Phase behavior of star-shaped polystyrene-block-poly(methyl methacrylate) copolymer

Phase behavior of star-shaped 18-arm polystyrene-block-poly(methyl methacrylate) copolymers ((PS-b-PMMA)$_{18}$) with various volume fraction of PS block (f$_{PS}$) was investigated via transmission electron microscopy and small angle X-ray scattering. (PS-b-PMMA)$_{18}$ was synthesized by atom transfer radical polymerization from $\alpha$-cyclodextrin ($\alpha$-CD) having 18 functional groups for the initiation. We also prepared corresponding linear PS-b-PMMAs by cutting the ester groups connecting $\alpha$-CD and PS chains in (PS-b-PMMA)$_{18}$ through the hydrolysis. The microdomains of (PS-b-PMMA)$_{18}$ changed from body-centered cubic spheres, hexagonally packed cylinders, perforated lamellae, and lamellae with decreasing fPS from 0.7 to 0.2. Interestingly, (PS-b-PMMA)n with f$_{PS}$ of 0.23 showed highly asymmetric lamellar microdomains, while corresponding linear PS-b-PMMA with the same volume fraction exhibited spherical microdomains. Thus, the microdomains are highly affected by the molecular architecture of block copolymer.