Surface properties adhesion

An atom or molecule that approaches the surface of a solid always experiences a net attractive potential ). As a result there is a finite probability that it is trapped on the surface and the phenomenon that we call adsorption occurs. Under the usual environmental conditions (about one atmosphere and 300 K and in the presence of oxygen, nitrogen, water vapor and assorted hydrocarbons) all solid surfaces are covered with a monolayer of adsorbate and the build-up of multiple adsorbate layers is often detectable. The constant presence of the adsorbate layer influences all the chemical, mechanical and electronic surface properties. Adhesion, lubrication, the onset of chemical corrosion or photoconductivity are just a few of the many macroscopic surface processes that are controlled by the various properties of a monolayer of adsorbates. 

Leenars has shown both from theoretical considerations and experimental measurements that the passage of a liquid-gas phase boundary along the substrate may result in particle removal. It is shown that since both Ep/g (Equation (9.2)) and (Equation (9.5)) are both proportional to R, the surface tension effects can be utilized to clean particles of all sizes. Theoretical considerations are described in Reference (11). By adopting a suitable combination of the cleaning liquid, particle, and surface properties, adhesion forces can be overcome by surface-tension-related forces — the adhesion of the particles to the liquid-gas phase boundary is stronger than their adherence to the substrate. This leads to subsequent removal of the particle into the moving liquid. McConnell has shown that isopropyl alcohol (IPA) can be used to achieve this type of... 

Physical characterization of macromolecular systems strives to determine chemical structure/property relationships. This subfield includes study of thermomechanical performance viscoelastic properties surface properties, adhesion science thermal transitions morphological analysis, including semicrystalline, amorphous, liquid-crystalline, and microphase-separated structures. Structural analysis employs electron microscopy, con-focal microscopy, optical microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, and x-ray and neutron scattering of macromolecular compositions. 

Conventional polymers do not always possess the combination of desired bulk and surface properties for a specific application. The polymer materials used for microfluidic devices are innately hydrophobic, low-surface-energy materials and thus do not adhere weU to other materials brought into contact with them. This necessitates their surface modification/treatment to render them adhesive. This has prompted the development of a variety of polymer modification techniques, with the aim of developing new materials from known and commercially available polymers that have desirable bulk properties (elasticity, thermal stability, permeability, etc.) in conjunction with newly tailored surface properties (adhesion, biocompatibUity, optical reflectivity, etc.). 

The picture shows stem cells from bone marrow adhered to a solid substratum, where they may differentiate into tissue cells. In vitro differentiation of stem cells is strongly influenced by interactions between the cell and the substratum and, hence, by their surface properties. Adhesion in biological systems is discussed in Chapter 20, based on principles and concepts from colloid and interface science presented in the foregoing chapters. (Figure courtesy of Leibniz Institute for Polymer Research, Dresden, Germany.)... 

It should be noted that for an adhesive which is behaving primarily as an elastic solid, the detachment failure criterion used by Derail et al. is equivalent to a stored elastic energy density criterion but under conditions where the deformation is primarily viscous, the two criteria are quite different. None of these authors has been able to successfully link the values of these failure criteria to fundamental interfacial properties or the thermodynamic work of adhesion. Clearly, much remains to be done to complete our understanding of the relationships among surface properties, adhesive rheology, and peel force. 

Plasma processing technologies ate used for surface treatments and coatings for plastics, elastomers, glasses, metals, ceramics, etc. Such treatments provide better wear characteristics, thermal stability, color, controlled electrical properties, lubricity, abrasion resistance, barrier properties, adhesion promotion, wettability, blood compatibility, and controlled light transmissivity. 

The exceUent adhesion to primed films of polyester combined with good dielectric properties and good surface properties makes the vinyhdene chloride copolymers very suitable as binders for iron oxide pigmented coatings for magnetic tapes (168—170). They perform very weU in audio, video, and computer tapes. 

As is true for macroscopic adhesion and mechanical testing experiments, nanoscale measurements do not a priori sense the intrinsic properties of surfaces or adhesive junctions. Instead, the measurements reflect a combination of interfacial chemistry (surface energy, covalent bonding), mechanics (elastic modulus, Poisson s ratio), and contact geometry (probe shape, radius). Furthermore, the probe/sample interaction may not only consist of elastic deformations, but may also include energy dissipation at the surface and/or in the bulk of the sample (or even within the measurement apparatus). Study of rate-dependent adhesion and mechanical properties is possible with both nanoindentation and... 

A chemical property of silicones is the possibility of building reactivity on the polymer [1,32,33]. This allows the building of cured silicone networks of controlled molecular architectures with specific adhesion properties while maintaining the inherent physical properties of the PDMS chains. The combination of the unique bulk characteristics of the silicone networks, the surface properties of the PDMS segments, and the specificity and controllability of the reactive groups, produces unique materials useful as adhesives, protective encapsulants, coatings and sealants. 

Even the void fraction together with particle size distribution does not provide all of the necessary information on the kind of flow. The mutual forces between distinct particles depend not only on the distance between the particles but also on the surface properties of the particles. The strength of the attractive forces between particles depends on conditions. For instance, the moisture content of the solid is essential for determining the attractiv c forces between particles, especially for hydroscopic materials such as wood. Airflow between particles usually tends to separate particles, whereas the surface forces, adhesion forces, tend to bring them together. 

Electron beam-initiated modification of polymers is a relatively new technique with certain advantages over conventional processes. Absence of catalyst residue, complete control of the temperature, a solvent-free system, and a source of an enormous amount of radicals and ions are some of the reasons why this technique has gained commercial importance in recent years. The modification of polyethylene (PE) for heat-shrinkable products using this technique has been recently reported [30,31]. Such modification is expected to alter the surface properties of PE and lead to improved adhesion and dyeability. 

A surface is that part of an object which is in direct contact with its environment and hence, is most affected by it. The surface properties of solid organic polymers have a strong impact on many, if not most, of their apphcations. The properties and structure of these surfaces are, therefore, of utmost importance. The chemical stmcture and thermodynamic state of polymer surfaces are important factors that determine many of their practical characteristics. Examples of properties affected by polymer surface stmcture include adhesion, wettability, friction, coatability, permeability, dyeabil-ity, gloss, corrosion, surface electrostatic charging, cellular recognition, and biocompatibility. Interfacial characteristics of polymer systems control the domain size and the stability of polymer-polymer dispersions, adhesive strength of laminates and composites, cohesive strength of polymer blends, mechanical properties of adhesive joints, etc. 

Majumder and Bhowmick [381] have investigated the influence of the concentration of TMPTA on the surface properties of EPDM mbber, modified in the presence of EB. The surface energy of the TMPTA-modified EPDM mbber has been observed to increase as compared to the unmodified one. Table 31.4 displays the contact angles and the work of adhesion of water, for the control and the modified EPDM surfaces. 

Nature takes various approaches to adhesion, the most common being to control contact area through deformation of a multitude of hairlike fibrils or alternatively to modify the surface property in a way analogous to chemical force microscopy. These processes have been... 

Adhesion of non-polar polymers is sometimes improved by oxidation, so essentially what is normally thought of as degradation may be used to introduce an enhancing surface property. Nevertheless, in both cases the oxidation process modifies the polymer surfaces, and the analysis tools can be rather similar, see, e.g., Ref. [101]. 

What one considers included in surface properties depends on what one considers the surface. (We are not in this chapter considering specific coatings per se that may be applied to achieve a particular effect or property). For adhesion... 

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