Rheology of model polyurethanes at the gel point

Abstract

The universality of the gel equation, a recently suggested rheological equation for polymers at the gel point, was tested on three well-defined cross-linking polyurethanes (PU). Stoichiometric amounts of triisocyanate cross-linker (DRF) were mixed with a,w-dihydroxypoly(propylene oxides) (PPO) of nominal molecular weights 425, 1000, and 2000. As the cross-linking reaction progressed at 30 °C the evolution of the viscoelastic properties was measured. At the gel point, the storage modulus G' and the loss modulus G" were found to be congruent and proportional to w112, where w is the frequency of the oscillatory shear experiment. The same behavior was previously observed with an end-linking poly(dimethylsiloxane), however, with tetrafunctional cross-linking points.1•2 This suggests universal validity of the rheological equation for stoi­chiometrically balanced end-linking polymers.