3. Forces Involved in Adhesion

The establishment of interfacial bonds through forces at the interface causes materials to attract one another. Therefore adhesion between materials and adhesive in assemblies are intimately related to the interatomic and intermolecular interactions at the interface of the two considered surfaces. Describing the mechanism responsible for adhesion in simple terms is difficult due to the complexity and evolving understanding of the subject. Nevertheless, when considering adhesion phenomena it is important to consider both the bulk and surface mechanical properties of the materials in contact and the type of interfacial forces established at the interface. High adhesion can only be obtained if the interface can sustain sufficient stress to induce dissipative forms of deformation, such as flow, yield, or crazing, in the polymer. Under most circumstances such dissipative processes can only be obtained when the interface is coupled with sufficient density of bonds. This chapter focuses on the description of the main forces responsible for adhesion, from strong covalent bonds to weak van der Waals forces, also considering some more specific interactions such as acid-base (like hydrogen bonding) or capillary forces (that could, as an example, influence adhesion of nanoparticles). The second part of this chapter concerns recent developments in experimental scanning probe techniques that may give assess to direct adhesion forces determination at the nanoscale. A case study is presented.