Internal Structure of Ultrathin Diblock Copolymer Brushes

Abstract

The internal structure of as-deposited, high grafting density, ultrathin (thickness < 25 nm) diblock copolymer brushes (DCBs) is resolved using neutron reflectivity (NR) and grazing incidence small-angle X-ray scattering (GISAXS). DCBs of various thicknesses containing deuterated polystyrene (dPS) blocks and poly(methyl acrylate) (PMA) blocks with dPS (dPS-b-PMA) or with PMA (PMA-b-dPS) adjacent to the substrate were synthesized by atom transfer radical polymerization (ATRP). For the thinnest films, a model of two layers with a smooth interfacial gradient provides a good description of the data. For thicker dPS-b-PMA samples of sufficiently asymmetric composition, a third layer must be included. This is consistent with the presence of lateral ordering in the center of the brush, as evidenced by GISAXS data. For the thinnest DCBs, the gradient in composition perpendicular to the surface extends through nearly the entire thickness of the brush, consistent with the conjecture that the gradient is imposed by the presence of the surface field and tethering on a material that, in the absence of tethering, would be disordered. The interface widths for brushes with a PMA block tethered to the substrate are smaller than for brushes with a dPS block tethered to the substrate. In general, the region adjacent to the substrate is found to have a substantial composition of the “top” block in contrast to expectations from theory. Experimental interface width values are consistent with expectations from self-consistent field theory for brushes with a dPS bottom block. A scaling theory for the interfacial width in a DCB identifies a crossover as (d/Rg)2 > χN from the classical Helfand−Tagami regime, where w≈b/(χ1/2), to a new stretched interface regime, where w ≈ d/(χN). The scaling theory provides insight into how interface width in the DCBs should vary with grafting density, interaction parameter, and chain molecular weight and is qualitatively consistent with the experimental data and suggests directions for further work.