Inverted Cylindrical Microdomains by Blending Star-Shaped and Linear Block Copolymers

Abstract

We investigated the morphology formed in the binary blend of six-arm star-shaped (poly(methyl methacrylate)-block-polystyrene)6 copolymer [(PMMA-b-PS)6] and PMMA-b-PS linear diblock copolymer by varying their molecular weights as well as volume fractions of the blocks. When the molecular weight of PMMA-b-PS is much larger (> ∼4) than that of one arm of (PMMA-b-PS)6, PMMA-cylindrical microdomains are formed even though the volume fraction of PMMA (fPMMA) in both (PMMA-b-PS)6 and PMMA-b-PS is nearly symmetric (fPMMA ∼0.5). On the other hand, when the ratio of molecular weights between these two copolymers is not large, lamellar morphology is observed in the blend as expected. Very interestingly, we found that even for a binary blend with the overall volume fraction of the PMMA block (f̅PMMA) as large as 0.71, the major PMMA blocks still aggregate into cylindrical microdomains, and thus, “inverted cylinders” are formed, although PS-cylinders are observed in the neat (PMMA-b-PS)6 and PMMA-b-PS melts. This interesting inverted cylinder is mainly stabilized by two factors. On the one hand, the long linear diblock copolymer swells the domain significantly, thus preventing the short (PMMA-b-PS)6 star copolymer from forming the favorable bridging configurations in order to avoid the high stretching energy. As long as the bridging configurations are prohibited, the PMMA-core blocks of (PMMA-b-PS)6 prefer to stay inside the curvature, amplifying the tendency of forming a spontaneous curvature toward PMMA-blocks. On the other hand, the radial distribution of the long PMMA-block of the diblock and the short PMMA-block of the star increases the spontaneous curvature. The experimental results as well as the formation of the inverted cylinders have been verified by self-consistent field theory (SCFT).