Nano-scale Assembly of Metal Clusters
in Block Copolymer Films

@@ The few nanometer (1 nanometer = 10^-9 meter) sized fine metal particles are a group of more than ten atoms called metal cluster known to show electric, optical and catalytic properties different from bulk state. But because of the difficulty in stabilizing ultra fine particles, the researches on dispersing and stabilizing them in different materials are being carried out. Moreover, the technique to achieve spatially ordered arrangements of metal nano clusters in many different materials is an important method to be applied to chemical, optical, magnetic and electronic devices.
@@ We developed a dry route with a single step. This process does not require the help of solvents to provide nanometer-scale assembly of metal clusters with a narrow size distribution within self-assembled microdomain space of block copolymers. The method is explained in Figure 1. The bottom of the glass tube loaded with 10 mg of bis (acetylacetonato) Pd(II), [Pd(II)AA] under reduced pressure (in vacuo) is dipped into the oil bath at 180��. Then Pd (II) AA is sublimed and cooled down around upper side of the tube and condensed on the glass wall again. A polymer film is loaded on the bottom of this glass tube with Pd(II)AA condensed on the wall, and then dipped into the oil bath at 180�� under a nitrogen atmosphere for certain periods. The sublimed Pd (II) AA vapor penetrates into the polymer film and thermally decomposes, reduces simultaneously, forms Pd metal clusters and disperses in the film with stability.
@@ By applying this method to the block copolymer films having microdomain structure, it became clear that the Pd metal clusters of approximately 3.5 nm diameter on average are oriented spatially in the films in a controlled manner as it is shown in Figure 2. The method had been applied to the diblock copolymers of polystyrene (PS) and polymethylmethacrylate (PPMA) and the cross section of the film was observed by a transmission electron microscopy (TEM).
@@ Block copolymer is a polymer composed by different polymers, where the polymer chains are chemically joined at the ends of them, and shows nano scale phase separation with regularity. The repeating distance and the patterns of phase separation structures are decided primarily according to the length and ratio of the block chain. Because the diblock copolymers used here are symmetric with each block having average 70,000 molecular weight, they form the parallel orientation of lamellae of the PS and the PMMA phases. The reason for the formation of such regular assembly of the Pd cluster is due to the different reducing power of the polymers against the Pd (II) AA penetrated in the polymer film, and the Pd cluster forms selectively in the the PS domain simultaneously. It is not clear why the reduction of the Pd (II) AA in the PMMA is slower comparing to that in the PS. In other words, the PMMA has poor efficiency for the production of the Pd cluster. We infer that the interaction between the carbonyl groups of PMMA and Pd (II) ions works to moderate the reduction of the PD (II) AA and that the interaction between carbonyl group of the PMMA side chains and Pd (II) ions contributes to its stability. Also, the growth of clusters from the Pd (II) AA in the polymer film is much faster than thermal decomposition of the Pd (II) AA bulk, which suggests that the polymer itself has catalytic function, but its mechanism is not clear.

The advantages of our method are as follows:

1. The dry process employed here does not require the help of solvents, and therefore have a great advantage in view of environmental issues. In addition, the clusters can be induced while maintaining polymer structure.

2. If there are differences in reducing power on the metal complexes among component polymers forming block copolymers, it is applicable and possible to induce metal clusters in variety of polymers.

3. Block copolymers form various ordered nonadomain structures, such as lamellar, cylindrical, or spherical structures whose morphology and characteristic length scale can be tailored by their molecular weight and composition.

4. It is also expected that the arrangement and the repeating distance of the ordered structure of metal clusters are tunable by the selection of block copolymers not limited to amphiphilic block copolymers.