THE FAST AND SLOW MODE THEORIES OF INTERDIFFUSION IN POLYMER MIXTURES- RESOLUTION OF A CONTROVERSY

The theory of interdiffusion of a pair of components in multicomponent polymer mixtures is reviewed from a statistical point of view, and th e foundation of the ''fast'' and ''slow'' mode theories, as well as th e more recent ''ANK'' theory of interdiffusion is critically examined. The ANK theory reproduces the results of the slow and fast mode theor ies as the two limits when the vacancy concentration is varied from ze ro to a large value, and shows that the interdiffusion coefficient in a binary compressible mixture at finite vacancy concentrations can not in general be expressed only in terms of the tracer diffusion coeffic ients of the components, but it involves in addition the cooperative d iffusion coefficient which characterizes the relaxation of total densi ty fluctuations. The predictions of the ANK expression for the molecul ar dependence of the kinetic factor is compared with recent scattering experiments.