Dynamic viscoelastic characterization of a polyurethane network formation

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' similar to omega(Delta') and G'' similar to omega(Delta''), for all the reaction times investigated. Gel times are taken as the critical times where a frequency independent loss tangent (tan(delta) = G''/G') is observed. At these critical times, the frequency dependencies of G' and G'' follow the same power law with: the same exponent Delta' = Delta'' = 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 Delta'' = 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.