The random phase approximation is summarized for compressible polymer
blend mixtures in the homogeneous phase region. Similarities with the
polymer reference site interaction model are described. The effect of
compressibility is also discussed in terms of 'free volume' by introdu
cing an extra component to represent 'small voids'. Variation oft he e
ffective Flory-Huggins interaction parameter with composition is found
to agree with results obtained using the lattice cluster theory.