DYNAMIC VISCOELASTIC MODULUS OF ASSOCIATIVE POLYMER NETWORKS - OFF-LATTICE SIMULATIONS, THEORY AND COMPARISON TO EXPERIMENTS

An off-lattice simulation model for associative polymer gels as introd uced recently (Groot, R. D.; Agterof, W. G. M. J. Chem. Phys. 1994, 10 0, 1649) has been applied to obtain the mechanical spectrum as a funct ion of frequency and polymer concentration, using a Green-Kubo relatio n for the time-dependent modulus. Two stages of relaxation are observa ble in our simulations. The early-time decay is consistent with a -2/3 power law, whose form is insensitive to large variations in polymer c oncentration, association lifetime, and degree of association. The lat e stage, which relaxes like the end-to-end vector in the Rouse model, has a characteristic stress that scales as the cube of the concentrati on and a relaxation time that is proportional to the monomer-monomer d issociation rate. The simulation results have been compared with exper iments found in the literature for several physical gels. The quantita tive agreement calls into question other postulated mechanisms involvi ng hydrodynamic interaction or reptation, since the simulation contain s neither of these features. As an alternative explanation for the obs erved early-time decay, an explicit relation between the power law exp onent and the polymer fractal dimension is given.