Local composition of nanophase-separated mixed polymer brushes

Polystyrene-poly(methyl methacrylate) (PS-PMMA) mixed brushes synthesized by surface-initiated polymerization show nanophase separation into defined pattern depending on the molecular parameters of the brushes. Two sets of mixed brushes are studied: (i) with fixed grafting density and molecular weight of the PS chains, but differing in the molecular weight of PMMA polymer, and (ii) with varying grafting density of the PMMA chains while that of the PS chains and the molecular weight of PS and PMMA chains are kept constant. The local distribution of PS and PMMA chains within the monolayer and the size, shape, and position of the domains constituting the nanopattern are found to vary with the nature of the solvent to which the brushes are exposed. The brushes are treated cyclically first with either a good solvent for both blocks, leading to strong swelling and structure erasure, and then with a selective solvent, which induces the nanophase separation. It was found that in the case of the brush exposed to toluene solvent (good solvent for both polymers) the brush surface exhibiting a small variation in topography has a heterogeneous surface composition, with the nanoscopic areas having only PS or PMMA chains at the surface. When the brush is treated with acetone solvent, which is better for the PMMA chains, the surface consists only of PMMA chains, and the topography assumes a more pronounced relief. We introduce the concept of the local domain memory effect of the brushes, i.e., whether the brush locally forms always the same pattern or if the local assembly of the domains emerges in different places every time the transition to the structured state occurs.