Pseudo-Single-Crystalline Self-Assembled Structure Formed from Hydrophilic CdSe and Hydrophobic Au Nanoparticles in the Polystyrene and Poly(4-vinylpyridine) Blocks, Respectively, of a Polystyrene-b-poly(4-vinylpyridine) Diblock Copolymer

The composites formed from diblock copolymers and nanoparticles (NPs) have attracted much attention recently because their dimensions are comparable and their applications are versatile; for instance, diblock copolymers can have ordered structures with periodic thicknesses ranging between 10 and 100 nm, while NPs having sizes between 1 and 10 nm have the most interesting optical, electrical, and magnetic properties. Hence, using diblock copolymers as templates to control the spatial locations of NPs is a natural approach to producing hierarchically ordered structures for various applications. In block copolymers, the location of NPs, which critically affects their resulting properties, has been the focus of a number of theoretical and experimental studies. For example, using self-consistent field and density functional theories, NPs have been predicted to decorate the interface of two blocks or be positioned in the center of one particular block, depending on the ratio between the size of the NPs and the periodic thickness of the diblock copolymer. Experimentally, controlling the spatial distributions of NPs in specific domains of block copolymers can be achieved in situ or ex situ, using external fields or directed chemical-binding methods. The embedding of NPs into a copolymer matrix, however, often occurs with NP-induced morphological transformations in diblock copolymer. For instance, simple cubic structures of polystyrene-b-poly(ethylene oxide) (PS-b-PEO)/CdS NPs composites were obtained when using an originally hexagonally packed cylindrical structure of PS-b-PEO. Similarly, the morphological change of the PS-b-P4VP/CdS composite from the hexagonally packed P4VP cylinders into a lamellar structure was induced by the formation of hydrogen bonds between CdS NPs and the P4VP domains. Moreover, NP-induced phase transformation resulting from the choice of solvent or the depth of the bulk film are also critical factors affecting the particle-concentration gradient along the film depth. Most studies into the ordered morphologies formed from diblock copolymer/NP composites involve only one type of NP; indeed, we are aware of only two reports concerning the formation of ordered structures from two different kinds of NPs and a diblock copolymer: the self-assembly of ex situ-synthesized Au NPs and in situ-synthesized iron oxide NPs on a monolayer film of PS-b-P4VP micelles and the self-assembly of small Au NPs and large Si NPs, both coated with alkyl ligands, along the interface and within the poly(ethylene propylene) core, respectively, of a poly(styrene-b-ethylene propylene). Note, however, that long-range order in the ternary composites consisting of a diblock copolymer and two kinds of NPs has not been reported previously. Herein, we demonstrate, for the first time, that long-range ordered tetragonal single-crystal structures can be formed by loading optimal amounts of hydrophobic Au NPs and hydrophilic CdSe NPs into the respective blocks of a PS-b-P4VP. We used high-resolution synchrotron small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) to characterize the structures of the PS-b-P4VP/CdSe/Au ternary composites. A lamellar PS-b-P4VP diblock copolymer [molecular weight (Mn): 20 000 PS/19 000 P4VP; polydispersity index (Mw/Mn): 1.09; Polymer Source, Inc.] was used in all of the experiments. Hydrophilic pyridine-coated CdSe (CdSe-Py) NPs having a core size of 3.5 nm were synthesized using a modified procedure; hydrophobic dodecanethiol-coated Au NPs having a core size of 2.5 nm were synthesized using a two-phase method described by Brust et al. Scheme 1 displays the procedures used to fabricate the ternary composites using a two-step process. First, CdSe NPs and PS-b-P4VP were dissolved together in pyridine. After evaporation of pyridine, the residue was subject to thermal annealing for a binary PS-b-P4VP/CdSe composite. Subsequently, the binary composite and Au NPs were dissolved together in THF. After evaporating THF, the residue was furthermore annealed in dichloromethane for the final product of PS-b-P4VP/CdSe/Au ternary composites. Instead of THF,