In the present work, we report the results of an investigation of the temperature and cross-link density dependences of the swelling activity parameter S, or dilational modulus, which represents a difference in free energy between the cross-linked and the uncross-linked systems, for PDMS and polyisoprene within the framework of the Frenkel-Flory-Rehner (FFR) theory. We find that the values of S (lambda(2) -> 1), where lambda is the swelling stretch, are nonzero and increase as the cross-linking density increases, implying that the Flory-Huggins interaction parameters, which represents the energy contribution from interspersing polymer and solvent molecules, of the cross-linked (chi(c)) and uncross-linked (chi(u)) systems are different The full analysis of S vs lambda(2)(s) within the FFR model also leads to the new result that the difference between chi(c) and chi(u) decreases as the degree of swelling increases. Results from torsional measurements for network elasticity determination of the PDMS end-linked networks are also presented.