  The two approaches to analyzing the large strain behavior of rubbery networks
are phenomenologically, using strain energy functions drawn from continuum
mechanics, and molecular models, which apply statistical mechanics to compute
the effect of chain orientation on the entropy. The early rubber elasticity
models ignored intermolecular interactions, whereas later developments
("constraint models") included the effect of entanglements or steric
constraints on the mechanical stress. These constitutive equations for rubber
elasticity are compared to experimental results, and the connection of network
elasticity to the relaxation behaviour is discussed. For conventional
elastomers there is a compromise between stiffness and strength. Different
methods to circumvent this limitation are described. Examples are given of the
properties obtained with novel network architectures, including
interpenetrating networks, double networks, bimodal networks, miscible
heterogeneous networks, and deswollen networks.
