Effect of Overall Grafting Density on Microphase Separation of Mixed Homopolymer Brushes Synthesized from Y-Initiator-Functionalized Silica Particles

Abstract

We report in this article a systematic study on the effect of overall grafting density (σ overall ) on microphase separation of mixed poly( tert -butyl acrylate) (P t BA)/polystyrene (PS) brushes synthesized from asymmetric difunctional initiator (Y-initiator)-functionalized silica particles. The initiator particles were made by the immobilization of a triethoxysilane-terminated Y-initiator on the surface of 173 nm silica particles via an ammonia-catalyzed hydrolysis/condensation process in ethanol. Mixed P t BA/PS brushes were then grown from initiator particles by surface-initiated atom transfer radical polymerization of t BA at 75 °C and nitroxide-mediated radical polymerization of styrene at 120 °C. By changing the mass ratio of Y-initiator to silica particles in the initiator immobilization step, we found that the overall grafting density of mixed brushes can be systematically tuned while the individual grafting densities of two polymers remained comparable. A series of mixed P t BA/PS brushes with molecular weights of ∼23 kDa for both P t BA and PS and σ overall from 1.06 to 0.122 chains/nm 2 were prepared. Their microphase separation behavior was studied by transmission electron microscopy after the samples were drop-cast from chloroform onto carbon-coated TEM grids, annealed with CHCl 3 vapor, and stained with RuO 4 . CHCl 3 is a good solvent for both grafted polymers. When the σ overall was 0.34 chains/nm 2 and above, the mixed P t BA/PS brushes underwent lateral microphase separation, forming “rippled” nanostructures. The ripple wavelength ( D ) increased with decreasing σ overall; the normalized D scaled with σ overall –0.47 in the σ overall range of 1.06–0.54 chains/nm 2 . With further decreasing the grafting density, the phase separation became weaker, and no microphase separation was observed in the sample with σ overall of 0.122 chain/nm 2 .