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 ( λ 2 → 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 λ s 2 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.