Mushrooms and Brushes in Thin Films of Diblock Copolymer/Homopolymer Mixtures

The configuration of polymer chains end-adsorbed at a surface from the melt is probed by taking advantage of the interfacial segregation of diblock copolymers to the substrate/polymer interface from their mixtures with homopolymers in thin films as a function of the ratio of block lengths. The segment density profiles of either PV2P-PS or PMMA-PS diblocks adsorbed at the PS/substrate interface are evaluated by neutron reflectivity as a function of the relative size of the anchoring block vs that of the dangling block. Moreover, the composition profiles and conformational characteristics of all chain species present in the interfacial region are evaluated using a lattice-based self-consistent field model inspired by the work of Scheutjens and Fleer and extended to incorporate chain conformational stiffness. Inputs to the model are the molecular characteristics of the various species, interaction parameters extracted from experimental binary interfacial widths, and experimental data on the total surface excess. When the ratios of the block-lengths are changed, both experiment and theory reveal evidence for the existence of three regimes regarding the configuration of the dangling chains: a "wet brush" regime, a "mushroom" regime, and a broad transition regime in between.