Tailoring Core−Shell Polymer-Coated Nanoparticles as Block Copolymer Surfactants

Abstract

The effect of core−shell polymer-coated nanoparticles on block copolymer morphology is investigated systematically by controlling particle positioning within a block copolymer template. Our approach of varying the areal chain density (Σ) of short thiol terminated polystyrene (PS-SH) brushes on the particles where the molecular weight (Mn) of the PS ligands (2.5 kg/mol) is used to control the fraction of the particles adsorbed to the interface of PS-b-P2VP diblock copolymer domains (Mn = 196 kg/mol). The change in Σ from 2.38 to 0.49 chains/nm2 brings systematic control over the fraction of Au particles bound to the PS/P2VP interface (fi), this fraction ranging from 0 to 95%. Particles with Σ of 2.38 chains/nm2 (fi ∼ 0) were observed to localize within the PS domains and further produce a macroscopic particle aggregated phase separate from the PS-b-P2VP mesophase. In contrast, decreasing the Σ of PS chains provides for strong binding of Au nanoparticles to the PS/P2VP interfaces as a consequence of preferential wetting of one block of the copolymer (P2VP) to the Au substrate. Interestingly, the addition of low volume fractions (ϕP) of such nanoparticle surfactants (Σ < 1.1 chains/nm2) to lamellar diblock copolymers initially leads to a decrease in lamellar thickness, a consequence of decreasing interfacial tension, up to a critical value of ϕP beyond which the block copolymer adopts a bicontinuous morphology. The relationship between domain spacing and morphology of block copolymer with the level of nanoparticle surfactants will be discussed based on a strong-segregation model. In addition, the rational design of nanoparticle surfactants based on the nature and density of the grafted chains will be described.