Hierarchical Structures in Thin Films of Macrophase- and Microphase-Separated AB/AC Diblock Copolymer Blends

Interesting hierarchical structures are generated in thin films of the AB/AC diblock copolymer (diBCP) blends of poly(dimethylsiloxane)-b-poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PDMS-b-PMPCS, DMPCS) rod-coil diBCP and poly(dimethylsiloxane)-b-poly(methyl methacrylate) (PDMS-b-PMMA, DMMA) coil-coil diBCP with the common block as the minor component in both diBCPs. The macrophase separation and microphase separation occur in the DMPCS/DMMA BCP blends in bulk, confirmed by small angle X-ray scattering (SAXS) results. Moreover, the macrophase- and microphase-separated morphologies in thin films of the DMPCS/DMMA BCP blends are directly observed by transmission electron microscopy experiments owing to the different electron densities among the three different blocks. For the blends of DMPCS and DMMA, both of which have the nanostructures of hexagonally packed cylinders (HEX) (DMPCSHEX/DMMA(HEX)), when the blend contains 75 wt % of one diBCP, subordered macrophase-separated structures with ordered nanostructures in the macrodomains develop in the thin film. When the matrix of the macrophase is the coil-coil DMMA(HEX) diBCP which has the nanostructure of vertically oriented cylinders in the thin film, the macrophase-separated submicrometer structures become more ordered, and the interfaces of the macrodomains become more smooth. For the blends of the lamellar DMPCS and the HEX-structured DMMA having similar volume fractions of PDMS (DMPCSLAM/DMMA(HEX)), with 75 wt % of lamellar DMPCS in the blend, hamburger-like structures form in the DMPCSLAM macromatrix of the thin film, which is ascribed to the solubility of DMMA in the lamellar DMPCS on the segmental length scale. When the weight fraction of the lamellar DMPCS in the blend is 25%, the short DMPCS lamellae with a few layers are uniformly dispersed in the HEX-structured DMMA macromatrix.