Dynamic Viscoelastic Characterization of a Polyurethane Network Formation

Abstract

The evolution of the frequency dependence of the viscoelastic properties during the gel formation of polyurethane has been monitored as a function of time. Over the frequency range investigated, the frequency dependence of the shear moduli G‘ and G‘‘ are well described by the power laws G‘ ∼ ωΔ‘ and G‘‘ ∼ ωΔ‘‘, for all the reaction times investigated. Gel times are taken as the critical times where a frequency independent loss tangent (tan(δ) = G‘‘/G‘) is observed. At these critical times, the frequency dependencies of G‘ and G‘‘ follow the same power law with the same exponent Δ‘ = Δ‘‘ = 0.69 ± 0.04, which, within the experimental error, is independent of the initial stoichiometry r. Measurements of fully cured networks (far above the gel point) show that G‘‘ follows a power law behavior with exponent Δ‘‘ = 0.54, while in the same frequency range, G‘ has already reached its plateau value. This low-frequency behavior of G‘‘ appears to be independent of r (for r > 0.6) and is not as yet included in the theoretical models.