Analytic stress-strain relationship for isotropic network model of rubber elasticity

James and Guth [3] have used the inverse Langevin function-based expression of tension in a polymer chain to build the so-called 3-chain model of uncompressible rubber elasticity. It is an analytical model, but it is not isotropic. If one considers an isotropic superposition of an infinite number of chains in all the directions, one obtains an isotropic model, but it is no more tractable analytically. Following Cohen [12], we propose to replace the inverse Langevin function by its Padé approximant, the two functions being very close. The isotropic model is then analytically integrable, and yields large strain strain-stress relationship.