Adhesion of Polymers
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trolled by both the bulk properties and the molecular-scale coupling at the material interface. High adhesion requires that the coupling at the interface is sufficiently strong that the stresses around a crack tip are large enough over a sufficient region to initiate considerable energy dissipation in the bulk material. In glassy polymers, the energy is mainly dissipated in the formation and growth of a crack-tip craze. A craze is a type of planar, cracklike, deformation zone where the two surfaces are joined by fine fibrils of highly drawn and strainhardened material. Energy is dissipated by d r a w i n g the fibrils from bulk material. Changing the properties of the bulk material without changing the properties of the interface can profoundly change the adhesion. This effect can easily be seen by crosslinking a rubbery polymer such as a siloxane, as shown schematically in Figure I.3 When uncrosslinked, the material is not very sticky. As the degree of crosslinking is increased to the gel point, the material becomes very sticky. If the material is cross-linked suf-
Effects of Bulk Mechanical Properties A joint is tough if a large amount of energy is dissipated on propagating a crack along it. The energy per unit crack area Gc is typically dissipated in plastic or viscoelastic processes in the material adjacent to the interface. However, the size of the region of energy dissipation is strongly material-dependent. Although the energy is dissipated within the bulk of the material and is therefore dependent on bulk material properties, the amount of dissipated energy is con-
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Crosslink Density Figure 1. Schematic variation of peel strength with crosslink density of a polymer at a temperature above its glass-transition temperature.
ficiently to form a tight network, it becomes a controllable release material such as a backing on an adhesive label. The bulk mechanical properties of a polymer glass are very different from those of an elastomer so it is not surprising that their adhesive properties are also very different. For example, van der Waals forces are large enough to cause high adhesion in some elastomeric adhesives (consider adhesive tapes) but glassy polymers normally require stronger atomic-scale bonding, such as covalent bonds, to give a tough interface. Here I will first discuss the adhesion of glassy materials and then later consider elastomers.
Interface Fracture Mechanics The fracture mechanics of interfaces differs from that of bulk material because the materials on either side of the interface are different and the interface controls the direction of crack propagation. Both effects ensure that one can no longer ignore the shear or mode II cracktip stress patterns. The ratio between the shear and opening mode crack-tip stress can be described approximately by the "mixity" ^ Values of the mixity exist for many layered geometries4 and also for some standard test geometries such as the peel test.^ Here I am concerned with the effect of mixity on the measured toughness of an interface, as this effect
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