MOLECULAR INFLUENCES ON MISCIBILITY PATTERNS IN RANDOM COPOLYMER HOMOPOLYMER BINARY BLENDS/

The lattice cluster theory (LCT) is used to study the microscopic mole cular factors affecting the miscibilities of A(x)B(1-x)/C binary mixtu res (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 i n describing gross departures of LCT predictions from the prevailing r andom copolymer Flory-Huggins (FH) theory. These departures are illust rated by analyzing computed constant pressure spinodal (and binodal) c urves, and some computations are compared with experimental data. Diff erent 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 measu re 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 pres sure on the phase behavior of random copolymer/homopolymer systems. Ap plications to polyolefins employ a model for interaction energies base d on Lennard-Jones parameters for these olefins.