IDENTIFICATION OF MODES IN DYNAMIC SCATTERING FROM TERNARY POLYMER MIXTURES AND INTERDIFFUSION

It is shown that the two exponential modes in dynamic scattering from ternary polymer mixtures cannot in general be identified with the interdiffusion and cooperative diffusion processes, contrary to recent suggestions in the literature. Conditions under which such an identification is possible are obtained. General expressions for the interdiffusion and cooperative diffusion coefficients associated with a given pair of components A and B in a ternary or in general in a multicomponent polymer mixture are obtained in terms of the partial mobilities mu-alpha-beta and partial scattering functions S-alpha-beta(q) of the A and B homopolymers. When the latter are expressed in terms of the bare mobilities and static structure factors by the random phase approximation, a new expression of the interdiffusion coefficient in an incompressible ternary mixture of A and B homopolymers in a matrix of C homopolymers is found in terms of the tracer diffusion coefficients of the three components in the mixture. It is shown that the results of the controversial "fast mode" and "slow mode" theories are obtained as the two limits of this new expression as a function of the concentration and bare mobility of the matrix. The approach used in the paper is compared with the Onsager formalism.