Mechanical and swelling behaviors of end-linked PDMS rubber and randomly cross-linked polyisoprene

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 (λ² → 1), where λ 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 (χ_c) and uncross-linked (χ_u) systems are different. The full analysis of S vs λs² within the FFR model also leads to the new result that the difference between χ_c and χ_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.