Network Recovery from Uniaxial Extension: I. Elastic Equilibrium

The elastic response of rubber has a direct and controllable dependence on structure. The behavior of the various networks described herein is seen to be consonant with the well-established roles of spatial fluctuations of crosslinks and the inhibition of this Brownian motion by steric and entanglement constraints from neighboring chains. Although elasticity also governs to a large extent the nonequilibrium mechanical response, interpretation of the latter in terms of network structure is obscured by viscoelasticity. The junctions of real networks, having some freedom to reconfigure themselves away from strictly affine displacement, exhibit lower equilibrium stresses than predicted by the statistical model for affine chains. To the extent that this overestimation of the model and the compressive component of the retraction response mimic the effect of departures from equilibrium, some conformance of network behavior during retraction to Equation (1) may be realized. It is such a fortuitous circumstance that likely underlies the various proposals commending such a relationship. A measure of junction density is generally not obtainable from deformation experiments in which dissipative processes are active.