ABSTRACT
In this chapter we consider one of the three general classes of polymers in the solid state: infinite
networks. The other two categories, glassy polymers and semicrystalline polymers, will be taken
up in Chapter 12 and Chapter 13, respectively. We will shortly define the term network more
precisely, but we have in mind a material in which covalent bonds (or other strong associations)
link different chain molecules together to produce a single molecule of effectively infinite
molecular weight. These linkages prevent flow and thus the material is a solid. There are two
important subclasses of network materials: elastomers and thermosets. An elastomer is a crosslinked polymer that undergoes the glass transition well below room temperature; consequently,
the solid is quite soft and deformable. The quintessential everyday example is a rubber band. Such
materials are usually made by cross-linking after polymerization. A thermoset is a polymer in which multifunctional monomers are polymerized or copolymerized to form a relatively rigid
solid; an epoxy adhesive is a common example. In this chapter we will consider both elastomers
and thermosets, but with an emphasis on the former. The reasons for this emphasis are that the
phenomenon of rubber elasticity is unique to polymers and that it is an essential ingredient in
understanding both the viscoelasticity of polymer liquids (see Chapter 11) and the swelling of
single chains in a good solvent (see Chapter 7). In the first two sections we examine the two
general routes to chemical formation of networks: cross-linking of preformed chains and poly-
merization with multifunctional monomers. In Section 10.3 through Section 10.6 we describe
successively elastic deformations, thermodynamics of elasticity, the ‘‘ideal’’ molecular description
of rubber elasticity, and then extensions to the idealized theory. In Section 10.7, we consider the
swelling of polymer networks with solvent.
