Phase behavior of the blend of rod-coil diblock copolymer and the corresponding coil homopolymer

Blending with a homopolymer is an effective approach to tailor the microphase-separated morphology of block copolymers. It has been established for the blends of coil-coil diblock copolymer (A-b-B) with the corresponding homopolymer (h-A) that h-A can be solubilized uniformly into A mcirodomain to induce structure transformation when its molecular weight is smaller than that of the A block, i.e., α = M_h-A/M_b-A < 1. Here we examine if the microdomain structure of a rod-coil diblock copolymer, poly(2,5-diethylhexyloxy-1,4-phenylenevinylene)-block-poly(methyl methacrylate) (DEH-PPV-b-PMMA), may be systematically tuned by blending with PMMA hompolymer (h-PMMA). The blends over the major composition window were found to undergo macrophase separation even when the value of α was as low as 0.3. The phase separation led to the formation of a copolymer-rich phase and a homopolymer-rich phase, in which microphase separation occurred, yielding a well-ordered lamellar structure and a sponge structure, respectively. The phase behavior of the blend of rod-coil diblock with the corresponding coil homopolymer is hence fundamentally different from that of the conventional blend of coil-coil diblock in that microdomain morphology transformation induced by completely uniform solublization of coil homopolymer into the selective microdomain is essentially inaccessible. This fact is attributed to the large positive free energy of mixing between the whole rod-coil diblock and the coil homopolymer due to strong rod-rod attraction, the intrinsically strong repulsion between DEH-PPV and PMMA, and the low entropy of mixing.