Modeling of stress-strain relations of non-Gaussian chains in swollen networks

The results of the Monte Carlo (MC) simulations on a tetrahedral lattice are reported about the elastic response of model chains with variable segmental interaction and flexibility. The single-chain elastic functions were calculated either through distribution function W(R) of end-to-end distances or the displacement R was directly computed for a given value of external force F. The computed force-displacement curves are strikingly affected by the chain-end constraints chosen, either of the "spring deformation" or of the "coil deformation" type, which correspond to the different physical situations. In all cases the elastic response is nonlinear and influenced by the solvent quality. The distribution function W-F(R) of a chain prestretched by external force F and the corresponding relations f(F) vs R were evaluated. By using the "three-chain model" of polymer networks the stress-strain isotherms were computed, which are directly relevant to the mechanical properties of the so-called c* gels. The isotherms show non-Gaussian behavior with an upturn in stress at higher elongation modulated by the solvent quality. The consequences of the solvent-sensitive elastic functions of a network to the separability of the mixing and elastic terms in the Helmholtz energy of swelling are pointed out.