Nanoparticle-Induced Phase Transitions in Diblock-Copolymer Films

Incorporation of nanoparticles into self-assembled block copolymers has been explored as an efficient way for improving the mechanical strength, electrical conductivity, and optical properties of materials at the nanometer scale. Recent computer simulations predict that the cooperative self-organization of nanoparticles and block copolymers should yield a wide variety of structures with well-controlled particle arrangements. Such nanostructures may make it possible to fabricate novel functional materials, such as photonic bandgap materials, nanostructured solar cells, highly efficient catalysts, and high-density magnetic storage media. In particular, gold nanoparticles have attracted much interest for applications as catalysts as well as building blocks for electronic devices that operate at the single-electron level. Over the years, several experimental methods have been developed for incorporating inorganic nanoparticles into polymeric nanostructures. These can be divided into two approaches. The first approach involves synthesizing nanoparticles in situ within preformed block-copolymer structures. Preformed micelles of block copolymers containing metal precursors are used as nanoreactors to synthesize nanoparticles selectively in block copolymers. For example, Boontongkong et al. produced micropatterned silver nanoparticles in a poly(styrene-b-acrylic acid) block-copolymer template using this approach. Such an approach is simple and can be easily extended to yield large-area samples. However, controlling the arrangement of the nanoparticles within the periodic structure of the block copolymer is difficult. A second approach, recently proposed as a way to avoid some of the drawbacks of the method described above, uses cooperative self-organization of nanoparticles and block copolymers. Based on theoretical predictions of the morphology of inorganic–organic hybrid materials by Balazs and coworkers, Bockstaller et al. demonstrated hierarchical pattern formation using block copolymers and binary mixtures of different sized hydrophobic nanoparticles. More recently, our previous work showed how grafting different polymers to particle surfaces can be used to precisely control the placement of particles either at the center of one of the block-copolymer domains or at the interface dividing the two block-copolymer domains. This approach exploits enthalpic interactions between the block copolymer and functionalized nanoparticle surfaces to achieve precise particle placement. Such precise control of nanoparticle position in block-copolymer thin films is important for developing highly organized hybrid materials. A particularly interesting example is the work of Russell and co-workers, who showed that the orientation of block-copolymer domains in thin films could be controlled by introducing surface-active nanoparticles that preferentially segregated to the surface of high-surface-energy domains. However, the introduction of particles into block copolymers can also have profound effects on the overall morphology of nanocomposites, especially if the particle loading is high. Nevertheless, the effect of high particle loading on the overall composite morphology has not been explored in previous experimental studies. In this communication, we report a nanoparticle-induced phase transition in a block-copolymer thin film as well as the formation of different structures of gold nanoparticles. When gold nanoparticles are coated with short polymer chains that are chemically identical to one of the copolymer blocks (in what follows we refer to such particles as simply Au particles, but it should be understood that in all cases they are coated with short polymer chains), the particles can self assemble near the center of that copolymer domain, as shown in Figure 1a. In this case, due to the coating of polystyrene (PS) chains on gold particles, the interaction between gold particles and one of the polymer blocks (PS) is neutral, but the interaction between the gold particles and the other block (P2 VP) is very unfavorable. In this report, we show that the lamellar morphology remains unchanged only if the gold nanoparticle volume fraction, cau, is below some critical value, cau,cr. For cau > cau,cr, the local concentration of gold nanoparticles varies along the thickness of the film, and different coexisting morphologies emerge as a function of sample depth. For these experiments, symmetric block copolymers of polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) with total molecular masses, Mn, of 59 kg mol –1 and 114 kg mol were used. Gold nanoparticles with an average diameter of 2.5 nm were stabilized by short thiol end-functionalized PS chains (PS-SH; Mn = 1.3 kg mol ) that were grafted to the nanoparticle surfaces to make them compatible with the PS block and to prevent aggregation. The transmission electron microscopy (TEM) image in Figure 1a shows the cross-sectional morphology of the film for cau = 0.14 (< cau,cr) cast from toluene. The lamellae lie parallel to the film surface, as expected. Since the P2VP was preferentially stained by iodine vapor, it appears darker than the PS in C O M M U N IC A TI O N S