Introduction

Many applications of polymers rely on their adhesion, between different pieces of identical polymer, between chemically different polymers or between a polymer and a non-polymer. Examples of situations in which identical polymers are joined include the wide variety of packaging materials that are sealed by heat, the welding of plastic pipes and, less obviously, the integrity of weld lines in injection moulded objects. Adhesion between different polymers is important in determining the mechanical properties of polymer mixtures and of coextruded polymer sheets, whereas polymer/nonpolymer adhesion determines the mechanical properties of filled polymers and composites, as well as being the basis of the widespread use of polymers in glues and adhesives. Given the increasing degree of understanding of the microscopic structure of polymer interfaces that we have discussed in earlier chapters, can we predict a macroscopic property such as the strength of such an interface?

A natural starting point would be to enumerate the bonds crossing an interface, whether chemical or physical, multiplying the average force needed in order to break each bond by the number of bonds per unit area to yield a maximum stress that the interface can sustain. In considering the bulk strength of materials, however, one rapidly finds that this is not always the most useful approach; flaws and cracks lead to values of the local stress that are greatly in excess of the average stress by the mechanism of stress concentration; this usually leads to brittle failure at values of the applied stress much lower than the calculated theoretical ultimate tensile stress (see for example Ashby and Jones (1980)).