  Constitutive modelling of the stress-strain response of rubbery polymers is
described, with an emphasis on the limits to linearity for both neat and filled
rubber (the latter due to the Payne effect). Deviations from the Boltzmann
superposition principle (time-strain invariance) under reversing strain
histories are reviewed, including a discussion of the Mullins effect. The
chapter also reviews the application of the stress optic law and the conditions
causing its breakdown: internal stress, the glass transition, orientational
coupling, and creep recovery. Different empirical rules for nonlinear flow are
considered, including the Cox-Merz rule and the relations of Laun and Gleissle.
