Phase Diagrams for the Blends of a Styrene-Butadiene Diblock Copolymer and a Styrene-Butadiene Random Copolymer: Theory

The phase diagram of the blends of a diblock copolymer (A−B) and a random copolymer (ABR) is constructed by the modified confined-chain model developed earlier where the morphology of A−B block copolymer is lamellae and the composition of ABR random copolymer is close to that of A−B. These theoretical results predict a relatively simple phase behavior for the A−B/ABR blends in comparison to the A−B/A blends, A being a homopolymer, studied previously. Unlike in A−B/A blends, the order−disorder transition temperature in A−B/ABR blends is lowered by the addition of the random copolymer regardless of its molecular weight. It is also found that the fraction of ABR solubilized into the A−B is very limited due to the endothermic mixing interaction between ABR and each block of A−B.