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 (f PMMA) in both (PMMA-b-PS)6 and PMMA-b-PS is nearly symmetric (f PMMA ∼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 (f̅ PMMA) 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).

Phase Behavior of PS-(PS-b-P2VP)3 Miktoarm Star Copolymer

“…Moreover, because PS L is attached to the junction point of a three-arm star-shaped block copolymer (PS S - b -P2VP) 3 , the arrangement that all PS S - b -P2VP chains are aligned along the same direction causes a configurational entropy penalty. Because the star polymer has a tendency to maintain its own star shape, this entropy penalty can be reduced by forming a curvature toward the PS microdomain. However, for the formation of P2VP cylinders (or P2VP spheres), the curved interface is aligned toward the P2VP microdomain (Figure d2), which is the opposite conformation to the radial distribution of PS chains.…”

“…For instance, symmetric AB diblock copolymer shows lamellar microdomain, while cylindrical microdomains composed of the minor A block are observed at 0.2 < f A < 0.35, and double gyroids consisting of A channels are found at a narrow range of f A (∼0.35). Many research groups have extensively tried to shift the above boundary of volume fraction between microdomains. − …”

“…The introduction of a complex-shaped chain architecture, such as miktoarm star copolymers, grafted copolymers, and star-shaped block copolymers, is an efficient way to shift the boundary of the phase diagram. − Gido and co-workers studied the morphologies of six-arm miktoarm star block copolymers consisting of five arms of polyisoprene (PI) and one arm of polystyrene (PS) (I 5 S). Because of the chain asymmetry, the curved interface was energetically more preferred, resulting in a skewed phase diagram toward a higher volume fraction of the PI block.…”

Inverted Cylindrical Microdomains from Binary Block Copolymer Blends Capable of Hydrogen Bonding

“…22 Recently, we observed inverted cylindrical microdomains consisting of poly(methyl methacrylate) (PMMA) chains at f PMMA ∼ 0.7 by blending six arm star-shaped (PMMA-b-PS) 6 copolymer and high-molecular-weight PMMA-b-PS diblock copolymer. 23 However, these complex architectures of BCPs require a large molecular weight and rather complicated synthesis compared with linear BCPs, which limits preparing inverted cylinders with smaller microdomain size. Thus, an effective strategy to form inverted cylindrical microdomains with smaller microdomains based on diblock copolymers is needed.…”

Accelerated Method of Self-Consistent Field Theory for the Study of Gaussian Ring-Type Block Copolymers

“…In past decades, a few highly efficient numerical methods have been developed to solve SCFT equations, enabling the accurate calculation of the free energy of various ordered structures. − In addition, SCFT has another remarkable advantage; that is, it can deal with various chain architectures readily . Accordingly, SCFT has been widely used to construct the phase diagrams of a variety of block copolymers. − No doubt, SCFT interplaying with experiments plays an irreplaceable role in understanding the self-assembly mechanism of block copolymers, especially the influence of the molecular architecture on the self-assembly. ,,− …”