Viscosity critical behaviour at the gel point in a 3d lattice model

Within a recently introduced model based on the bond-fluctuation dynamics, we study the viscoelastic behaviour of a polymer solution at the gelation t hreshold. We here present the results of the numerical simulation of the mo del on a cubic lattice: the percolation transition, the diffusion propertie s and the time autocorrelation functions have been studied. From both the d iffusion coefficients and the relaxation times critical behaviour a critica l exponent k for the viscosity coefficient has been extracted: the two resu lts are comparable within the errors giving k similar to 1.3, in close agre ement with the Rouse model prediction and with some experimental results. I n the critical region below the transition threshold the time autocorrelati on functions show a long-time tail which is well fitted by a stretched expo nential decay.