DYNAMIC VISCOELASTIC CHARACTERIZATION OF A POLYURETHANE NETWORK FORMATION

The evolution of the frequency dependence of the viscoelastic properti es during the gel formation of polyurethane has been monitored as a fu nction of time. Over the frequency range investigated, the frequency d ependence of the shear moduli G' and G'' are well described by the pow er laws G' similar to omega(Delta') and G'' similar to omega(Delta''), for all the reaction times investigated. Gel times are taken as the c ritical times where a frequency independent loss tangent (tan(delta) = G''/G') is observed. At these critical times, the frequency dependenc ies of G' and G'' follow the same power law with: the same exponent De lta' = Delta'' = 0.69 +/- 0.04, which, within the experimental error, is independent of the initial stoichiometry r. Measurements of fully c ured 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 b ehavior of G'' appears to be independent of r (for r > 0.6) and is not as yet included in the theoretical models.