Viscoelastic properties of networks with low concentration of pendant chains

A good knowledge of the molecular structure of polymer networks allows to relate rheological properties with different molecular parameters. In this work we analyze the influence of low concentrations of pendant chains on the viscoelastic properties of polymer networks. Model networks, with a well defined structure, were synthesized by reacting a commercial α,ω-divinyl poly(dimethylsiloxane) (B₂) with a trifunctional cross-linker bearing silane groups (A₃) and known amounts of an anionic ω-vinyl poly(dimethylsiloxane) (B₁). The structure of the networks was predicted with a molecular model based on a mean field approach (recursive model) taking into account the initial composition of the reactants. Rheological characterization was carried out in a rotational rheometer by dynamic and stress relaxation test. Viscoelastic properties of the networks depend on both concentration and molecular weight of pendant chains. Relaxation modulus was adjusted by the empirical Chasset-Thirion equation. It was found that it provides a very good fit to the behavior of these networks prepared by end-linking. The fitting parameter m in the Chasset-Thirion equation shows a strong dependence with the molecular mass of pendant chains, but it is rhougly independent of concentration. The results agree remarkably well with the predictions of a theoretical model previously reported by our group.