The Conformational Exhaustion Model

Another approach to rubber elasticity has been developed by Stepto and Taylor [25,26], which also accounts for the Mooney-Rivlin softening. Their approach is not phenomenological, but is based on structural considerations that give an accurate description of the molecular conformational states of the units in the polymer chains as the network is stretched. They have proposed a method for calculating the free energy of a stretched molecular network based on the rotational isomeric state of the network chains with conformational energies determined from observations of conformational properties. 

Comparison of the Monte Carlo p r) with those predicted on the basis of the Gaussian distribution function (Equation (4.18)) and the Langevin function (Equation (4.36)) shows clear differences. In particular, the molecular structure-based Monte Carlo p r) reflects clearly the limited extensibility of chains in the true network. 

In the Stepto theory, the free energy of the network is given by 

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