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Direct Visualization of Three-Dimensional Morphology in Hierarchically Self-Assembled Mixed Poly(tert-butyl acrylate)/Polystyrene Brush-Grafted Silica Nanoparticles

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posted on 27.08.2013, 00:00 by Saide Tang, Ting-Ya Lo, Jonathan M. Horton, Chunhui Bao, Ping Tang, Feng Qiu, Rong-Ming Ho, Bin Zhao, Lei Zhu
Three-dimensional (3D) morphology of hierarchically self-assembled mixed poly­(tert-butyl acrylate) (PtBA)/polystyrene (PS) brush-grafted 67 nm silica nanoparticles cast from chloroform on a carbon-coated transmission electron microscopy (TEM) grid was directly visualized using electron tomography (i.e., 3D TEM). The number-average molecular weights for PtBA and PS were 22.2 and 23.4 kDa, and their grafting densities were 0.51 and 0.34 chains/nm2, respectively. For a dense monolayer of mixed brush-grafted silica particles, a rippled phase structure was observed, and the monolayer consisted of protruded top portions from large nanoparticles and a bottom continuous film. In the top protruded part of large nanoparticles, isolated cylindrical PS microdomains were formed in the PtBA matrix due to the low PS volume fraction (i.e., 37%). The morphology in the lower continuous film was strongly influenced by interparticle interactions via mixed polymer brushes. As a result, bicontinuous nanostructures instead of isolated PS microdomains in the PtBA matrix were formed, even though the PS was a minor phase. In the continuous film, the top and bottom regions gave better microphase separation due to the larger interstitial spaces, and the microphase-separated pattern was dictated by the hexagonal packing of hairy silica particles. For isolated individual particles cast from chloroform, different morphology was observed because of the lack of interparticle interactions. Clear lateral microphase separation was observed only in the bottom part of the particle, likely because the mixed brush layer in the top portion was too thin. In the bottom part, isolated PS microdomains with a truncated wedge shape radiated out from the projected center of the individual particle. This is the first time that detailed 3D morphology was thoroughly characterized for self-assembled mixed brush-grafted nanoparticles cast from a nonselective good solvent.