Molecular influences on miscibility patterns in random copolymer/homopolymer binary blends

The lattice cluster theory (LCT) is used to study the microscopic molecular factors affecting the miscibilities of A(x)B(1-x)/C binary mixtures (where the homopolymer C is either different or identical to the A(x)B(1-x) random copolymer species). A prime goal of this study lies in describing gross departures of LCT predictions from the prevailing random copolymer Flory-Huggins (FH) theory. These departures are illustrated by analyzing computed constant pressure spinodal (and binodal) curves, and some computations are compared with experimental data. Different miscibilities are predicted for several A(x)B(1-x)/A and A,A(x)B(1-x) systems with x = 1/2, departing considerably from predictions of FH random copolymer theory. These differences are partially explained in terms of the entropic structural parameter that provides one measure of blend structural asymmetry. The computed phase diagrams of A(x)B(1-x)/C not equal A,B blends exhibit richer miscibility patterns than those derived from FH random copolymer theory. The illustrations focus on the influence of monomer structure, interaction energies, and pressure on the phase behavior of random copolymer/homopolymer systems. Applications to polyolefins employ a model for interaction energies based on Lennard-Jones parameters for these olefins.