Comparison of Diffusion Coefficients in Ternary Polymer Solutions Measured by Dynamic Light Scattering and Forced Rayleigh Scattering

Several dynamic light scattering (DLS) studies of ternary solutions (polymer A/polymer B/solvent) have recently been reported.^1-14 By selecting a solvent isore-fractive with polymer B and employing sufficiently small concentrations of (chemically distinct) polymer A, the resulting autocorrelation functions are reasonably interpreted as reflecting the dynamics of the A chains. Furthermore, the measurements have generally been restricted to the range qR < 1, and thus the measured diffusion coefficients have (with one exception²) been identified with the translational diffusion of A. This technique has been demonstrated as an effective means to investigate two areas of great current interest: diffusion mechanisms in semidilute and concentrated solutions and thermodynamics of multicomponent systems. Nevertheless, there are several reasons for caution in this identification of the measured diffusion coefficient as the tracer or pure center-of-mass translational diffusion of component A. These include (i) the concentration of A is necessarily nonvanishing in order to effect the measurements, (ii) the refractive index increment for component B in the solvent may not be exactly zero, (iii) the inevitable molecular weight polydispersity of the A chains has a substantial influence on the correlation function as the matrix concentration increases, even for nearly monodisperse samples,⁷ and (iv) the DLS properties for semidilute binary solutions are as yet incompletely understood, as illustrated both by the variety in number of modes reported and in their differing assignment to dynamical processes.