Phase Behavior of Block Copolymer/Homopolymer Blends

Abstract

We examine weakly segregated blends of AB diblock copolymer and A homopolymer with similar degrees of polymerization. The relative stability of numerous phases is examined and phase diagrams are constructed using self-consistent field theory. While the pure diblock system is only found to exhibit the body-centered cubic (spherical), hexagonal (cylindrical), bicontinuous Ia3d cubic (gyroid), and lamellar ordered phases, we find that the addition of homopolymer stabilizes close-packed spherical, bicontinuous Pn3m cubic (double-diamond), and hexagonally-perforated (catenoid) lamellar phases. We find that, in general, the minority-component region of a microstructure can only accommodate a limited amount of homopolymer before macrophase separation occurs. On the other hand, the majority-component regions can swell indefinitely with the addition of homopolymer, eventually resulting in an unbinding transition. We associate the region of highly-swollen microstructures with the micellar region observed in real systems. For the lamellar and hexagonal phases, we examine the distribution of homopolymer within the microstructure, and for the lamellar phase, we calculate the effect of homopolymer on the dimensions of the A- and B-rich microdomains.