The solution of the many-body statistical mechanical theory of elasticity formulated by James and Guth in the 1940s is presented. The remarkable aspect of the solution is that it gives an elastic free energy that is essentially equivalent to that developed by Flory over a period of several decades.
IntroductionRubber elasticity is the first bulk property of polymers that yielded to theoretical analysis. The identification of the relation between the Gaussian distribution of the end-to-end distance of a random walk with the quadratic strain dependence of phenomenological stress-strain theory was key to this success. Of course, the underlying physics that makes this connection inevitable and viable is that a polymer chain in the bulk amorphous melt phase is unperturbed by intermolecular interactions.[1] This fact allows one to realize the force that a chain delivers to the cross-links that terminate it, and which tie it to other chains in the three-dimensional space filling random network, is determined solely by the chain's intramolecular potential. While high elasticity theory was the first to successfully predict bulk polymeric materials behavior, it remains one of the few, and perhaps the only, analytical theory of polymers to do so, computer simulations notwithstanding. This is reason enough to justify efforts to improve upon the theory. Given the history of the subject, rubber elasticity is one of the first soft materials to admit an atom-based theoretical analysis.The extent to which the elastic equation of state is determined by the interaction between network connectivity and chain statistics has been a point of contention from the earliest days of polymer theory. The theory initiated by Kuhn,[2] elaborated by Wall [3,4]and Flory and Rehner, [5,6] and discussed extensively in treatises, [1,7,8,9,10] is constructed by adding together the contributions to the stress from independent chains. This requires the so-called affine assumptionthe displacement of the ends of an average network chain is congruent to the macroscopic strain.This theory is relatively easy to execute, but by treating the chains as independent it incurred the criticism of James and Guth. [11,12,13] In their many-body theory the individual chains obey Gaussian statistics, just as in the independent chain theory, but James and Guth emphasized the fact that in tying the chains together with cross-links they become an indissoluble whole that must be treated as a single entity. This insight carried a heavy price -their many-body formulation was too difficult to be convincingly solved. To make progress, James and Guth [11] introduced the