DYNAMIC LIGHT-SCATTERING AND VISCOELASTICITY OF A BINARY POLYMER-SOLUTION

The effect of viscoelasticity on the dynamic light scattering spectrum of a binary polymer solution is examined using a hydrodynamic theory. It is shown that when the coupling of the concentration fluctuation to the viscoelasticity is present, the time correlation function of the concentration fluctuation is bimodal, consisting of a single-exponential decay and a group of relaxation modes, characterizing the relaxation of the longitudinal stress tensor of the binary solution. The rate constant of the single-exponential decay is equal to q2D(c), where q is the amplitude of the scattering vector and D(c) is the cooperative diffusion coefficient. The viscoelastic modes have rate constants spanning a wide dynamic range from the fast cooperative diffusion modes to the slowest stress relaxation modes. In appropriate limits, the present theory reduces to the result previously obtained by Brochard and de Gennes and that by Adam and Delsanti for the semidilute binary polymer solution using a transient gel model. In addition to binary solutions consisting of polymers of high molecular weight, the present result is also expected to apply to viscoelastic nonpolymeric liquids as well.