Symmetric Diblock Copolymer Thin Films on Rough Substrates: Microdomain Periodicity in Pure and Blended Films

Abstract

The lamellar dimension, D, for pure and blended block copolymer (BCPs) thin films of symmetric poly(styrene)-block-poly(methyl methacrylate) was measured using atomic force microscopy analysis of surface patterns of perpendicularly oriented lamellar structures. It was approximately verified, using SAXS and AFM analysis, that perpendicular structures in lamellar thin films bounded by a neutral and a roughened interface did not significantly alter the bulk block copolymer phase separation thermodynamics. In the case of blends, it was also verified that there was a uniform distribution of blend components throughout the thin film. These checks allowed a detailed comparison of D as a function of the number of statistical segments (N) and blend composition using existing bulk phase predictions and experiments. In pure BCPs we observed the two characteristic regimes; strong (D ∼ N0.66) and intermediate segregation regimes (D ∼ N0.85). In addition, we observed a more complicated, weakly segregated regime where D does not scale as N0.5 as expected for a random coil. In blended BCPs there was good agreement with existing theory when BCP components from the strongly segregated regime were used. In cases where intermediate or weakly segregating BCP components were used, an alternative semiempirical analytical function was developed. In cases where the ratio of N for the BCP blend components exceeds ∼5, macrophase separation was observed in the thin film with the smaller BCP component remaining at the air surface.