterated (PS) d-PS, can be used as a model system that will capture the fundamental physics of the problem. Initially, the bilayer will have a sharp interface, which upon annealing will broaden with time. 

Note that many of these surface reactions involve the conversion of a hydrophophic polymer to one with a hydrophilic surface or vice versa. For example, the replacement of trifluoroethoxy groups at the interface by hydroxyl units changes a non-adhesive, highly hydrophobic surface to an adhesive hydrophilic one. Variations in the reaction conditions allow both the depth of transformation and the ratios of the initial to the new surface groups to be controlled. A possible complication that needs to be kept in mind for all of these surface transformations is that polymer molecular motions may bury the newly introduced functional units if the polymer comes into contact with certain media. For example, a hydrophilic surface on a hydrophobic polymer may become buried when that surface is exposed to dry air or a hydrophobic liquid. But this process can be reversed by exposure to a hydrophilic liquid. 

As the nature of the electrified interface dominates the kinetics of corrosive reactions, it is most desirable to measure, e.g., the drop in electrical potential across the interface, even where the interface is buried beneath a polymer layer and is therefore not accessible for conventional electrochemical techniques. The scanning Kelvin probe (SKP), which measures in principle the Volta potential difference (or contact potential difference) between the sample and a sensing probe (which may consist of a sharp wire composed of a conducting, stable phase such as graphite or gold) by the vibrating condenser method, is the only technique which allows the measurement of such data and therefore aU modern models which deal with electrochemical de-adhesion reactions are based on such techniques [1-8]. Recently, it has been apphed mainly for the measurement of electrode potentials at polymer/metal interfaces, especially polymer-coated metals such as iron, zinc, and aluminum alloys [9-15]. The principal features of a scanning Kelvin probe for corrosion studies are shown in Fig. 31.1. 

Therefore, the measurable Volta potential difference A p°f includes information on the corrosion potential at the buried metal/electrolyte interface only if the Donnan potential is known or small. Usually the Donnan potential is significant only for polymers with a high density of fixed charges (e.g., in ion-exchange membranes), as polymers with fixed cationic functional groups will exchange exclusively anions, and vice versa [27]. Adhesives or lacquers used for corrosion protec-... 

Generally, the interfacial region and the interfacial (interphase) material are difficult to characterize since they usually consist of a small amount of material buried under a relatively thick film. Figure 10.4 shows the RBS analysis of tungsten metallization of a Si-Ge thermoelectric element as deposited and after a furnace treatment, which diffused material at the interface. Before diffusion, the interface has no features discernible by RBS. Interdiffusion rejects the germanium and reacts to form a tungsten siUcide. After extensive diffusion the interface is weakened and the adhesion fails. 

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