A Phase Diagram for the Binary Blends of Nearly Symmetric Diblock Copolymers. 1. Parameter Space of Molecular Weight Ratio and Blend Composition

A phase diagram as a function of molecular weight ratio and blend composition was constructed for the binary blends of nearly symmetric polystyrene- block -polyisoprene (PS−PI) copolymers. In this study we prepared four series of binary blends, and every blend consists of a copolymer, hereafter designated as α, whose number-average molecular weight ( M n ) and volume fraction of PS-block ( f PS ) are 1.0 × 10 5 and 0.47, respectively. Four PS−PI copolymers (hereafter designated β), each of which is blended with α, have their M n and f PS in the range 1.2 × 10 4 −2.1 × 10 4 and 0.40−0.49, respectively, and therefore the molecular weight ratio of α to β, hereafter designated as r, ranges between 4.8 and 8.3. All the blends showed partial miscibility of the constituent copolymers; that is, at a certain blend composition the macrophase separation between α and β occurred. However, the range of blend compositions where the constituent copolymers phase-separate became small as r decreased. As for the two blend series where r = 4.8 and 6.9, the PS cylindrical morphology was observed over wide blend compositions, although the constituent copolymers have nearly symmetric compositions and by themselves are self-assembled into lamellar morphology.