Composition dependence of the viscoelasticity of end-linked poly(dimethylsiloxane) at the gel point

Precursor composition is shown to have a considerable impact on the viscoelastic properties of a polymer a t the gel point. End-linked poly(dimethylsi1oxane) polymers were prepared with various stoi-chiometry, chain length, and concentration. The power law relaxation modulus, G(t) = St-", identifies the gel point, and a small frequency window (2 decades) is sufficient to determine the relaxation exponent, n, and gel strength, S. Both are shown to be strongly composition dependent. The relaxation exponent ranges between 0.19 and 0.92 in this study, which is almost the entire possible range (0 < n < 1). The gel strength ranges over 5 decades. A prepolymer below the entanglement molecular weight produces critical gels with relaxation exponents in the range 0.4-0.7, depending on stoichiometry. A high molecular weight prepoly-mer, above the entanglement molecular weight, produces critical gels with lower n values, in the range of 0.2-0.4, depending on stoichiometry. Addition of a diluent also affects the relaxation exponent, and near the overlap concentration n approaches the limit of 1. In a series of experiments with different stoichiom-etry and concentration, S and n appear to be related by a material characteristic function S = GOA$, where Go and XO are characteristic constants of the precursor material and n depends on composition.