Rheology of model polyurethanes at the gel point

The universality of the gel equation, a recently suggested rheological equation for polymers a t 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(propy1ene oxides) (PPO) of nominal molecular weights 425, 1000, and 2000. As the cross-linking reaction progressed a t 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 d2, 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.** * This suggests universal validity of the rheological equation for stoi-chiometrically balanced end-linking polymers.