Interdiffusion in polymer blends: The effect of compressibility

The polymer/polymer interdiffusion in a binary compressible blend consisting of long and short chains is investigated within the framework of the dynamic random phase approximation (RPA). The relative contributions of two (fast and slow) stages into the total relaxation of compositional heterogeneities are explicitly shown to be strongly dependent on the initial conditions and the degree of asymmetry of the blend. Special emphasis is given to the influence of the initial distribution of free volume on the apparent rate of the composition relaxation. The evolution of the dynamic struture factor is described as well. It is shown that most of the peculiarities usually ascribed to the "fast-mode" behavior can be derived within the RPA approach.