Ideal adhesion experiment

Figure 7.13. The JKR experiment for measuring the ideal adhesion energy between a rigid substrate and an elastomer. A hemispherical cap of the elastomer of radius R is brought into contact with the substrate and loaded with a force P. This results in a displacement of the cap <5 and the formation of a circular area of contact radius a.

When attempting to relate the adhesion force obtained with the SFA to surface energies measured by cleavage, several problems occur. First, in cleavage experiments the two split layers have a precisely defined orientation with respect to each other. In the SFA the orientation is arbitrary. Second, surface deformations become important. The reason is that the surfaces attract each other, deform, and adhere in order to reduce the total surface tension. This is opposed by the stiffness of the material. The net effect is always a finite contact area. Depending on the elasticity and geometry this effect can be described by the JKR 65 or the DMT 1661 model. Theoretically, the pull-off force F between two ideally elastic cylinders is related to the surface tension of the solid and the radius of curvature by... 

In practice, this idealized experiment is impossible because, when the surfaces get close together, the adhesion foree increases very rapidly and an instability occurs such that the surfaces jump into contact. Essentially, the adhesion force is so strong that it overcomes the elastic resistance of the materials. So the situation shown in Fig. 7.8(b) cannot exist instead, the system goes to the position shown in Fig. 7.8(c), however much we try to control the positions of the surfaces. This is the crack geometry. Molecular contact is made over part of the surface, and there is no contact over the rest of the bodies. 

A direct relationship was found by Bradley [25] in experiments on the adhesion of quartz spheres under vacuum. Corn [89] also found a direct variation of adhesive force with microparticle size. In essence, in the work of Bradley and Corn, interaction was determined under vacuum between the fused ends of glass fibers and a sphere (autohesion) or a plane surface (adhesion), the contiguous bodies having ideally smooth, clean surfaces. The elimination of electrical charges and capillary forces under vacuum provided grounds for the assumption that the measured values reflected only the molecular component of adhesive force in the interaction of the fused ends of the fiber with a sphere or a fiat surface. 

An ideal buccal deUveiy system should stay in the oral cavity for at least few hours and release the active ingredients in a unidirectional way towards the mucosa in a controlled or sustained-release fashion. Muco- and bioadhesive polymers are supposed to prolong the residence time of the device in the oral cavity. One of the experiments developed a direct staining method to visualize the polymer adhesion to human buccal cells after exposure to an aqueous dispersion of chitosan. An in vivo study was performed on a test group of human subjects, who rinsed with... 

A different type of experiment was performed by Laurence et al. with a pair of compatible polymers A + B (where A is polyvinyl chloride and B is polycaprolactone). To their surprise, they found D N rather than D N. In fact, their results are quite normal (de Gennes, 1982) the mixture /1 + B is not ideal (in contrast with the experiments discussed previously). A molecule of type A prefers to be in contact with molecules of type B. Thus, a region which is rich in A can be more readily reached by B molecules than in an ideal mixture. When we include these thermodynamic effects, we get exactly D N. Similar changes are possible when studying the adhesion of two polymers A and B (de Gennes, 1982). 

Considering only the bulk adhesive properties, the ideal pressure-sensitive adhesive would have a broad molecular weight distribution with a fraction at very high molecular weight, but lightly cross-linked. Such a material will be highly extensible, but unable to flow. It will provide rheological loss over a broad spectrum of relaxation times, and thus be resistant to flow on various time scales (or alternatively temperatures). The Tg of the adhesive should be selected such that it is sufficiently below the lowest temperature/deformation rate it will experience in... 

It is clear from the situation just described that the latter situation is ideal for a stable chemical modification of the metal substrate and that molecules should be chosen that are able to interact strongly with the metallic substrate. However, there are some characteristic differences between corrosion inhibitors and molecular adhesion promoters whereas for corrosion inhibition, the composition and structure of the metal surface are defined by the corrosive medium, the surface properties can be changed and adjusted to the structure of the adhesion promoter. Inhibitors must be soluble in the electrolyte (e.g., water) and can be applied only for well-defined reaction conditions. Adhesion promoters, however, may be applied from aqueous or nonaqueous solvents or even from the gas phase and the reaction conditions can be optimized for the given substrate. Based on the broad knowledge gained from decades of experience with inhibitors, it seems reasonable that some characteristic molecular features of inhibitors such as heteroatoms S, P, and O should be incorporated into the structure of the adhesion promoter however, the molecule itself should show minimum solubility in water and the possibility to bind a polymer onto the adhesion promoter. 

Another method to study the contact mechanics and adhesion behavior of soft solids is the so-called JKR test using elastomeric poly(dimethylsiloxane) (PDMS) lenses that are brought in contact with flat surfaces or with each other [884]. The soft PDMS ensures almost ideal JKR behavior of the contacting surfaces. Apphed load, indentation and contact radius, and neck shape can be determined simultaneously, which allows comparison with the JKR predictions. The surfaces of the lenses can easily be modified by treatment with an oxygen plasma to induce a silica-like surface that can then be modified using silane chemistry. As long as these layers are kept thin, the mechanical properties will still be dominated by the bulk PDMS. This type of experiments have been used extensively to study the influence of separation rate on adhesion (for a review, see Ref. [885]). 

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