Morphology of binary blends containing diblock copolymer (AB) and homopolymer (hA) was studied as a function of homopolymer molecular weight, homopolymer concentration, and copolymer composition and molecular weight. Poly(styrene-b-isoprene) (SI) or poly(styrene-b-butadiene) diblock copolymers were blended with homopolystyrene (hPS), homopolyisoprene or homopolybutadiene. The equilibrium morphologies were studied primarily by transmission electron microscopy and small angle x-ray scattering.
The lamellar morphology was studied in blends with a lamellar SI diblock copolymer and low hPS concentrations. The area per junction measures the lateral separation between the copolymers, while the layer thicknesses measure the axial expansion. As hPS concentration increased the thickness of the PI layer decreased indicating polyisoprene chain relaxation. The area per junction increased as the homopolymer concentration increased and as the homopolymer molecular weight decreased indicating a greater extent of hPS penetration and lateral swelling of the PS blocks. The swelling of the PS layers becomes asymmetric as the homopolymer concentration or molecular weight increases.
The ordered bicontinuous double diamond (OBDD) morphology was observed in AB/hA blends using a lamellar or cylindrical AB. An overall polystyrene volume percent of $\sim$64-67 PSvol% and a relative homopolymer molecular weight of $\sim$0.15-1.00 were required to produce the OBDD morphology; the relative molecular weight is the molecular weight ratio of hA and the A block.
The overall blend composition generally dictated the type of ordered blend morphology. Increasing the homopolymer molecular weight at an overall composition of 65 PSvol% produced unique order-order transitions between cylinders, OBDD and lamellae. The intermaterial dividing surface was characterized by the mean curvature and the area per copolymer junction which increased as the homopolymer concentration increased and/or the homopolymer molecular weight decreased. These trends were generally obeyed both between and within ordered morphology types.
The morphologies observed in AB/hA blends were summarized in two new types of isothermal morphology diagrams. The constant molecular weight morphology diagrams illustrated the interdependence of the copolymer composition and the homopolymer concentration. The constant copolymer composition diagrams emphasized the importance of the relative homopolymer molecular weight and the overall blend composition. These morphology diagrams will guide future theoretical developments.
