Experimental studies of symmetric block copolymer blends in thin films.

Microdomain size control in thin films of blended symmetric poly(styrene-\textit{block-}methyl methacrylate) [PS-$b$-PMMA] was examined. Rough substrates and high temperatures were used to prepare perpendicularly oriented PS-$b$-PMMA lamellae. A large block copolymer [BCP], (with $N_{\mbox{large}} $ monomer units) and a small BCP (with N$_{small}$ monomer units) were mixed in a range of composition using a set of symmetric PS-$b$-PMMA BCPs (where $\chi N$ ranged from 20$\sim $100). Thus the blend was characterized by; blend composition, the ratio $\alpha =N_{\mbox{large}} /N_{\mbox{small}} $ and $\chi N_{\mbox{large}} $. AFM studies of the perpendicular surface structure determined the effect of blend composition on lamella periodicity, $D_{mix}$. Depth profile analysis and x-ray scattering studies determined the copolymer distribution in the film and the bulk lamellar period, respectively. The effect of composition on $D_{mix}$ was compared to an analytic mean field theory in the strong segregation limit. Whilst good agreement was observed when both $\chi N_{\mbox{large}} $ and $\chi N_{\mbox{s}mall} $ were large ($\sim $70-100), there were significant deviations from the theory when $\chi N_{\mbox{s}mall} \sim $ 20-60. However in agreement with bulk blend studies, block copolymer macrophase separation was observed at the polymer air surface when the relative size of the block copolymer pairs exceeded $\alpha \sim $5.