Adhesion force, viscoelastic contribution

Nanomechanical mapping has been applied to several material systems to date, as introduced in Section 3.3. However, in these applications we adopted Hertzian theory and argued only elastic modulus, and therefore the analyses were subject to many restrictions. More seriously, practical measurements must be performed under appropriate conditions to avoid other complex interactions, such as adhesion and viscoelasticity, and to obtain precise and correct results. Measurement in an aqueous environment to avoid adhesion effects is a possible example, where we can suppress the water capillary effect, which is unavoidable, and the major contribution to the adhesion force under ambient conditions. 

Figure 6.11 shows an impressive result of practical adhesion its interpretation via the van Oss-Good parameters is shown here (from the work of McCafferty, 2002, mentioned in Chapter 3 and Section 6.2.2). The peel energy and thus the adhesion for the acidic pressure-sensitive (commercially available) adhesive is clearly highest for the basic oxide film of aluminium. This means that there is greater affinity for this basic oxide film to donate electrons to the acidic polymer. On the other hand, the very basic PMMA is attracted by the acid oxide Si film. These increased acid—base interactions lead to increased practical adhesion. Although the measured pull-off force for PMMA includes both interfacial and viscoelastic contributions, since the polymer is the same for each metal surface, the viscoelastic contributions may be taken to be the same in all cases. Thus, differences in the measured pull-off forces may be ascribed to differences in interfacial adhesion. 

CR 3nd tp are the contributions from chain recoiling and interfacial dynamics (i.e. drag forces and disentanglement), respectively, and / ve is the viscoelastic loss function which has interfacial and bulk parts. / is a characteristic length of the viscoelastic medium, t is the contact time and n is the chain architecture factor. Fig. 21 illustrates the proposed rate dependency of adhesion energy. 

The atomic force microscope (AFM) is a promising device for the investigation of materials surface properties at the nanoscale. Precise analysis of adhesive and mechanical properties, and particularly of model polymer surfaces, can be achieved with a nanometer probe. This study distinguishes the different contributions (chemical and mechanical) included in an AFM force-distance curve in order to estabhsh relationships between interfacial tip-polymer interactions and the surface viscoelastic properties of the polymer. 

For both PDMSs, for both substrates, and for all friction speeds, a great effect of normal force is observed. The higher friction coefficient observed at low normal force could be explained by the role of adhesion, which is magnified at low load (where the bulk contribution is lower). The contribution of interfacial interactions (or adhesive contact) is then magnified. These interfacial interactions will activate viscoelastic dissipation mechanisms, increasing the friction resistance. 

These surface energies are the basis for calculating the thermodynamic work of adhesion, which also represents the work required to separate two surfaces. It must be stressed that is far smaller than the force required to remove a PSA tape from a substrate the experimental peel force includes significant contributions from the bulk viscoelastic properties of the adhesive and the backing, as pointed out later. Nevertheless fV and peel force can be correlated. 

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